Growth mechanism of Ni 3 Sn 4 in a Sn/Ni liquid/solid interfacial reaction

Transcription

1 Available online at Acta Materialia 57 (2009) Growth mechanism of Ni 3 Sn 4 in a Sn/Ni liquid/solid interfacial reaction J. Shen a, Y.C. Chan a, *, S.Y. Liu b a Department of Electronic Engineering, City University of Hong Kong, Tat Chee Avenue, Kowloon Tong, Hong Kong b Department of Mechanical Engineering, University of Hong Kong, Pokflum Road, Hong Kong Received 1 November 2008; received in revised form 6 July 2009; accepted 13 July 2009 Available online 12 August 2009 Abstract The chemical interfacial reaction of Ni plates with eutectic Sn 3.5Ag lead-free solder was studied by microstructural observations and mathematical calculations. Compared with the Sn 3.5Ag 0.75Ni/Ni interfacial reaction, based on a simple model of the growth of the liquid/solid chemical compound layer, the growth mechanism of Ni 3 Sn 4 in the Sn 3.5Ag/Ni interfacial reaction is discussed and presented. The growth process of Ni 3 Sn 4 in the Sn/Ni liquid/solid reaction interface 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. The growth time exponent n and morphology of Ni 3 Sn 4 were found to be dependent on these factors. Ó 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Sn/Ni; Interfacial reaction; Microstructure; Growth mechanism 1. Introduction Solders, substrate materials and their interfacial reaction products play crucial roles in the reliability of joint assemblies in microelectronic packages because they provide electrical, thermal and mechanical continuity in electronic assemblies [1,2]. The sustained trend towards miniaturization and functional density enhancement in electronic devices, the use of increasingly dense arrays and fine-pitch interconnections for microelectronic packaging, and the development of lead-free solders in response to the strict legislation of a ban on the use of lead-based solders have posed several new challenges to the microelectronic packaging industry [3]. Whether in the first-level (chip-to-module) or in the second-level (module-to-board) packaging technologies for advanced electronic applications, the most important issue in packaging is that the molten solders flow or spread on the substrate surfaces to form a proper metallic bond and thus achieve a perfect joint [1 3]. Hence a thin, continuous and uniform intermetallic compound * Corresponding author. Tel.: ; fax: address: eeycchan@cityu.edu.hk (Y.C. Chan). (IMC) layer formed between a solder and the substrate material is an essential requirement for good metallurgical bonding. However, due to the inherently brittle nature of IMC layers and their tendency to generate structural defects, IMC layers which are too thick at the solder/substrate material interface may degrade the fatigue and fracture strengths of solder joints, leading to poor reliability of electronic devices [2]. Copper has been the most widely used solderable metal substrate in the under bump metallization (UBM) for flip-chip and in the bond pad for ball grid array (BGA) applications [1 5]. Because Cu dissolves into molten Sn-rich solders very quickly during the soldering process and the Sn Cu IMC layers grow at a very high rate to become thick during thermal aging, this excessive growth of Sn Cu IMC layers may have a deleterious effect on the reliability of solder joints when electronic devices are used in service at high temperatures [5 8]. Often, Ni and Ni-based alloys, which are also solderable metals, are considered to be excellent alternatives for Cu substrates. The rate of dissolution of Ni in Sn-based solders is very low at the soldering temperature so that only a very thin IMC layer is generally observed between Ni and the Sn-based solders [9 11]. This is the reason that Ni/Au /$36.00 Ó 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi: /j.actamat

2 J. Shen et al. / Acta Materialia 57 (2009) and Ni/Pd metallization schemes are widely applied as substrates for solder joints in advanced microelectronics packaging. Because of the prime importance of the interfacial reaction between solder alloys and metal substrates, the morphologies and the growth kinetics of the IMC layer in the Sn/Ni system have been widely researched and reported [9,10,12 26]. However, it is difficult to achieve a clear image regarding the phases, their morphologies and growth kinetics (in particular, the time exponent) of the IMC layer in the Sn/Ni interfacial reaction system from the literature because there are ambiguous results reported and some results are in conflict. It is widely reported that Ni 3 Sn 4 is the only phase formed that can be detected in the interfacial reaction layer between molten pure Sn or Sn X (X is one of the chemically inert metallic elements with Ni, such as Pb, Ag, etc.) alloys and Ni [9,15,17,22,25]. However, Ni 3 Sn and/or Ni 3 Sn 2 phases, which are thermodynamically stable phases, have also been reported to exist in the Sn/Ni couple reaction interface [13,14]. In