Microelectronics Reliability

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1 Microelectronics Reliability 49 (2009) Contents lists available at ScienceDirect Microelectronics Reliability journal homepage: Research advances in nano-composite solders J. Shen, Y.C. Chan * Department of Electronic Engineering, City University of Hong Kong, Tat Chee Avenue, Kowloon Tong, Hong Kong article info abstract Article history: Received 14 July 2008 Received in revised form 6 October 2008 Available online 17 November 2008 Recently, nano-composite solders have been developed in the electronic packaging materials industry to improve the creep and thermo-mechanical fatigue resistance of solder joints to be used in service at high temperatures and under thermo-mechanical fatigue conditions. This paper reviews the driving force for the development of nano-composite solders in the electronic packaging industry and the research advances of the composite solders developed. The rationale for the preparation of nano-composite solders are presented at first. Examples of two nano-composite solder fabrication methods, a mechanical mixing method and an in-situ method, are explained in detail. The achievements and enhancements in the nano-composite prepared solders are summarized. The difficulties and problems existing in the fabrication of nano-composite solders are discussed. Finally, a novel nano-structure composite solder, which attempts to solve the problems encountered in the fabrication of nano-composite solders, is introduced in detail. Guidelines for the development of nano-composite solders are then provided. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Solder materials play crucial roles in the reliability of joint assemblies in electronic packaging because they provide electrical, thermal and mechanical continuity in electronic assemblies [1]. Lead-based solders have been in use for a relatively long time, resulting in an extensive database for the reliability of these materials [2]. Recently, increasing environmental and health concerns over the toxicity of lead combined with strict legislation to ban the use of lead-based solders have provided an inevitable driving force for the development of lead-free solder alloys. However, the knowledge base on lead-based solders gained by experience is not directly applicable to lead-free solders. In other words, a database for modeling the reliability of lead-free solders is not currently available [1,3]. With the development of lead-free solders to be used in the electronic packaging industry, there are several challenges to be met. The sustained trend towards miniaturization and functional density enhancement requires much smaller solder joints and fine-pitch interconnections for microelectronic packaging in electronic devices. For example, portable electronic devices, such as portable computers and mobile phones, have become thinner and smaller with more complicated functions. The miniaturization of these electronic devices demands better solder-joint reliability. Hence, flip chip (FC) and ball grid array (BGA) technologies are widely used in these portable devices because of their high density input/output (I/O) connections in a limited space [4]. These ultrafine solder joints packaged in a narrow space will lead to a high * Corresponding author. Tel.: ; fax: address: eeycchan@cityu.edu.hk (Y.C. Chan). homologous temperature during service which may lead to coarsening of the microstructure of the solder joints. So, solder alloys to be utilized in high density packaging electronic devices must stand up to relatively high service temperatures. That is, high temperature solder alloys must be developed to face the challenge of the extensive miniaturization of microelectronic packaging in electronic devices. During the switching on and off electronic devices, the electrical circuits in electronic devices heat up and cool down, thus experiencing low cycle thermo-mechanical fatigue (TMF) due to stresses that develop as a consequence of the coefficient of thermal expansion (CTE) mismatch between the solder, the substrate and the components [5]. These thermal cycles cause plastic straining of solder joints which experience long hold times at stress extremes at two significantly different temperatures. So, this stress may be sufficient to cause low strain rate creep deformation of the solder during the hold time at the higher temperature [6,7]. The creep resistance of solder joints is essential in high precision packaging (such as optoelectronic packaging) because of the need to maintain positional accuracy of the optoelectronic components over extended periods of time. For instance, because of the creep of solder, time-dependent and gradual misalignment between a solid state laser chip and a spherical lens on a fibre in lightguide ocean cables will reduce the transmission intensity or even cause complete loss of the lightwave communication signals in the cables [8]. In defense applications, the mechanical vibration of tanks, aeroplanes or missiles will create high frequency vibrational fatigue conditions for solder joints in the attached electronic components and lead these weapons to fail. Hence, highly creep and fatigue resistant lead-free solders must be developed to solve these problems [9 11] /$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi: /j.microrel

2 224 J. Shen, Y.C. Chan / Microelectronics Reliability 49 (2009) It is well known that solders are low-melting alloys with room temperature being 40 50% of their melting temperature in Kelvin. Most solder alloys are used in service at relatively high temperature conditions compared with their melting temperatures [12]. So, any deformation imposed on the solder joints during their operation is a creep deformation in the metallurgical sense [7,13,14]. The creep of solders has not received much attention when they were used in traditional large size consumer electronic devices, in which they are usually not required to suffer relatively high temperatures and thermo-mechanical fatigue environmental conditions. However, in recent years, the trend towards extreme miniaturization and precise electronic packaging has promoted the development of creep and thermo-mechanical fatigue resistance solders which can withstand high temperature environmental conditions. Of the lead-free solders developed, only the high melting point Sn 80Au alloy can withstand high temperature service conditions. However, this alloy cannot be utilized in a wide range of manufacturing situations due to its cost and the degradation of the properties of electronic components by high temperature soldering [9,12]. In addition, although the fine-grained microstructure developed in lead-free solders may be beneficial from a mechanical fatigue stand point, it may not be ideal for creep resistance since creep deformation at the service temperature may be by grain boundary sliding. Hence, an attractive and potentially viable method of enhancing the creep and thermo-mechanical fatigue resistance of solder materials is to adopt a composite approach. that a specific interaction must take place between the liquid solder and the solid surface of the parts to be soldered. The ability of molten solders to flow or spread during the soldering process is of prime importance for the formation of a proper metallic bond and thus to achieve a perfect joint. Hence, in order to obtain solder joints of good quality, the modified nano-composite solders should function in a similar manner to monolithic solders by appropriately demonstrating good wetting of the substrate, proper flow characteristics, and reliable bonding Strengthening effect The purpose of using composite processing technology to prepare modified composite solders is to improve the creep and thermo-mechanical fatigue resistance of solder materials by the strengthening effect of the reinforcement. To obtain a good strengthening effect, the reinforcement particles should be appropriately fine and distributed homogeneously throughout the solidified solder joints to promote stabilization of a fine-grained microstructure in the solder matrix, to reduce the probability of crack initiation and enhance the fatigue resistance. Furthermore, the reinforcement particles should provide sufficient locking of the grain boundaries to limit grain boundary sliding to improve the creep resistance, since dispersions of very fine particles that are of uniform size and homogeneously distributed can provide creep resistance by impeding grain boundary sliding and also dislocation movement [7,17]. According to the Wagner Lifschitz Slivoz equation for Ostwald ripening [18]: 2. Rationale for the development of nano-composite solders d 3 ðtþ d 3 0 ¼ adcc 0t ð1þ Solder alloys with intentionally incorporated reinforcements are termed composite solders. When the reinforcements are on the nano scale, the composite solders are referred to as nano-composite solders. In these solders, the composite approach has been developed mainly to improve the service performance, especially the creep and thermo-mechanical fatigue resistance of solder joints. Nano-sized reinforcement particles have been selected to be added in solder matrices due to their effectiveness in improving the creep resistance by being distributed at the grain boundaries to limit grain boundary