1. Introduction. Keywords Chemical elements and inorganic compounds, Solder, Melting point, Hardness, Solder paste. Paper type Research paper

Transcription

1 Addition of cobalt nanoparticles into Sn-38Ag-07Cu lead-free solder by paste mixing SL Tay, ASMA Haseeb and Mohd Rafie Johan Department of Mechanical Engineering, University of Malaya, Kuala Lumpur, Malaysia Abstract Purpose The purpose of this paper is to investigate the effects of addition Co nanoparticles on the characteristic properties of Sn-38Ag-07Cu solder Design/methodology/approach Cobalt (Co) nanoparticles were added to Sn-Ag-Cu solders by thoroughly blending various weight percentages (0-20 wt%) of Co nanoparticles with near eutectic SAC387 solder paste Blending was done mechanically for 30 min to ensure a homogeneous mixture The paste mixture was then reflowed on a hot plate at 2508C for 45 s The melting points of nanocomposite solder were determined by differential scanning calorimetry Spreading rate of nanocomposite was calculated following the JIS Z standard The wetting angle was measured after cross-sectional metallographic preparation Findings No significant change in melting point of the solder was observed as a result of Co nanoparticle addition The wetting angles of the solder increased with the addition of nanoparticles, while the spreading rate decreased Although the wetting angle increased, the values were still within the acceptable range Scanning micrograph observations revealed that the as-solidified microstructure of the composite solder was altered by the addition of Co nanoparticles Microhardness of the solders slightly increased upon Co nanoparticles addition to SAC387 Originality/value The paper demonstrates that a simple process like paste mixing can be used to incorporate nanoparticles into solder Keywords Chemical elements and inorganic compounds, Solder, Melting point, Hardness, Solder paste Paper type Research paper 1 Introduction Owing to worldwide environmental legislation to ban toxic materials in solder and demands for green products, a variety of lead-free solders have been developed (Gao and Takemoto, 2006; Gao et al, 2006; Humpston and Jacobson, 2004; Amagai et al, 2002) Sn-Ag-Cu is one of the most suitable candidates to replace lead solders (Yao et al, 2008; Takemoto, 2007; Alam et al, 2003) However, the performance of Sn-Ag- Cu alloys is not considered good enough to meet severe boardlevel reliability requirements in some applications (Zhong and Gupta, 2008) In an effort to improve the properties of lead-free solders, researchers have been investigating the effects of particle reinforcement Because of the miniaturization of electronic products, the pitch size of IC chips has reduced rapidly Nanoscale reinforcements are, therefore, more suitable for solders intended for such fine pith applications (Liu et al, 2008; Kumar et al, 2006) A number of researchers are working on the development of new lead-free nanocomposites by adding various types of nanoparticles, including carbon nanotubes and ceramics (eg SiC, TiO 2, ZrO 2,Al 2 O 3 )(Tayet al, 2009; Zhong and Gupta, 2008; Liu et al, 2008; Shen and Chan, 2008; Shen et al, 2006; Li and Gupta, 2005; Lin et al, 2003a, b; Mavoori and Jin, 1998) and metals (Cu, Ag, Co, Ni, Pt, Au, Zn, Al, In, Ge, Sb) (Shen and Chan, 2008; Amagai, 2006, 2008; The current issue and full text archive of this journal is available at wwwemeraldinsightcom/ htm 23/1 (2011) q Emerald Group Publishing Limited [ISSN ] [DOI / ] Lin et al, 2002; Suganuma, 2001) Researchers have found that Co additions have beneficial effects on tin-based solders (Lin et al, 2009; Eu et al, 2008; Anderson et al, 2001, 2008) A number of researchers have investigated the effect of nanoparticle additions on the morphology of the intermetallic layer or solder matrix Shen et al (2006) reported that the addition of nano ZrO 2 into Sn-35Ag solder alloy tended to suppress the growth of Ag 3 Sn in the solder matrix Amagai (2008) reported that the growth of intermetallic compounds (IMCs) tended to be suppressed with the addition nano Co into Sn-35Ag solder alloy after 4 reflow The IMC scallop and spacing of the eutectic lamella tended to be reduced with the addition of nano Cu (Lin et al, 2002, 2003a, b) or nano TiO 2 (Lin et al, 2003a, b) into