particular, the morphology of the Ni 3 Sn 4 phase has been described differently, such as whisker-like [13], scallop-like with round and smooth surfaces (a non-faceted structure) [14,15,22], scallop-like with cusps (a faceted structure) [9,22], a continuous thin layer [17,22], a nonuniform and fractured layer [17], a chunk type [10] and a faceted rod type [10], etc. Generally, alloying elements, which are not chemically inert with Ni, influence the phases of the IMC layers formed between Sn-based solders and Ni. For example, a minor Cu addition to a Sn Ag solder changed the interfacial reaction between molten solder and Ni dramatically to form ternary (Cu,Ni) 6 Sn 5 or/and (Ni,Cu) 3 Sn 4 IMCs phases in the IMC layers [19,20,23]. The effect of minor Zn additions in a Sn Ag solder is to nucleate a Ni 5 Zn 21 IMC layer and to suppress the formation of Ni Sn IMC layers [24]. It is interesting that, although Bi reacts with Ni to form both soft and brittle NiBi 3 IMC particles in the Ni/Bi couple, except for Ni 3 Sn 4, none of the other Ni Sn IMCs or Ni Bi IMCs was observed when molten 58Bi 42Sn solder reacted with Ni [22]. It is easy to understand that the concentration of Sn in Sn X (X is one of the chemically inert metallic elements with Ni) solders influences the thickness of the Ni 3 Sn 4 layer because a larger amount of Sn atoms react with the Ni atoms, and of course, this results in a thicker Ni 3 Sn 4 layer [16,25]. The most ambiguous description of Ni 3 Sn 4 is its growth kinetics during thermal aging. According to the literature, various growth kinetics of the Ni 3 Sn 4 layer have been reported, which could be roughly classified as parabolic kinetics [14,15,25,26], linear and parabolic kinetics (in different thermal aging stages) [22] and nonparabolic growth kinetics [9,12,13,18]. Hence, a question arises: what is the intrinsic growth mechanism of Ni 3 Sn 4 in the Sn/Ni liquid/solid interfacial reaction system? Because of the significance of the Sn/Ni interfacial reaction in advanced microelectronics packaging technology, there is a particular need to develop a comprehensive understanding of this system. In this paper, a eutectic Sn 3.5Ag solder alloy, which is regarded as the most recommended lead-free solder candidate to replace leadbased solder [1 4], was used to investigate the interfacial reaction between liquid solder and Ni plates. Moreover, minor amounts of Ni were introduced into the Sn 3.5Ag solder to assist in clarifying the growth mechanism of Ni 3 Sn 4. This paper aims to provide a clear picture of the growth mechanism of Ni 3 Sn 4 in the Sn/Ni interfacial reaction system. 2. Experiments In order to avoid the influence of impurities in commercial solder alloy bars/pastes on the interfacial reaction, Sn 3.5Ag and Sn 3.5Ag 0.75Ni (mass%, hereafter) solder alloys were prepared from bulk rods of pure Sn, Ag and Ni (their purities were all above 99.99%). After weighing the individual pure metals, they were mixed and melted in a vacuum arc furnace under a high-purity argon atmosphere to produce button-like specimens with a diameter of about 3.5 cm. In order to get a homogeneous composition, all ingots were remelted four times. Finally they were solidified in a water-cooled copper mold with a cooling rate of about 20 K s 1. Pure Ni plates with a thickness 0.15 mm (the purity was above 99.99%) were adopted as substrates for interfacial reaction tests. The pure Ni plates were cut to 1 1 cm specimens and were polished lightly with diamond powder and degreased in a solution (99 vol.% C 2 H 5 OH + 1 vol.% HCl) so as to remove surface oxides and contaminants. Both multiple reflow tests and thermal aging experiments were carried out in an air environment. In the multiple reflow tests, Sn 3.5Ag and Sn 3.5Ag 0.75Ni alloys were machined into /10 mm 5 mm size samples and placed on the prefluxed Ni plates (using water-soluble EP9301 flux) to be reflowed 1, 3, 5, and 7 times in a five-zone forced convection reflow oven (BTU VIP-70 N) with the highest temperature of 523 K for 1 min. In the thermal aging experiments, the machined Sn 3.5Ag and Sn 3.5Ag 0.75Ni alloy samples were placed on the prefluxed Ni plates and subjected to high-temperature aging at 523 K and 553 K for times up to 10 h. After multiple reflows and thermal aging tests, the specimens were sectioned carefully using a slow speed diamond saw and mounted in an epoxy. The cross-sections of the solder/ni interfaces were prepared using standard metallographic procedures (grinding and polishing). The microstructures were characterized by scanning electron microscopy (Philips, Inc. XL 40 FEG SEM) in the back-scattered electron (BSE) mode and transmission electron microscopy (Philips Tecnai G2 20 S-TWIN). Energy dispersive X- ray spectroscopy (EDX) (OXFORD, Inc. ISIS300), using a standard atomic number, absorption, fluorescence (ZAF) correction, and X-ray diffraction (Siemens D500 XRD), were used to determine the phase composition and the crystal structure of the IMC layers.