sliding [7,15]. Several composite processing methods may be used to prepare nano-composite solders and some conditions should be obeyed during the composite operation [11] Melt temperature The melting temperature is perhaps the first and most important factor for the development of new solder materials in the microelectronic packaging industry. Most of the assembly equipment in use today is designed to operate at 456 K (the eutectic temperature of Sn Pb is 456 K) as a base reference temperature. If the melting points of the nano-composite solders developed are significantly higher, then new equipment will have to be purchased by manufacturers, leading to significant capital expenditure and product cost increases [1]. Hence, an important feature in the development of nano-composite solders is that the reinforcements should not increase the melting points of the solder alloys significantly (if possible, it is desirable that the reinforcements will reduce the melting points of the solder alloys), but may effectively increase the service temperatures of the base solder materials Wettability/solderability During soldering, in order to form a proper metallurgical bond between two materials, wetting must take place [16]. This means where d(t) = particle size at time t, d 0 = initial size, a = a constant, D is the diffusion coefficient, c is the interfacial energy, and c 0 is the solubility of elements of which the particles are made in the matrix. For maximum dimensional stability, the dispersoid particles should be chosen to have minimum diffusivity, a low interfacial energy, and a low solubility in the matrix. Hence, to be effective obstacles to dislocation motion, the particles must be sufficiently fine, stable with respect to size and inter-particle spacing, have a higher flow resistance than the matrix, and be undeformable and resistant to fracture [17]. That is, nano-sized particles or fibres are seen as the best candidate as reinforcement particles Interfacial stability A thin, continuous and uniform intermetallic compound (IMC) layer between a solder and the substrate material is an essential requirement for good bonding. However, due to the inherently brittle nature of IMCs and their tendency to generate structural defects, IMC layers which are too thick at the solder-substrate material interface may degrade the reliability of solder joints [19]. An important characteristic of the reinforcement particles is that they should be compatible with the solder alloy matrix to avoid excessive growth of interfacial layers that could deteriorate the properties of the joint. In particular, in order to improve the reliability of a solder joint under thermo-mechanical fatigue conditions, such reinforcement particles should be designed to retard the growth of the IMC layers that form at the solder substrate interface Ductility In order to obtain a solder joint with high creep resistance, one may believe the solder material should be as strong as possible. This is not ideal in electronic device applications although it is easy to add rigid reinforcements into solder matrices to improve the strength of solder alloys. This is because the CTE of the solder itself

3 J. Shen, Y.C. Chan / Microelectronics Reliability 49 (2009) may be altered to reduce the thermal stresses that could develop with low CTE substrates by adjusting the volume fraction of the reinforcement. If the solder in the joint is not able to dissipate the stresses that develop, failure of the electronic component will result [5,6,20]. So, a composite solder possessing a modest increase in strength with a significant enhancement in ductility and creep resistance under heavy mechanical overloads will be the most suitable compromise. 3. Fabrication of nano-composite solders In the existing literature [21 42], methods to introduce the desired reinforcement particles into solder matrices to prepare nanocomposite solders may be classified as two: a mechanical mixing method and an in-situ method. Foreign nano-sized particles and fibres which have been added to solder matrices to strengthen solder alloys depend on the effect of dispersion hardening in the mechanical mixing method. By contrast, in the in-situ approach, precipitation hardening is the strengthening mechanism to improve the creep and thermo-mechanical fatigue resistance of these nano-composite solders Mechanical mixing method The mechanical mixing method involves extrinsically adding foreign nano-particles as dispersoids into solder matrices to form composites. These dispersoids usually have very little solubility in the matrices even at elevated temperatures. Several variants of this method have been developed to fabricate nano-composite solders Mixing solder alloy powders with nano-sized reinforcement particles In early research activities to comprehensively improve the properties of lead-based solders using composite technology, Lin et al. [21 23] have added nano-sized TiO 2 and Cu powders into conventional Sn Pb solder to obtain nano-composite lead-based solder pastes by the following sequence. They precision weighed a quantity of Sn Pb solder alloy powder and different percentages of TiO 2 and Cu nanopowders and blended them very well prior to the addition of a water-soluble flux to the mixtures. Then, the resultant composite mixture was stirred for a full 30 min so as to ensure a homogeneous distribution of the nanopowders in the composite matrix. Then, the composite mixture was placed on a designed heating system to melt it under the protection of argon gas and then it was cooled down to room temperature to obtain a solidified composite sample. A similar method was adopted by Liu et al. [42] to prepare nano-sized Ag particle-reinforced Sn Pb composite solders. In their study, the composite solder paste was prepared by mechanically mixing eutectic 63Sn 37Pb solder powder, reinforcement Ag nano-particles, and the solder flux for at least 15 min to ensure a uniform distribution of the reinforcements. Nai et al. [24 27] have produced multi-wall carbon nanotube (MWCNT) reinforced Sn Ag Cu composite solders by use of a powder metallurgical synthesis approach. This procedure involved first preweighing the desired amount of solder alloy powders and MWCNTs. The preweighed materials were then placed in a plastic container, which was then tightly sealed and placed into a V-blender. The blender was then subjected to a rotational motion at 50 rpm for 10 h. The mixture was then uniaxially compacted at 140 bar and sintered at 448 K for 2 h in an inert argon atmosphere. Finally, the billet of the mixture obtained was extruded at room temperature into an 8-mm diameter composite solder rod, employing an extrusion ratio of 20:1. Kumar et al. [28,29] have prepared single-wall carbon nanotube (SWCNT) functionalized Sn Ag Cu lead-free composite solders by the same method as that adopted by Nai et al. Li and Gupta [30] have prepared nano-al 2 O 3 strengthened Sn In Ag Cu composite solders by mechanically mixing nano-al 2 O 3 powders and Sn In Ag Cu solder alloy powder. Shi et al. [31] have blended preweighed Sn 37Pb and Sn 0.7Cu solder alloy powders with different volume percentages of nano-sized and microsized Cu and Ag reinforcement particles. A rosin mildly activated (RMA) flux was added into the resultant composite solders which were mechanically stirred for 30 min to ensure a homogeneous distribution of the reinforcement particles in the solder matrix pastes. In order to distribute the dispersoids into solder matrices relatively homogeneously, Mavoori and Jin [32,33] have developed an approach of coating particles and then repeatedly plastically deforming them to achieve a distribution of nano-particles in a solder matrix. In this approach, eutectic Sn 37Pb solder alloy powders (35 mm) and nano-sized TiO 2 (5 nm)/al 2 O 3 (10 nm) powders were thoroughly dispersed in ethanol with vigorous shaking and mixed in the desired proportions (3% by volume of dispersoids), with constant stirring, to form a slurry of Sn Pb particles coated with the finer oxide particles, as illustrated schematically in Fig. 1. The amount of ethanol and the viscosity of the slurry were controlled to minimize gravity-induced segregation of the oxide particles. After drying, they were compacted in a die (cold pressing at room temperature) at a pressure of 196 MPa to form 50 mm 10 mm 5 mm pellets. These were then subjected to repeated compressive mechanical deformation (with a mean-compressive stress of 915 MPa) at 393 K in an inert argon atmosphere. Each mechanical deformation step resulted in a reduction of thickness by a factor of 6, after which the flattened piece was cut into six pieces lengthwise, the pieces stacked, and pressed again to get a sixfold reduction in thickness. This process was repeated six times to achieve a final calculated interdispersoid spacing, in the thickness direction, of 0.8 nm. For maximum strengthening, it is desirable to have the interdispersoid spacing reduced as much as possible, and at least in theory, reduced to less Fig. 1. Powder mixing approach for dispersion strengthening of solder: powder compact consisting of solder particles coated with dispersoid particles is repeatedly pressed, cut, and restacked to yield a fine-scale dispersion of the particles. (After Mavoori and Jin [33] with permission of American Institute of Physics.)