tin-based solder alloys The majority of researchers have focused on the effects of nanoparticles on the microstructure, as well as IMCs However, little information is available on the effect of nanoparticles on wetting angle and spreading rate Wetting and spreading behaviour is very important for solders A good reliable interconnection requires a small wetting angle (Mavoori and Jin, 1998) Kripesh et al (2001) suggested that when the wetting angle of a solder is between 08 #u # 208, the wetting quality is very good When the wetting angle is between 208 #u # 408, The authors would like to acknowledge financial support from the Ministry of Science, Technology and Innovation with the Project No and Institute of Research Management and Consultancy (IPPP), University of Malaya with the Project No PS C The support of DSC analysis by Mr Said Sakat, and SEM by Mr Mohd Zulhizan bin Zakaria also appreciated Received: 1 February 2010 Revised: 12 July 2010 Accepted: 13 August

2 it is good and acceptable However, when it exceeds 408, the solder is not acceptable (Kripesh et al, 2001) The wetting angle is influenced by a number of factors, including metal surface condition, alloy composition, temperature of soldering, reflow time and type of flux (Liang et al, 2007; Humpston and Jacobson, 2004; Abtew and Selvaduray, 2000) In general, there are two main methods for producing nanocomposites and these are a mechanical mixing method and an in situ method The mechanical mixing method is to strengthen the solder alloys, which depends on the effect of dispersion hardening during the process On the other hand, the strengthening mechanism for the in situ method is precipitation hardening to improve thermo-mechanical fatigue resistance and creep of the nanocomposite There are a few methods for fabricating nanocomposite solders by the mechanical mixing method The simplest and most direct method for producing nanocomposites is by mixing the solder paste with nanosize reinforcement particles (Shen and Chan, 2008) In this research, composite solders were synthesized by mechanical mixing of various weight percentages of cobalt (Co) nanoparticles with Sn-38Ag-07Cu (SAC) solder paste The effects of Co nanoparticles on spreading rate and wetting angle were investigated The influence of the addition of Co nanoparticles on the morphology and micohardness of the reflowed solder was also studied 2 Experimental procedure Composite solders were prepared by mixing various weight percentages (0-20 wt%) of Co nanoparticles (,28 nm, Accumet Materials, Co, USA) with the near eutectic SAC solder paste (Indium Corporation of America, Singapore) The particle size of the as-received Co nanopowder was investigated using transmission electron microscopy (TEM, Leo Libra 120, German) The crystallite size was determined by X-ray diffraction (XRD, Philips X Pert MPD PW3040, USA) The crystallite size was calculated by Scherrer s formula (Cullity, 1956): kl 57:3 d xrd ¼ W size cos u ; where Addition of cobalt nanoparticles into Sn-38Ag-07Cu solder SL Tay, ASMA Haseeb and Mohd Rafie Johan W size ¼ W b 2 W s ; 2 W b is the full width at half maximum intensity (FWHM) of the (111) peak; W s is the FWHM for a standard reference sample and k is a constant equal to 09 and l is nm (the Cu K a line) The particle size of the as-received SAC solder was determined by scanning electron microscopy (Philips SEM 515, USA) The average particle size was calculated from the diameters of, 100 particles The mixture was manually blended for half an hour to ensure uniform distribution of the reinforcing particles The melting points of the mixed solder were determined by using differential scanning calorimetry (DSC, Mettler 820, Switzerland) at a heating rate of 108C/min Copper sheets ( mm) were used as substrates for solder preparation Before reflow, the copper sheets were cleaned with detergent to remove any greasy material, chemically cleaned with a solution of 50 per cent HNO 3 and 50 per cent H 2 O to remove oxide, and then rinsed thoroughly in distilled water, followed by drying with ethanol After that, around 02 g of the as-prepared solder paste-co nanoparticles mixture was placed on the copper sheet through a mask with an opening of 65 mm and a height of 124 mm The copper sheet was placed over a hotplate