3 5198 J. Shen et al. / Acta Materialia 57 (2009) Results 3.1. Microstructural evolution of the interfacial reaction product Both in the multiple reflow samples and in the thermal aging test samples, interfacial reaction product layers were clearly observed (see Figs. 1 3). EDS analysis results show that these IMC layers are a composite of Sn and Ni elements. Furthermore, XRD tests were performed in the as-reflowed sample and a sample after 10 h thermal aging (they are the representative of other samples) (see Fig. 4). It was found in our study that only Ni 3 Sn 4 was detected as the reaction product. This result is in accord with some experimental results reported in the literature [9,15,17,22,25], while it is inconsistent with other test results in the literature [13,14], which reported that Ni 3 Sn and/or Ni 3 Sn 2 phases were also formed by the Sn/Ni interfacial reaction. Experiments with different numbers of reflow cycles or periods of thermal aging gave Ni 3 Sn 4 with different morphologies. The microstructure of Ni 3 Sn 4, produced in the as-reflowed Sn 3.5Ag/Ni sample, was loose-like, including a thin main layer at the interface and some discontinuous Ni 3 Sn 4 particles embedded inside the Ni plate (see Fig. 1a). By contrast, a continuous and uniform Ni 3 Sn 4 layer was formed in the as-reflowed Sn 3.5Ag 0.75Ni/Ni sample (see Fig. 1c). The microstructures of Ni 3 Sn 4 in Sn 3.5Ag/Ni and Sn 3.5Ag 0.75Ni/Ni samples after being reflowed for seven cycles were representative of all multiple reflows samples. As seen in Fig. 1b and d, they show almost the same microstructure compared with the corresponding as-reflowed samples (note the thickness of the Ni 3 Sn 4 layers did not increase dramatically). In the thermally aged test samples, between the solder matrices and Ni plates, Ni 3 Sn 4 layers were clearly observed, showing different morphologies and thicknesses. Obviously, both in the Sn 3.5Ag/Ni and in Sn 3.5Ag 0.75Ni/Ni samples, the thickness of the Ni 3 Sn 4 layers increased with an increase in the thermal aging time. However, the evolution of morphology of these Ni 3 Sn 4 layers was different in the Sn 3.5Ag/Ni and Sn 3.5Ag 0.75Ni/ Ni samples. A thin and continuous Ni 3 Sn 4 layer was formed in the Sn 3.5Ag/Ni sample after aging for 5 min at 523 K (see Fig. 2a). After thermal aging for several hours, some isolated Ni 3 Sn 4 particles were found to have departed from the main Ni 3 Sn 4 layer and entered into the solder matrices (see Fig. 2b and c). The interfaces of the solder/ni 3 Sn 4 layers were not flat and some bulges appeared with rounded edges (this bulging morphology of Ni 3 Sn 4 usually is described as a scallop structure). The Ni 3 Sn 4 layers formed in the Sn 3.5Ag 0.75Ni/Ni samples were thicker than those in the Sn 3.5Ag/Ni samples for the same thermal aging time. As seen in Fig. 2d, some Ni 3 Sn 4 particles, originating from the solidification of the solder alloy, were found to be distributed in a wide region of the solder matrices showing faceted structures in the Sn 3.5Ag 0.75Ni/Ni sample thermally aged for 5 min. However, a few solidified Ni 3 Sn 4 particles appeared in the solder matrices of Sn 3.5Ag 0.75Ni/Ni samples which had been subjected to thermal aging for several hours (see Fig. 2e and f). The surfaces of the Ni 3 Sn 4 layers in Sn 3.5Ag 0.75Ni/Ni samples were also scallop-like but with faceted Fig. 1. SEM micrographs of cross-sections of IMC layers formed in reflowed samples: (a) Sn 3.5Ag/Ni as-reflowed solder joint, (b) Sn 3.5Ag/Ni solder joint reflowed for seven cycles, (c) Sn 3.5Ag 0.75Ni/Ni as-reflowed solder joint, and (d) Sn 3.5Ag 0.75Ni/Ni solder joint reflowed for seven cycles.