4 226 J. Shen, Y.C. Chan / Microelectronics Reliability 49 (2009) than the average dispersoid diameter. While the experimentally determined interdispersoid spacing obtained is larger than the calculated 0.8 nm spacing due to the plastic flow of the matrix solder alloy and the limited number of added dispersoid particles, a nanoscale, relatively random dispersion with a particle size distribution of 5 50 nm was achieved Mixing solder component powders with nano-sized reinforcements particles A variant of the process adopted by Mavoori and Jin [32,33] is to use conventional mechanical alloying to produce nano-composite solders. Kao et al. [34] and Lee and Duh [35] have adopted a mechanical alloying (MA) process to produce nano-composite Sn Ag Cu and Sn Ag Ni solders by adding nano-sized Cu 6 Sn 5 / Ni 3 Sn 4 IMC particles into pure Sn and pure Ag powders and then mixing them together to form nano-composite solders. An RMA flux was added directly into the composite powders prepared at room temperature and mixed on a glass plate with a plastic spatula until uniform mixtures of flux and composite solder pastes were achieved Mixing solder pastes with nano-sized reinforcement particles The most simple and direct approach to prepare nano-composite solder pastes is mechanically incorporating nano-sized reinforcement particles into solder pastes. Liu et al. [36] have mechanically mixed moissanite SiC particles (20 nm) with a Sn 3.8Ag 0.7Cu solder paste to prepare a nano-composite Sn Ag Cu solder paste. Tai et al. [37] have prepared a nano-composite solder paste by mechanically mixing a Sn 0.7Cu solder paste and oxide-free Ag nano-particles ( nm) for at least 15 min to ensure the a uniform distribution of the reinforcement particles Mixing molten solder alloy with nano-sized reinforcement particles In a previous study, Shen et al. have developed an approach to prepare a nano-composite solder alloy by mixing ZrO 2 nano-particles with a molten Sn Ag solder alloy [38]. In this approach, the Sn 3.5Ag ZrO 2 composite solder was prepared by adding 2% ZrO 2 nano-particles into a Sn 3.5Ag alloy in a box-like arc-melting furnace. The Sn 3.5Ag alloy ingot and the preweighed ZrO 2 nanoparticles were first put in an Al 2 O 3 crucible, which was placed on a water-cooled copper plate. When a vacuum of 10 4 Pa was achieved, high-purity argon was introduced into the vacuum chamber and maintained at 10 2 Pa. Then, a temperature-controlled arc generator was used to melt the Sn Ag ZrO 2 alloy mixture. At the same time, an electromagnetic stirrer was activated to mechanically stir the molten alloy for 30 min at 523 K to ensure a homogeneous distribution of the reinforcing ZrO 2 nano-particles in the molten alloy. After this, the molten alloy was cooled to room temperature with a cooling rate of about 20 Ks 1 to obtain a Sn 3.5Ag ZrO 2 composite solder sample. solder by this in-situ process technology. They added Cu powder (1 lm) coated with RMA type flux into a Sn 3.5Ag eutectic solder and melted it in an Al 2 O 3 crucible under an argon atmosphere. The molten solder with the Cu powder was mechanically stirred, while its temperature was increased to 573 K in order to promote the reaction between the Sn and Cu powders. The solder was then kept at this temperature for 1 min and then quenched into alcohol to cause solidification. The solidified ingot was then hot-rolled into thin sheets, which were punched into disks. The disks were remelted in a column of silicon oil, which had a temperature gradient with the highest temperature 523 K at the top. During the sedimentation through the column, the molten disks became spherical balls about 760 lm diameter due to the surface tension of the solder. By this process, a composite solder with a very fine uniform distribution of dispersoids (Cu 6 Sn 5 phase) was achieved. Hwang et al. [40] have developed an in-situ method to prepare a composite solder as follows: pure Sn, Ag, and Cu ingots were melted in a porous porcelain crucible under an argon atmosphere, to prevent oxidation, and then the resultant solder mixtures were cast in a steel mold. The solder strips, reinforced with IMC particles, were fabricated by rolling the cast plates as shown in Fig. 2. Each cast plate was rolled to 0.07 mm thick strips, which were then punched to discs of 1.5 mm diameter. This size was adopted to make 630 lm diameter solder balls after the spheroidizing process in a hot oil bath. Since the primary IMC dendrites (Cu 6 Sn 5 phase) formed in the solder matrix during casting can be crushed into fine particles by means of plastic working, after rolling, the IMC particles were redistributed uniformly throughout the solder matrix. Another method Shen et al. have developed to prepare a nanocomposite solder is by use of rapid solidification technology [41]. This method can be employed by the careful selection of the alloy composition of the solder. In this process, a Sn 3.5Ag composite solder was prepared from bulk rods of pure Sn and Ag. The process of melting was carried out in a vacuum arc furnace under a high purity argon atmosphere to produce button-like alloy samples with a diameter of approximately 35 mm. In order to get a homogeneous composition within the ingot, the alloy was remelted four times. After this, the molten alloy was cast into a water-cooled copper mold to obtain a rod (with a diameter of 5 mm) at cooling rate of about 10 4 K min 1. Since the rapid cooling rate promotes the nucleation of Ag 3 Sn IMC particles and leads to rapid heat dissipation during solidification making the long-range diffusion of Sn and Ag atoms impossible, the nucleation of Ag 3 Sn IMC particles was greatly prompted but their growth in the eutectic matrix was suppressed, thus yielding a large number of uniformly distributed spherical nano-sized Ag 3 Sn IMCs in the solidified structure (as seen in Fig. 3) In-situ method The in-situ method refers to a technique by which reinforcing nano-particle phases are formed upon processing the bulk solder alloys themselves. Here, the reinforcing nano-particles embedded in the solder matrix have not come from the introduction of foreign reinforcements. Details of the in-situ procedures used to obtain nano-composite solders are not available in some published literature because of technical secrecy [10,11]. Based on the literature which introduced the processing route, there are two main in-situ methods to prepare nano-composite solders. One approach to preparing composite solders reinforced with nano-sized IMCs particles is by hot-rolling and pressing technology. Lee et al. [39] have prepared a Cu 6 Sn 5 reinforced composite Fig. 2. The crushing of intermetallic dendrites into particles and the redistribution of the crushed particles. (After Hwang et al. [40].)