maintained at 2508C for 45 s After reflow, chemical analysis of solder and flux residue was performed for Co by inductively coupled plasma (ICP) mass spectrometry The spreading rate of the solder was calculated according to the Japanese Industrial Standard (JIS Z , 2003) using the following equations (1) and (2): S R ¼ D 2 H 100 ð1þ D D ¼ 1:24V 1=3 ð2þ where S R is spread rate (per cent), H is the height of the solder (mm), D represents a diameter when the solder material used for a test is assumed to be in the form of a ball (mm), and V is mass/density of the solder sample used for the test In this study, the density of the solder was taken as 75 g/cm 3 (Ahmad et al, 2007) Five samples were prepared for each composition and the average spreading rate was calculated After spreading rate measurement, the specimens were cut by diamond cutter and prepared for cross-sectional metallographic examination The specimens were then ground and polished using standard metallographic techniques ICP mass spectrometry was carried out to identify the amount of Co nanoparticles in the flux residue and solder after reflow The wetting angle was measured under an optical microscope The as-polished samples were then chemically etched in a mixture of 5 per cent hydrochloric acid and 95 per cent of ethanol for a few seconds The microhardness of the solder was measured using a Mitutoyo MVK-H2 hardness-testing machine at a load of 10 g force 3 Results and discussion Figure 1a shows the morphology of the as-received SAC387 solder particles in the paste The micrograph was taken after the flux in the solder paste was dissolved in isopropanol SAC387 solder particles were observed to be spherical in shape The average diameter of the particle was calculated to be 3250 ^ 6 mm Figure 1b shows a TEM image of Co nanoparticles used as reinforcement in this study The Co particles appear agglomerated in the TEM image The agglomeration was believed to have occurred during TEM sample preparation Co nanoparticles attach themselves edgeon to appear as fibres The width of the fibres measured in the TEM image was about 24 ^ 7 nm, which is close to that specified by the supplier (28 nm) The measured crystallite size by XRD was found to have a similar order of magnitude, 21 nm Figure 1 Morphology of as-received materials (a) (b) 01 mm 500 nm Notes: (a) SEM image of Sn-38Ag-07Cu solder particle in paste; (b) TEM image of Conanoparticles 11

3 Table I shows the melting points of composite solders containing various weight percentages of Co nanoparticles, as measured by DSC The melting point of SAC is also shown for comparison Given the accuracy of DSC, it can be safe to assume that the addition of the Co nanoparticles did not result in a significant change in melting point Figure 2 shows the actual percentage of Co in solder and in flux residue, as determined by ICP after reflow These percentages are shown as a function of the amount of Co nanoparticles added to the paste before reflow A reference line is also added which represents the case where all nanoparticles enter and remain in the solder after reflow It can be seen in the figure that the amount of nanoparticles in the solder, as well as in the flux residue, increases linearly as the amount of particles in the paste increases However, the actual amount of nanoparticles is smaller in solder than in the flux residue For nominal concentrations of 05, 10, 15 and 20 per cent Co, the reflowed solder contains 018, 041, 075 and 079 per cent of Co, respectively It may be noted that the nanoparticles were added to the solder paste, which contains spherical balls of SAC, a viscous mass of flux and other additions In such a case, the nanoparticles are dispersed within the viscous flux During reflow, as much as 34 per cent of the added Co nanoparticles enter into the molten solder pool and the rest remains in the flux residue The nanoparticles that enter the molten solder get entrapped during solidification The flux residue, which separates out and rises to the surface due to the density difference, contains the rest of the nanoparticles During reflow, Co nanoparticles entrapped in the solder may undergo partial dissolution at the surface Table I Melting point of SAC-xCo nanoparticles Solder Figure 2 Percentage of Co in solder and in flux