4 J. Shen et al. / Acta Materialia 57 (2009) Fig. 2. SEM micrographs of cross-sections of IMC layers formed in samples thermally aged at 523 K: (a) Sn 3.5Ag/Ni solder joint thermally aged for 5 min, (b) Sn 3.5Ag/Ni solder joint thermally aged for 6 h, (c) Sn 3.5Ag/Ni solder joint thermally aged for 10 h, (d) Sn 3.5Ag 0.75Ni/Ni solder joint thermally aged for 5 min, (e) Sn 3.5Ag 0.75Ni/Ni solder joint thermally aged for 6 h, and (f) Sn 3.5Ag 0.75Ni/Ni solder joint thermally aged for 10 h. edges. Similar microstructures of Ni 3 Sn 4 layers were found in the Sn 3.5Ag/Ni and Sn 3.5Ag 0.75Ni/Ni samples which were thermally aged at 553 K (see Fig. 3). It is worth noting that by careful observation, in Sn 3.5Ag/Ni samples after thermal aging for 10 h at 523 K and 553 K, both faceted surfaces and round edge surface Ni 3 Sn 4 scallops appeared to a greater or lesser extent (see Figs. 2c and 3c). This phenomenon should be regarded as a transition from round edge surface Ni 3 Sn 4 particles to faceted surface Ni 3 Sn 4 particles during their growth Growth of Ni 3 Sn 4 layers Generally, there are three methods to obtain the average thickness of the IMC layers in solder joints. The first method (method I) is to determine the average thickness of IMC layers by calculating the arithmetical mean thickness through measuring the layer thickness at several equally spaced points. This method was not applied in our tests since it will result in a relatively large error due to the ragged edge of the solder/ni 3 Sn 4 interface. The second method (method II) to achieve the average thickness of IMC layers is by using software to measure the integrated area and the length of IMC layers first. Then, the average thickness of IMC layers may be calculated by dividing the integrated area by the length of the IMC layers. Since the interfaces of the Ni/Ni 3 Sn 4 were flatter than the interfaces of the solder/ni 3 Sn 4 in the IMC layers (see Figs. 1 3), a third method (method III) was developed to obtain the average thickness of the IMC layers by measuring the consumption of the Ni plates. From the volume transformation during the interfacial reaction, the average thickness of the Ni 3 Sn 4 layers may be calculated as follows: h Ni3 Sn 4 ¼ 1 q Ni h Ni ð1þ f Ni q Ni3 Sn 4 where h Ni3 Sn 4 is the average thickness of the Ni 3 Sn 4 layers, h Ni is the average thickness of the Ni plates consumed, f Ni

5 5200 J. Shen et al. / Acta Materialia 57 (2009) Fig. 3. SEM micrographs of cross-sections of IMC layers formed in samples thermally aged at 553 K: (a) Sn 3.5Ag/Ni solder joint thermally aged for 5 min, (b) Sn 3.5Ag/Ni solder joint thermally aged for 6 h, (c) Sn 3.5Ag/Ni solder joint thermally aged for 10 h, (d) Sn 3.5Ag 0.75Ni/Ni solder joint thermally aged for 5 min, (e) Sn 3.5Ag 0.75Ni/Ni solder joint thermally aged for 6 h, and (d) Sn 3.5Ag 0.75Ni/Ni solder joint thermally aged for 10 h. Fig. 4. X-ray diffractograms of the interfacial reaction product layers in the (a) as-reflowed and (b) thermally aged Sn 3.5Ag/Ni samples (553 K, 10 h). (Note the IMC layers are so narrow that when the X-ray beam is put over the layers, it takes in data from around the layers, such as the solder matrices.) is the weight fraction of Ni in the Ni 3 Sn 4 IMC, q Ni and q Ni3 Sn 4 are the densities of pure Ni (8.9 g cm 3 ) and Ni 3 Sn 4 (8.64 g cm 3 [26]), respectively. It should be stressed that in this equation, the