5 J. Shen, Y.C. Chan / Microelectronics Reliability 49 (2009) With the development of fine-pitch interconnection solder joints, the electrical conductivity of a solder alloy is an important issue. The electrical resistivity of a nano-composite solder is affected by a combination of the following factors [43]: (i) the presence of the reinforcement, (ii) the volume fraction of the reinforcement, (iii) the reinforcement shape, (iv) the reinforcement size, and (v) the type of reinforcement and matrix. Nai et al. [24] reported that their resistivity results revealed that for all the MWCNT reinforced composite solders studied, there was no statistically significant change in the electrical resistivity value. They believed this could be attributed to the low volume fraction of porosity found in the composite solders and the small amount of reinforcement additions. In the microelectronic packaging industry, Ag nano-particles have been widely used to improve the electrical conductivity of isotropically conductive adhesives (ICA) [44]. It is also desirable that Ag nano-particles increase the electrical conductivity of a nanocomposite solder due to the high electrical conductivity of Ag (1 108 X 1 m 1 ). Liu et al. [42] prepared composite solder with Ag nano-particles and reported this nano-composite solder had a better electrical conductivity ( X 1 m 1 ) than the eutectic 63Sn 37Pb solder ( X 1 m 1 ), which will produce less Joule heat when used in microelectronic circuitry Reduction in the degree of undercooling Fig. 3. TEM micrograph depicting the Ag 3 Sn nano-particles formed in a rapidlycooled Sn 3.5Ag solder. (After Shen et al. [41].) 4. Achievements with the nano-composite solders developed The original driving force for the preparation of nano-composite solders was to improve the creep and thermo-mechanical fatigue resistance of the solder alloys so as to utilize them in high temperature, harsh service conditions. However, in the nano-composite solders developed, more improvements were achieved in other characteristics Reduction in density For some electronic devices which need high portability, the weight of the electronic device is one of the concerns. A decrease in the density of a solder will reduce the weight of solder joints in electronic devices and thus help to improve the portability of electronic devices. Li and Gupta [30] have investigated the change of the density by incorporating Al 2 O 3 particles, by extrusion at room temperature, in a nano-composite solder and found that with the incorporation of the lighter Al 2 O 3 nano-particles into the heavier Sn alloy matrix, the density of the nano-composite solders was reduced from ± ± gcc 1 (1% Al 2 O 3 ), ± gcc 1 (3% Al 2 O 3 ) to ± gcc 1 (5% Al 2 O 3 ) Improvement of electrical conductivity The melting temperatures of most of the lead-free solders developed are higher than those of the traditional lead-based solders, which may lead to significant capital expenditure on modified or new plant and product cost increases. So, in the development of new nano-composite solders to be used in the microelectronic packaging industry, a major requirement is that the reinforcement particles should not increase the melting point of the solder alloy. Doubtless incorporating reinforcing particles will change the components in solder alloys and then vary the fusion and solidification processes of the solders significantly. Lin et al. [22] have reported the influence of the addition of Cu nanoparticles on the solidification of a Sn Pb solder. The cooling curves of the composite solders indicated that a decrease of the solidification temperature was achieved during the solidification of the composite solder. The reason for this is that the part of the Cu nano-particles dissolved in the molten solder to change the alloy composition of the solder. Research reports regarding the influence of the addition of inert nano-particles or nano-fibres (ceramic or IMC) on the melting temperature of solder alloys are ambiguous, and some results even are in conflict. Nai et al. [25,26] have reported from differential scanning calorimetry (DSC) experiments that no significant changes in the melting point of nano-composite solders reinforced with MWCNTs were observed. However, Kumar et al. [28,29] have reported that the melting temperatures of SWCNT-based nano-composite solders were slightly decreased compared with an unreinforced solder. This variation in melting temperatures could be ascribed to the high surface free energy and interfacial instability of the SWCNT-based nano-composite solders compared to that of the un-reinforced solders. Liu et al. [36] have obtained a reduction in the melting point of a Sn 3.8Ag 0.7Cu solder reinforced with SiC nano-particles. The melting points for the nano-composite solders were found to be lower than that of the un-reinforced solder by about 1 K. They also suggested that this reduction of the melting point is possibly ascribable to an increase in the surface instability of the composite solder caused by the addition of the SiC nano-particles with a higher surface free energy. A question is will the addition of inert nano-particles influence the melting temperature of an alloy? The answer should be no. The melting temperature of an alloy is an inherent physical property of the alloy which depends on the alloy itself. The Lindemann criterion [45] says that a crystal will melt when the root mean-square displacement of the atoms in the crystal exceeds a certain fraction of the inter-atomic distance. Because the surface atoms of a crystal usually have low coordination numbers they thus experience different bonding forces to that of atoms in the bulk. As a result, the combined effect of increasing the number of surface atoms and the surface phonon softening will significantly increase the atomic mean-square displacements, and then very slightly decrease the melting temperature of the alloy. So, when an alloy is on a nano-size scale, namely a nanocrystal, the melting temperature of the nanocrystal alloy is very slightly lower than that of the bulk [46 49]. However, the incorporation of a small amount of inert nanoparticles into solder alloys will not increase the number of surface atoms of the alloy significantly. In other words, the reinforcement particles can not divide the alloy on a very much smaller size scale

6 228 J. Shen, Y.C. Chan / Microelectronics Reliability 49 (2009) to greatly enhance the number of surface atoms of the alloy. So, inert reinforcement particles will not influence or alter the melting temperature of the alloy. Shen et al. have investigated the melting and solidification temperatures of a Sn 3.5Ag ZrO 2 nano-composite solder with a high precision DSC apparatus [38]. To avoid impurities which would influence the test results, the solder alloy was prepared by melting high purity Sn and Ag ingots. The isochronal measured differential thermal analysis (DTA) signals were calibrated and corrected using the Curie point of pure Fe according to the method described in [50]. The tests results indicated that the melting temperature of the Sn Ag solder was unaffected by the addition of the ZrO 2 nano-particles (as seen in Fig. 4). However, the addition of ZrO 2 nano-particles increased the solidification temperature of the Sn 3.5Ag solder and then reduced the degree of undercooling needed for solidification of this solder. This result can easily be explained by classical solidification theory: during the solidification of the composite solder, the ZrO 2 nano-particles acted as nucleation centres to promote the nucleation of solid in the molten alloy which increased the solidification temperature of the