residue after reflow as a function of Co nanoparticles added to the paste % of Co in solder or flux residue Addition of cobalt nanoparticles into Sn-38Ag-07Cu solder SL Tay, ASMA Haseeb and Mohd Rafie Johan Flux-residue Reference Melting point (8C) Sn-38Ag-07Cu ^ 004 Sn-38Ag-07Cu-05 nano Co a ^ 299 Sn-38Ag-07Cu-10 nano Co a ^ 265 Sn-38Ag-07Cu-15 nano Co a ^ 287 Sn-38Ag-07Cu-20 nano Co a ^ 243 Note: a Nominal weight percent of Co nanoparticles Solder Amount of Co nanoparticles added (Wt%) The variation of wetting angle and spreading rate of solder is shown in Table II as a function of weight percentage of Co nanoparticles in the solder It is observed that, in spite of the scatter in the data, the wetting angle increases as the percentage of Co nanoparticles increases The spreading rate, on the other hand shows the opposite trend Pure SAC solder has the best wettability compared with the composites The effects of the nanoparticles on the wettability of leadfree solder have been investigated by Nai et al (2006) They found that, in the case of TiB 2 nanoparticles, the wetting angle first decreased and then increased beyond a threshold volume fraction of about 2 per cent However, for carbon nanotubes, the wetting angle decreased monotonically for up to 004 wt% The increase in wetting angle with Co nanoparticles addition, as found in the present study, could be linked to the difficulty of spreading of the leading edge of the molten solder This could be attributed to a possible increase in viscosity caused by the presence of nanoparticles in the solder (Nai et al, 2006) A number of researchers have investigated the wetting angle of Sn-based solders on copper substrates The wetting angle obtained by different researchers varies over a wide range,, (Tay et al, 2009; Guo, 2007; Humpston and Jacobson, 2004; Wu et al, 2004; Yu et al, 2004; Kripesh et al, 2001) The wide variation is probably related to factors such as the alloy composition, processing atmosphere, type of flux and substrate metallurgy, etc (Liang et al, 2007; Humpston and Jacobson, 2004; Abtew and Selvaduray, 2000) In the present study, the wetting angle was found to be 1958 for SAC, which is well within the range for Sn-based solder reported in the literature On the other hand, the wetting angle for Co nanoparticle-added solders was found to be in the range of Although higher than that for SAC, the value is still considered good and acceptable, as it is within the range 208 #u # 408 (Kripesh et al, 2001) The trend in the results is similar to that observed in a previous study involving the addition of nickel nanoparticles into SAC387 (Tay et al, 2009) Figure 3(a) reveals the interfacial microstructure between the SAC solder and the Cu substrate after reflow A scallop-shaped intermetallic layer was formed after reflow In the case of SAC solder doped with Co nanoparticles, a planar type of intermetallic layer formed It was found that, during the growth of the IMC in the Co-containing solder, some pockets of solder were entrapped in the IMC layer Similar observations have also been made by other researchers (Gao et al, 2006) The etching reagent, however, dissolves such solder pockets, leaving pits in the IMC which are visible in Figure 3(b) as dark holes Energy dispersive spectroscopy was used to determine the composition of the intermetallic layer and it was identified as Cu 6 Sn 5 for the solder not doped with Co nanoparticles Table II Wetting angle and spreading rate of SAC with various addition of Co nanoparticles Solder Wetting angle (8) Spreading rate (%) Sn-38Ag-07Cu 1946 ^ ^ 242 Sn-38Ag-07Cu-018 nano Co 2695 ^ ^ 485 Sn-38Ag-07Cu-041 nano Co 2576 ^ ^ 532 Sn-38Ag-07Cu-075 nano Co 2671 ^ ^