solubility of Ni in the eutectic Sn 3.5Ag solder was neglected (since the solubilities of Ni in the eutectic Sn 3.5Ag solder at 523 K and 553 K are very small; compared with the consumption by the interfacial reaction, it is negligible) and the total Ni 3 Sn 4 layer formed in a solder joint was regarded as arising from the consumption of the Ni substrate. So, in theory, the average thickness of the Ni 3 Sn 4 layers calculated by methods II and III should be the same in the Sn 3.5Ag/Ni samples. However, because the Ni atoms contained in the solder alloy reacted with Sn atoms to form more Ni 3 Sn 4 which adhered to the Ni 3 Sn 4 layer and increased the thickness of Ni 3 Sn 4 layer, there must be a difference between the value of average thicknesses of Ni 3 Sn 4 layers in Sn 3.5Ag 0.75Ni/Ni samples determined by methods II and III.

6 J. Shen et al. / Acta Materialia 57 (2009) Using methods II and III, the average thickness of Ni 3 Sn 4 layers in Sn 3.5Ag/Ni and Sn 3.5Ag 0.75Ni/Ni samples were determined and are given in Fig. 5. Fig. 5a shows plots of the Ni 3 Sn 4 layer thicknesses against the number of reflow cycles in Sn 3.5Ag/Ni and Sn 3.5Ag 0.75Ni/Ni samples. It can be seen that, calculated by method II, the thickness of Ni 3 Sn 4 layers in the Sn 3.5Ag 0.75Ni/Ni sample was larger than that in the Sn 3.5Ag/Ni sample regardless of the number of reflow cycles, and their thicknesses did not change dramatically as a function of the number of reflow cycles. Using method III, the values of the thicknesses of Ni 3 Sn 4 layers in the Sn 3.5Ag 0.75Ni/Ni samples are close to those of the Sn 3.5Ag/Ni samples. Fig. 5b shows plots of the thickness of Ni 3 Sn 4 layers in Sn 3.5Ag/Ni and Sn 3.5Ag 0.75Ni/Ni samples during thermal aging at two different temperatures as a function of the aging time (the values of thickness were calculated by method II). It can be seen that in both the Sn 3.5Ag/ Ni and Sn 3.5Ag 0.75Ni/Ni samples, the thickness of Ni 3 Sn 4 layers increased with an increase of the thermal aging time at the two test temperatures. Moreover, the thickness of Ni 3 Sn 4 layers in the Sn 3.5Ag 0.75Ni/Ni samples was always larger than that for the Sn 3.5Ag/Ni samples. This is because the Ni atoms in the solder alloy reacted with Sn atoms directly at the interface of the solder/ni 3 Sn 4 layer without a requirement of long distance diffusion and formed additional Ni 3 Sn 4, which contributed to the value of the thickness of the Ni 3 Sn 4 layer when measured by method II. However, when measured by method III, the plots of the thickness of Ni 3 Sn 4 layers at two different temperatures as a function of aging time indicated that the thickness of Ni 3 Sn 4 layers in the Sn 3.5Ag 0.75Ni/Ni sample was only slightly smaller than that in the Sn 3.5Ag/Ni sample (as shown in Fig. 5c). In order to clarify the growth kinetics of the Ni 3 Sn 4 layers in the Sn 3.5Ag/Ni and Sn 3.5Ag 0.75Ni/Ni samples during thermal aging, an empirical power-law relationship was used as follows: h t h 0 ¼ kt n ð2þ where h t is the average thickness of the Ni 3 Sn 4 layer at time t, h 0 is the initial thickness (i.e., at the aging time of t = 0), Fig. 5. Variation of thickness of Ni 3 Sn 4 layers as a function of (a) number of reflow cycles, (b) thermal aging times (the values of thickness were calculated by method II) and (c) thermal aging times (the values of thickness were calculated by method III).