solder alloy. Because a suitable melting temperature is essential for the development of nano-composite solders, the resultant nano-composite solders presented above can be readily adopted using existing reflow conditions because of the suitability and stability of their melting temperatures. Moreover, the decrease of the degree of undercooling needed will help to reduce the paste time of these solders during reflow and promote the utilization of these composite solders in industry Improvement of the wettability It is encouraging that nano-sized reinforcements incorporated into solder matrices have been shown to improve the wettability of these solders on substrates. The wettability of Sn Ag Cu composite solders was improved by reinforcing with MWCNTs [25,26]. Also, the wettability of a Sn Ag Cu solder paste was enhanced by the presence of Cu 6 Sn 5 IMC particles [34]. Nickel and Ni 3 Sn 4 -doped composite solder pastes exhibit favorable wettability when compared with a commercial un-reinforced solder paste [35]. A Sn 0.7Cu composite solder reinforced with 0.5% Ag nanoparticles gave a better wettability compared to the Sn 0.7Cu solder. However, when the volume fraction of Ag nano-particles added was increased, the wetting of the Ag nano-particle reinforced Sn 0.7Cu composite solder on the Cu substrate was degraded [37]. For TiB 2 nano-particle reinforced Sn Ag Cu solders, the optimum wettability was achieved when 1.5 vol.% of TiB 2 nano-particles were added. When 5 vol.% of TiB 2 nano-particles were introduced, the wetting angle was also degraded [36]. Because nano-particles embedded into solder matrices inhibit the flow of molten solders by increasing its viscosity which pins the leading edge of molten solder from further spreading, the presence of too many nano-particles in composite solders degrades the wettability by solder alloys. However, the intrinsic mechanism of minor additions of nano-particles on the improvement of wettability between molten solder alloys and substrates is still not clear. No published literature, which has reported the enhanced wetting phenomenon, has discussed the reason for this in detail. Hence, further research should be carried out to clarify the mechanism for the improvement in wetting Suppression of the growth of IMC particles Fig. 4. DTA curves of the Sn 3.5Ag and the Sn 3.5Ag ZrO 2 solders in the (a) first and (b) second heat-treatment cycles. (After Shen et al. [38].) The effect of nano-particles on the growth of the IMC particles in solder matrices and between solders and substrates in solder joints has been extensively investigated. During soldering, the IMC phase particles in solder matrices are formed due to the precipitation of an interphase during solder alloy solidification. At the same time, IMC layers are formed due to an interfacial reaction between the solder alloy and metal substrate. Both reactive metal nano-particles and inert nano-particles were found to suppress the growth of IMCs particles in solder matrices and solder joints during reflow or aging. Amagai [51] has studied the influence of Co, Ni, Pt, Al, P, Cu, Zn, Ge, Ag, In, Sb and Au nano-particles on the growth of IMC layers between Sn Ag based lead-free solders and organic solderability preservative (OSP) Cu pads after four reflow processes and with thermal aging. The results indicated that Co, Ni and Pt were very effective in suppressing the growth of IMC layers and improved the drop test performance compared with Cu, Ag, Au, Zn, Al, In, P, Ge and Sb. Yu et al. [52] have investigated the morphologies of the IMC layers formed during the wetting reaction between molten Sn 3.5Ag and Sn 3.5Ag 0.7Cu lead-free solders and pure Cu and Ni substrates. Interestingly, it was found that nanosized Ag 3 Sn particles were formed on the IMC layer surface during solidification. The existence of these particles would decrease the interfacial energy and hamper the excessive growth of the IMC layer. Kao et al. [34] have added nano-sized Cu 6 Sn 5 IMC particles into a Sn Ag Cu solder to control the growth of (Cu,Ni) 6 Sn 5 IMC particles in SnAgCu/Ni P solder joints. Shen et al. have successfully added ZrO 2 nano-particles into a Sn Ag solder to suppress the formation of bulk Ag 3 Sn IMC particles during solder alloy solidification [38]. As effective surface-active materials, the inert nanoparticles will accumulate at the phase boundaries in solder alloy matrices or in IMC layers because they do not react with the metallic elements in the solder. Hence, surface absorption theory can be used to explain the controlling mechanism of the suppression of

7 J. Shen, Y.C. Chan / Microelectronics Reliability 49 (2009) the growth of IMC particles in solder matrices and solder joint layers during reflow and aging by nano-particles. According to the theory of adsorption [53] of a surface-active material, the surface free energy of a whole crystal is: X c k ðcþ A k ¼ X k k ¼ X k c k ð0þ RT Z c 0 c k ð0þ A k RT X k C k c dc A k Z c 0! A k C k c dc where C k is the adsorption of surface-active material at crystal planes k, c is the concentration of the surface-active material, R is the ideal gas constant, T is the absolute temperature, c k ðcþ is the surface tension of crystal planes k with adsorption of the active material, c k ð0þ is the surface tension of the initial crystal planes k without adsorption, and A k is the area of the crystal planes k. Given that the volume of a crystal is constant, the surface energy of the crystal planes must be kept to a minimum in the equilibrium state. That is: X c k ð0þ A k RT X Z c A k k k 0 C k dc! min c ð3þ Here, P kc k ð0þ A k is assumed to be constant because it is independent of the concentration of surface-active materials. Thus, P k A R c C k k dc should be maximized, which implies that the effect 0 c of the crystal plane with the maximum amount of adsorption, C k, is most active. Generally speaking, the crystal plane with the maximum surface tension (equivalent to maximum surface energy) grows rapidly, while the amount of surface-active material adsorbed is maximized. However, an increase in the amount of elements adsorbed decreases its surface energy and, therefore, decreases the growth velocity of this crystal plane. Compared with the IMC particles, the size of nano-particles is very small. So, they are regarded as effective surface-active agents, which may be adsorbed on the surface of the IMC particles to suppress their excessive growth. This explanation of the mechanism of the suppression of IMC growth by the effect of nano-particles on the surface energy is not universally accepted since the explanation is qualitative. The growth process of IMC in the liquid/solid reaction interface in solder joints involves the net effect of several interrelated phenomena, such as volume diffusion, grain boundary diffusion, grain boundary grooving, grain coarsening, and dissolution into the molten solder [54]. Hence, there is a particular need to understand how the nano-particles embedded in solder matrices influence these factors. Stated differently, the location and distribution of nano-particles must be investigated and observed directly to clarify if they are located in the grain boundaries and/or grain grooves to impede the diffusion of atoms. Otherwise, if the nano-particles were found to be distributed uniformity in solder matrices, do they also influence the diffusion of the atoms and the dissolution of the IMC layers? So, deeper research, in particular a quantitative analysis of the diffusion kinetics including the effective of nano-particles, needs to be performed Microstructural modification As stated in the literature, all types of nano-particles change