4 Figure 3 Interfacial microstructure between (a) Sn-38Ag-07Cu; (b) Sn-38Ag-07Cu doped with 075 wt% Co and the Cu substrates after reflow (a) For the solder doped with Co nanoparticles, the composition of the intermetallic layer formed was (Cu,Co) 6 Sn 5 The total thickness of SAC doped with Co nanoparticles was higher compared to the SAC solder without doped with Co nanoparticles Some researchers found that the IMC layer was thicker for Ni- as well as Co-doped Sn-based solder (Gao et al, 2006; Wang et al, 2009) It is thus seen that the presence of Co nanoparticles drastically changes the interfacial IMC and characteristics The IMC becomes flatter and thicker with the addition of Co nanoparticles Further investigations are necessary to determine the effect of such changes on the solder reliability The microhardness variation of the samples is shown in Figure 4 as a function of percentage of Co nanoparticles The average microhardness of pure SAC was found to be 1272 HV; this value is quite similar to the microhardness obtained by others (Sun et al, 2008) Results reveal that the microhardness of the solidified composite increases slightly with the addition of Co nanoparticles The increase of microhardness may be due to the refinement of the grains as well as due to dispersion strengthening caused by the addition of the Co nanoparticles Gao et al (2006) reported that Co addition for up to 02 wt% did not cause any detectable solid solution effects by nanoindentation Strengthening effects have been observed by various researchers for a variety of nanoparticle additions to solders (Zhong and Gupta, 2008; Kumar et al, 2008; Lin et al, 2003a, b) Figure 4 Variation of microhardness of solders as a function of amount of Co nanoparticles Hardness value (Hv) Addition of cobalt nanoparticles into Sn-38Ag-07Cu solder SL Tay, ASMA Haseeb and Mohd Rafie Johan (b) Amount of Co nanoparticles in solder (Wt%) 4 Conclusion In the present work, the effects of the addition of Co nanoparticles to SAC387 solder paste on the melting point, spreading rate, wetting angle and microhardness were investigated The following conclusions can be drawn from the investigation: A simple technique, like paste mixing, can be used to incorporate enough Co nanoparticles into SAC387 solder to slow their effects on the microstructure and properties There was no significant change in the melting point of composite solders upon the addition of Co nanoparticles As much as 34 per cent of the Co nanoparticles were entrapped in the solder matrix after reflow and the rest remained in the flux residue An increase in the weight percentage of Co nanoparticles added to the SAC387 solder paste caused an increase of wetting angle and a decrease of spreading rate The interfacial IMC layer became flatter and thicker as Co nanoparticles were added into the SAC solder alloy The microhardness of the composite solder increased slightly with the addition of Co nanoparticles References Abtew, M and Selvaduray, G (2000), A review journal leadfree solders in microelectronics, Materials Science and Engineering: R Reports, Vol 27 Nos 5-6, pp Ahmad, I, Jalar, A, Majlis, NY and Wagiran, R (2007), Reliability of SAC405 and SAC387 as lead-free solder ball material for ball grid array package, International Journal of Engineering and Technology, Vol 4 No 1, pp Alam, MO, Chan, YC and Tu, KN (2003), Effect of 05 wt% Cu addition in Sn-35Ag solder on the dissolution rate of Cu metallization, Journal of Applied Physics, Vol 94 No 12, pp Amagai, M (2006), A study of nano particles in SnAg-based lead free solders for intermetallic compounds and drop test performance, Proceedings of the 56th International Conference on Electronic Components and Technology, San Diego, CA, pp Amagai, M (2008), A study of nanoparticles in SnAg-based lead free solders, Microelectronics Reliability, Vol 48, pp 1-16 Amagai, M, Watanable, M, Omiya, M, Kishimoto, K and Shibuya, T (2002), Mechanical characterization of Sn-Ag based lead-free solders, Microelectronics Reliability, Vol 42 No 6, pp Anderson, C, Sun, P and Liu, J (2008), Tensile properties and microstructural characterization of Sn-07Cu-04Co bulk solder alloy for electronics applications, Journal of Alloys and Compounds, Vol 457 Nos 1-2, pp Anderson, IE, Foley, JC, Cook, BA, Harringa, J, Terpstra, RL and Unal, O (2001), Alloying effects in near-eutectic Sn-Ag-Cu solder alloys for improved microstructural stability, Journal of Electronic Materials, Vol 30 No 9, pp Cullity, BD (1956), Elements of X-Ray Diffraction, Addison-Wesley, Reading, MA, pp 98-9 Eu, PL, Ding, M, Wong, TL, Nowshad, A, Ibrahim, A, Mok, YL and Haseeb, ASMA (2008), A study of SnAgNiCo vs Sn38Ag07Cu C5 lead free solder alloy on mechanical strength of BGA solder joint, Proceedings of the 10th Electronic Packaging Technology Conference, Singapore, pp

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