7 5202 J. Shen et al. / Acta Materialia 57 (2009) k is the growth rate constant, and n is the time exponent. The values of k and n, for a particular aging temperature, can be obtained by means of multivariable linear regression analysis. In our study, a linear regression analysis of the average thickness of the Ni 3 Sn 4 layer as a function of thermal aging time was conducted to determine the best-fit n values for the two aging temperatures and the equation was rewritten as: logðh t h 0 Þ¼log k þ n log t where the time exponent n actually equals the slope of the logðh t h 0 Þ, vs log t plot for each temperature. The calculated values of the thicknesses of Ni 3 Sn 4 layers by method III reflect the actual growth process of the Ni 3 Sn 4 layers in Sn 3.5Ag/Ni and Sn 3.5Ag 0.75Ni/Ni samples by an interfacial diffusion reaction and only the data from Fig. 5c was used for log plotting (see Fig. 6). The value of the time exponent n of the Sn 3.5Ag/Ni sample at 523 K was 0.32 ± 0.013, while it was 0.43 ± at 553 K. The value of the time exponent n of the Sn 3.5Ag 0.75Ni/Ni sample was 0.30 ± at 523 K and was 0.42 ± at 553 K, which are slightly smaller than that in the Sn 3.5Ag/Ni sample at the corresponding temperatures. Meanwhile, the activation energy, Q, for the growth of Ni 3 Sn 4 in the Sn 3.5Ag/Ni and Sn 3.5Ag 0.75Ni/Ni samples, was calculated from an Arrhenius relationship of the growth rate: k ¼ k 0 e ð Q=RT Þ where R is the gas constant (8.314 J mol 1 K 1 ) and k 0 is a pre-exponential coefficient. The activation energies were calculated from the slope of the Arrhenius plots using a linear regression model and the values found were 10.7 kj mol 1 (Sn 3.5Ag/Ni sample) and 11.6 kj mol 1 (Sn 3.5Ag 0.75Ni/Ni sample), which are close to the values reported by others [9,15]. Fig. 6. Log plot of the growth of Ni 3 Sn 4 layers in Sn 3.5Ag/Ni and Sn 3.5Ag 0.75Ni/Ni samples during thermal aging tests (the thicknesses of Ni 3 Sn 4 layers were measured by method III). ð3þ ð4þ 4. Discussion In a simple model, analysis of the growth kinetics of the Ni 3 Sn 4 layer starts with the simplest case, that is, the growth of a solid layer between the molten solder and plate which forms only one IMC phase (Ni 3 Sn 4 ) and the interfacial reaction takes place in accordance with the reaction equation: 3Ni + 4Sn = Ni 3 Sn 4. On the assumption that the Ni 3 Sn 4 formed is a parallel-plate layer whose thickness is the same over the entire surface of contact of the Sn/ Ni 3 Sn 4 /Ni and the length of the Ni 3 Sn 4 layer in the direction normal to the direction of diffusion of Sn/Ni interface is considerably greater than its thickness, so that the edge effects on the growth of the Ni 3 Sn 4 layer can be neglected, and the growth of Ni 3 Sn 4 can be described by a kinetic equation [27]: dh dt ¼ k csn þ k cni 1 þ k csnh k dsn 1 þ k cnih k dni where k c is a chemical constant and k d is a diffusional constant. This equation representing the growth of the Ni 3 Sn 4 layer covers both the growth at the Sn/Ni 3 Sn 4 interface and the growth of the Ni 3 Sn 4 /Ni interface independently, and the growth at each interface includes two alternative processes the Sn and Ni atoms diffusing through the Ni 3 Sn 4 layer (a physical process) to react with the Ni and Sn atoms (a chemical process) to form the Ni 3 Sn 4 layer. It is obvious that in the very initial period the interaction of the Sn and Ni when the Ni 3 Sn 4 layer is very thin (several atoms in thickness), the diffusion of Sn and Ni atoms to react with Ni and Sn atoms can be regarded as almost instantaneous due to the very short diffusion distance. This corresponds to fulfilling the condition that k c k d =h in Eq. (5). Therefore, since the chemical reaction takes place at an almost constant rate, the growth of the Ni 3 Sn 4 layer is usually