the microstructure of monolithic solder matrices and, these variations were positive and helped to improve the mechanical properties of nano-composite solders. This is because the reinforcement particles decreased the size of grains in solder matrices and these fine grains gave an increase in the strength of the solder alloys according to the Hall Petch relationship [55]. Kumar et al. [28] reported the average size of the secondary phases (Cu 6 Sn 5 and Ag 3 Sn) were ð2þ about lm in the Sn Ag Cu solder without any reinforcement, whereas the size of the grains were lm in the 1 wt% SWCNT reinforced solders. Shen et al. [38] reported that the addition of ZrO 2 nano-particles into a Sn Ag solder reduced the average size and spacing of Ag 3 Sn particles significantly. Similar descriptions can be found elsewhere in other literature [21 23,32,33,36]. However, since the microstructure of lead-based solder is different from that of lead-free solders, the addition of reinforcement particles in the lead-free solders resulted in different microstructural evolutions. For example, the morphology of a eutectic tin lead solder consists essentially of fine alternating lamellae of the two constituent phases (tin-rich and lead-rich phases). The addition of a small amount of TiO 2 nano-particles to the eutectic Sn Pb solder was found to give some agglomerates but in the main distribute in the solder matrix uniformly [22]. However, for lead-free nano-composite Sn Ag Cu solders, two types of equiaxed IMCs (Ag 3 Sn and Cu 6 Sn 5 ) are observed dispersed uniformly in the b-sn matrix while with SWCNTs their distribution is at the corners of the Ag 3 Sn equiaxed grains [28] Enhancement of the mechanical properties As stated previously, creep resistance is one of the most important aspects of the development of nano-composite solders. However, the nano-composite solders developed have exhibited other desirable properties sought by the electronics packaging industry, such as high hardness, high tensile strength and high shear strength. All the reinforcements, including carbon nanotubes, ceramic, metal and IMC nano-particles, have been reported to enhance the strength of solder matrices significantly by composite processing. Nano-composite solders reinforced with MWCNTs and SWCNTs have been shown to be more stable dimensionally than the monolithic solders because of their lower CTE values. Mechanical characterization results revealed that the presence of larger weight fractions of MWCNTs and SWCNTs in solder matrices led to an improvement of microhardness, 0.2% yield strength (YS) and ultimate tensile strength (UTS) of solder joints but decreased the ductility of composite solders significantly [24 27,28,29]. The addition of ceramic nano-particles, such as ZrO 2 [38], SiC [36], Al 2 O 3 [30,32,33], TiO 2 [22,23,32,33], significantly enhanced the hardness, tensile strength and creep resistance of nano-composite solders. The shear strength of composite solder joints reinforced with Ag nano-particles was much higher than that of Sn 0.7Cu solder joints when the volume fraction of Ag nano-particles was >1%. The creep rupture life of Sn 0.7Cu solder joints was improved with the addition of 1% Ag reinforcement particles at all testing temperatures and stresses [37]. The data for the enhancement of the mechanical properties of nano-composite solders collected from some of the literature are listed in Table 1. Here, it should be stressed that although the addition of nano-particles into solder matrices improved the creep behavior of the nano-composite solders, these additions have a harmful effect on the shock loading resistance of solders since the solder matrices are strengthened and experience more and more stress [56]. According to the classic theory of dispersion strengthening, the enhancement of the strength of composite solders could be ascribed to the presence and distribution of fine strong particles within the matrices and at the grain-boundaries of solder matrices, which tend to alter the deformation characteristics of solder alloys by impeding grain-boundary sliding while concurrently retarding dislocation movement in solder matrices [17,56]. Based on the Wagner Lifschitz Slivoz equation (see Eq. (1), Section 2.3), in theory, sufficiently fine nano-sized reinforcements will be more effective obstacles to dislocation motion and improve the mechanical properties of solder alloys. Since in early studies, microsized metal,

8 230 J. Shen, Y.C. Chan / Microelectronics Reliability 49 (2009) Table 1 The data showing the enhancement of the mechanical properties of nano-composite solders. Solder matrix Reinforcement nano-particles Mechanical properties References Microhardness (MPa) USS (MPa) 0.2%YS (MPa) Ductility (%) UTS (MPa) Sn 37Pb None 153 / / / / [21] 1.0 wt% Cu 181 / / / / 2.0 wt% Cu 196 / / / / 5.0 wt% Cu 219 / / / / Sn 0.7Cu None / / / / [37] 0.5 wt% Ag / / / / 1.0 wt% Ag / / / / 2.0 wt% Ag / / / / 3.0 wt% Ag / / / / Sn 3.5Ag None / / / / [41] Ag 3 Sn / / / / Sn 3.5Ag None ± 2.0 / / / / [38] 2.0 vol.% ZrO ± 5.9 / / / / Sn 37Pb None 153 / / / / [23] 0.5 wt% TiO / / / / 1.0 wt% TiO / / / / 2.0 wt% TiO / / / / Sn 4In 4.1Ag 0.5Cu None ± 5.2 / 56 ± 6 37 ± 7 60 ± 8 [30] 1.0 vol.% Al 2 O ± 10.6 / 72 ± 6 21 ± 3 75 ± vol.% Al 2 O ± 7.9 / 73 ± 3 11 ± 3 77 ± vol.% Al 2 O ± 4.7 / 74 ± 3 10 ± 0 76 ± 2 Sn 3.8Ag 0.7Cu None / / / / [36] 0.01 wt% SiC / / / / 0.05 wt% SiC / / / / 0.2 wt% SiC / / / / Sn 3.5Ag 0.7Cu None ± 2.0 / 31 ± 2 41 ± 8 35 ± 1 [24] 0.01 wt% MWCNTs ± 3.9 / 36 ± 2 36 ± 2 47 ± wt% MWCNTs ± 2.0 / 36 ± 4 37 ± 5 46 ± wt% MWCNTs ± 1.0 / 33 ± 3 35 ± 4 43 ± 5 Sn 3.8Ag 0.7Cu None ± 2.4 / / ± ± 1.53 [28] 0.01 wt% SWCNTs ± 1.4 / / ± ± wt% SWCNTs ± 1.2 / / ± ± wt% SWCNTs ± 2.3 / / ± ± wt% SWCNTs ± 2.4 / / ± ± wt% SWCNTs ± 1.79 / / ± ± wt% SWCNTs ± 1.9 / / ± ± 1.39 USS, Ultimate shear strength; 0.2% YS, 0.2% yield strength; UTS, Ultimate tensile strength. ceramic and IMC particles have been widely used to prepare composite solders to enhance the mechanical properties of solder alloys [57 59], there is then a question. That is, what size of particles, nano-sized particles or micro-sized, is most effective in improving the mechanical properties of solder alloys by composite processing? Shi et al. [31] have investigated the effect of the particle size scale on the mechanical properties of the composite solders. They have studied the creep properties of Sn Cu and Sn Pb composite solders reinforced with nano-sized and micro-sized Cu and Ag particles and found that the composite solders reinforced with microsized particles exhibit better creep strength than the composite solders reinforced with nano-sized particles, although the tensile shear strength of composite solder joints reinforced with nanosized particles may be higher than that of composite solder joints reinforced with micro-sized particles. They believe the reason for this is that nano-sized reinforcement particles refine the grains of solder matrices and the fine grain size of these solders causes a decrease of the creep resistance which is more effective than the increase of creep resistance by fine particles impeding grainboundaries sliding. Is this result the case in other composite solders which have different compositions and microstructures? In order to achieve high creep resistance composite solders with optimum size reinforcement particles, more research investigations are needed. In particular, when the composition of a solder alloy and the type of the nano-sized reinforcement particles are determined, it is major importance to clarify which size of nano-particles will improve the mechanical properties of the nanocomposite solder most effectively. Furthermore, in each composite solder with different sizes of nano-particles, the critical mechanism controlling creep needs to be established. That is, even in a nano-composite solder, because the range of the nano-particle size could be from several nanometres to several hundred nanometres, there is a particular need to know what size nano-particles or what range of nano-particle size would give the optimum performance. To our knowledge, no research has been performed on this, and therefore, quantitative guidelines of the size effect of nano-particles on the mechanical properties of nano-composite solders cannot be summarized from the literature. 5. Problems in the development of nano-composite solders 5.1. Creating a uniform distribution of reinforcement particles in a solder matrix As effective surface-active materials, the nano-particles distributed at the grain boundaries of composite solders enhance the mechanical properties of solder joints. However, the nano-particles may also gather together to form brittle agglomerations in solder

9 J. Shen, Y.C. Chan / Microelectronics Reliability 49 (2009) matrices due to their high surface energy, which will degrade the strength of composite solders. Hence, the nano-particles must be distributed in a solder matrix homogeneously during composite processing. This may sound easy but really be difficult in the actual practice of the preparation of composite solders. In fact, by mechanically mixing nano-particles with solder powders or solder pastes, even when the composite mixtures are stirred for a long time, agglomerates are still formed in composite solder matrices [31,37]. To solve this problem, Mavoori and Jin [32,33] have developed a novel method to prepare a nano-composite solder by mechanically mixing accompanied with a repeated pressing process (see Section 3.1.1). With this approach, a nano-composite solder structure with a relatively uniform distribution of dispersoids was fabricated. In this process, the fracture of interfaces between the reinforcement particles and the matrix caused by repeated plastic deformation is a drawback. Moreover, how to transfer this relatively complicated and costly technique into industry is still a problem. Compared with the approach of mechanically mixing nano-particles with solder powders and solder pastes (an approach of mixing a solid with a solid), the approach of mechanically mixing nano-particles with molten solders may achieve relatively uniform distributions of particles in solder matrices (an approach of mixing a liquid with a solid). Shen et al. have successfully prepared a ZrO 2 nano-composite solder with a uniform distribution of the reinforcement particles by mixing ZrO 2 nano-particles with a molten Sn 3.5Ag solder alloy [38]. In this approach, a vacuum or an inert gas environment must be adopted to avoid the oxidation of the molten alloy and only high stability nano-particles, which withstand the high temperature conditions during preparation, may be used for this fabrication route. Because the reinforcement particles and the solder matrix densities are not matched, settling or floating of the reinforcement particles will cause the reinforcement particles to segregate within the solder matrix resulting in a nonuniform distribution. To achieve a more homogeneous distribution within the composite solder, the composite alloy prepared needs to be inverted and remelted several times during the fabrication. Considering the issue of distributing the reinforcement particles in a composite solder matrix homogeneously, the in-situ method has an advantage in the preparation of nano-composite solders. Because the reinforcement particles are formed by the precipitation of IMC phase particles during alloy processing, they are distributed in the solder matrix homogeneously without forming any agglomerates in their in-situ composite solders Generating reliable bonding between the solder matrices and the reinforcement particles The reinforcement particles must satisfy certain conditions so as to enhance the performance of nano-composite solders. One essential issue is that the reinforcement particles should be wetted by solder matrices well and reliable bonding, whether weak or strong, should be formed between the reinforcement particles and the solder matrices. Without this reliable bonding between solder matrices and reinforcement particles, gas pores will easily be formed at the surface of reinforcement particles during stirring. In addition, interfacial cracks between the reinforcement particles and the matrices will usually be caused by the rolling or pressing processes. Because the interfacial strength between reinforcement particles and the solder matrices could significantly alter the overall mechanical behavior of a composite solder alloy, reinforcement particles in terms of type, size and size distribution must be carefully considered before preparation of nano-composite solders. Generally speaking, there is no difficultly with some nano-particles, which will react with the solder alloy and bond well with solder matrix. For example, metallic and some IMC nano-particles react with a solder matrix during preparation or reflowing to form IMC layers between the nano-particles and the solder matrix. These reactions provide a good bond between the reinforcement particles and the solder matrix. However, the bonding between inert nano-particles and solder matrices is a problem because these inert nano-particles do not react with solder alloys to produce IMC layers to provide good bonding. In this case consideration must be given to the wetting between the inert nano-particles and the solder alloy matrices. Inert nano-particles, which are well wetted by solder matrices, may be used to prepare nano-composite solders of high quality. Also, in terms of achieving the reliable bonding between the reinforcement particles and the solder alloy matrices, the in-situ method is really a good choice since the reinforcement particles in these solders are formed by precipitation of IMC phase particles during processing of the alloys, so, the bonding between the precipitated IMC particles and the alloy matrices is sufficiently strong to enhance the mechanical behavior of these composite solders. It should be stressed that in the preparation of nano-composite solders, although nano-particles may be successful embedded into the solder matrices, during reflow, nano-particles (specially inert nano-particles) may be carried away from the solder and become trapped in flux residues. This means the solder reflow process and nano-composite solders do not go well with each other and this will raise some new problems of the reliability of solder joints, such as, the contamination of substrates and the deterioration of the mechanical properties of solder joints. Usually, these problems may be solved by solid state bonding techniques [60 62]. Solid state bonding techniques, for example, thermal compression, thermosonic processes and ultrasonic bonding, which have been developed for low-cost low-temperature fluxless bonding and can be integrated into electronic assembly and packaging facilities, are already widely adopted in industrial practice. Because there no melting is required in solid state bonding and the properties of the original solder are largely retained, the segregation of reinforcement particles which restricts the industrial applications of composite solders is no longer an issue Stability of the reinforcement particles during reflow As discussed above, one may believe that the in-situ method is the best approach to prepare nano-composite solders because the reinforcement particles are distributed in solder matrices homogeneously and good bonding exists between the reinforcement particles and the solder matrices. However, this may not be the ideal choice of preparation method because there is another important characteristic regarding the reinforcement particles, that is the stability of the reinforcement particles during reflow. Foreign dispersoid particles, especially inert nano-particles, introduced within solder alloys would not easily coarsen. However, the nano-sized reinforcement particles introduced by an in-situ process into nano-composite solder matrices are formed either by rapid precipitation from a supersaturated solid/liquid solution or by fracturing the IMC dendrites. Since these fine and uniformly distributed reinforcement particles are obtained by metallurgical processing, and formed through mass transport of the constituent elements from a supersaturated solid/liquid solution, they are generally prone to coarsening at high temperature. Moreover, when the electronic device is reflowed to form solder joints, the in-situ nano-composite solders prepared would experience fusion and solidification processes. This means the microstructure of solder joints after reflow is not the microstructure of the in-situ nano-composite solder asprepared, but, is a re-solidification microstructure of the in-situ nano-composite solders alloy after a fusion process. Shen et al. have prepared an in-situ nano-composite Sn 3.5Ag solder by a rapid solidification process [41]. In this composite

10 232 J. Shen, Y.C. Chan / Microelectronics Reliability 49 (2009) solder, due to the rapid solidification conditions, fine nano-sized Ag 3 Sn IMC particles were precipitated from the molten solder and distributed in the solidified solder alloy matrix uniformly to enhance the hardness of this solder. However, if this in-situ nano-composite Sn 3.5Ag solder were used for solder joints and reflowed, it would remelt and re-solidify with a relatively slow cooling rate. In this slow solidification process, the Ag 3 Sn IMC particles formed in solder joints will not be in the form of nano-sized particles but in the form of bulk plates. Due to the brittleness of these bulk IMC phase plates, they will adversely affect the plastic deformation properties of the solder and cause plastic-strain localization at the boundary between the Ag 3 Sn plates and the bounding b-sn phase [63]. Hwang et al. [40] have prepared an in-situ composite solder by plastic deformation (see Section 3.2). They stated that the IMC particles were distributed uniformly throughout the solder matrix and coarsened but not remelted during spheroidization and reflowing because the melting point of these IMC particles was higher than the spheroidization and reflow temperatures. In other words, the composite solder alloy melted at the spheroidization and reflow temperatures while the IMC particles within it did not melt because the melting point of the IMC phase was higher than that of the solder alloy. This explanation seems difficult to accept since thermodynamics and phase equilibria would suggest that on melting the matrix a new equilibrium would need to be established which would cause the IMC particles to dissolve in the molten matrix. It is just possible that some highly-stable IMC phases might resist this process thermodynamically or that in other cases the kinetics of the dissolution process are slow. So, in general it is to be expected that most nano-sized IMC particles would dissolve into the solder matrix during reflow. However, if solid state bonding techniques are adopted, these IMC particles may be retained in the in-situ composite solder matrix and thus play a strengthening role. Hence, further research regarding the development of in-situ composite solders, in particular, the application of in-situ composite solders in industry, should focus on this technique. embedded in an alloy during service. This problem has impeded these POSS molecules being used in some composite materials [65]. Luckily, the melting points of most solder alloys are relatively low and this gives an opportunity to incorporate POSS molecules in a Sn Ag solder alloy to prepare a nano-structure composite solder [1,12]. In their study, Lee and Subramanian [64] selected POSS molecules which contained one to three silanols (Si OH) as surface active groups, while the core of the POSS molecule itself was an inert, strongly bonded Si-O cage structure (as seen in Fig. 5). During preparation, the OH groups of the POSS silanol strongly bonded with the metallic matrix by the formation of Si O Sn bonds, within the grains and at the grain boundary regions of the solder alloy. The combination of Si O M (M = metal) bonding and the presence of seven inert R groups on the POSS cage prevented agglomeration of these reinforcement particles during processing. In addition, the Si O and Si O M bonds are thermodynamically favorable and, consequently, do not dissociate under service conditions. The microstructure of the POSS nano-particles enhanced composite solder prepared indicated that the POSS nano-particles were embedded in the Sn Ag solder matrix successfully (as seen in Fig. 6). Many nano-sized (about 50 nm) POSS particles are seen well dispersed and are present in a higher concentration along the grain boundaries. Shear and thermo-mechanical fatigue (TMF) tests were carried out on solder joint samples after reflowing. The test results of mechanical properties validated the concept of using these surface active, inert nano-structured chemical rigid cage molecules for pinning the grain boundaries of solder alloys, which leads to enhanced mechanical performance at elevated temperatures and improved service reliability of solder joints. 6. A novel nano-structure composite solder Facing the challenges in the development of nano-composite solders presented above, Lee and Subramanian [64] have developed a novel nano-structure composite solder by adding polyhedral oligomeric silsesquioxanes (POSS) nano-particles into a solder matrix. The materials methodology of this method is to obtain a nano-structure composite solder by a novel design of the reinforcement particles. To obtain a nano-structure composite solder, the reinforcement particles in a solder matrix should be surface active with an inner inert nano-particle which can facilitate the initial bonding of the reinforcement particles with the solder matrix during reflow, and then prevent the inert nano-particles from reacting with the solder matrix any further. During preparation or reflow, the initial bonding between the reinforcement particles and the solder matrix should be established quickly to avoid the agglomeration of the reinforcement particles. Once this initial bonding is achieved, due to the inert nature of nano-particles, they should no longer react with the solder matrix. As a result, coarsening of the reinforcement particles in the solder matrix during solder joint use in service or reflow will not occur. POSS molecules consist of a cage-like inorganic core made with a silicone and oxygen framework (SiO 1.5 ). It is possible to surround this inorganic core with selected active groups by appropriate materials engineering. The presence of these surface-active groups facilitates the bonding between the POSS molecules and the matrix material. The only problem in implementation of this materials approach is the high temperature stability of these POSS molecules Fig. 5. Anatomy of POSS trisilanol for use in modification of solders. (After Lee and Subramanian [64].) Fig. 6. The SEM image reveals the dimensions of POSS trisilanols and their distribution in a eutectic Sn Ag matrix. (After Lee and Subramanian [64].)