referred to as having linear kinetics (n = 1). Similarly, it is easy to understand that after a continuous increase of the Ni 3 Sn 4 layer, the growth of the Ni 3 Sn 4 layer becomes more and more dependent on the rate volume diffusion of Sn and Ni atoms through the bulk of the Ni 3 Sn 4 layer, whereas the effect of the rate of growth by the chemical reaction gradually decreases and eventually becomes insignificant. At this stage, k c k d =h should be fulfilled in Eq. (5) and the growth rate of the Ni 3 Sn 4 layer by diffusion is proportional to its existing thickness. In practice, the time dependence of the total thickness increase of the Ni 3 Sn 4 layer is described by parabolic growth kinetics (n = 0.5). The reflow test results revealed that even after only one reflow cycle, Ni 3 Sn 4 layers with thicknesses of about 1.33 lm were formed in both the Sn 3.5Ag/Ni and Sn 3.5Ag 0.75Ni/Ni samples and the thicknesses of these Ni 3 Sn 4 layers increased very slowly during multiple reflows (see Fig. 1). This indicates that in the very initial period of the interaction of the molten solders and Ni plates, the Sn and Ni atoms reacted directly without long distance diffusion to form Ni 3 Sn 4 layers quickly following linear kinetics ð5þ

8 J. Shen et al. / Acta Materialia 57 (2009) (although this growth process was not observed directly in our tests because this linear kinetic growth regime of the Ni 3 Sn 4 layer is of a short duration at the high aging temperature due to the rapid chemical reaction of Sn and Ni atoms. However, the research results regarding the linear kinetic growth of Cu 6 Sn 5 layers in a copper tin reaction couple which was reported by Tu and Thompson support the growth mechanism proposed here [28]). After the initial period, the Ni 3 Sn 4 layer formed grew slowly with parabolic kinetics because the diffusion rate of Sn and Ni atoms became slower due to the thicker Ni 3 Sn 4 layer. In this stage, in order to observe the variation of the thickness of the Ni 3 Sn 4 layer easily, a relatively long high-temperature thermal aging must be performed. The actual growth of the Ni 3 Sn 4 layer by the Sn/Ni interfacial reaction during soldering is more complex than the description in the simple model above because it involves the net effect of several interrelated phenomena, such as diffusion through the layer via bulk (volume diffusion) and grain boundary diffusion, grain boundary grooving, grain coarsening, and dissolution into the molten solder. For example, in the Sn 3.5Ag/Ni samples, the grain boundaries in the Ni plate act as rapid paths for the Sn atoms to pass through and react with Ni atoms to form Ni 3 Sn 4 particles, which became embedded in the Ni plate. Fig. 1a and b shows microstructures where some Ni 3 Sn 4 particles were separated from the main Ni 3 Sn 4 layer and were embedded in the grain boundaries of Ni plates. However, in the Sn 3.5Ag 0.75Ni/Ni samples, because the Ni atoms contained in the molten Sn 3.5Ag 0.75Ni solder reacted with Sn atoms directly and rapidly in the initial period of interaction of Sn and Ni, these formed Ni 3 Sn 4 particles which were nucleated adjacent to the surface of the Ni plate with a match in their crystallographic orientation relationships, and thus impeded the Sn atoms passing through grain boundaries to form discrete Ni 3 Sn 4 particles embedded in the Ni plate. The microstructure of the Sn 3.5Ag 0.75Ni/Ni samples show that no Ni 3 Sn 4 particles were observed to be embedded in the Ni plates, and therefore the Ni 3 Sn 4 layer was more uniform and compact than that in Sn 3.5Ag/Ni sample (compare Fig. 1c and d). Whether or not the Ni 3 Sn and/or Ni 3 Sn 2 phases form in the Sn/Ni interfacial reaction system is difficult to determine, although they are also thermodynamically stable phases in the Sn Ni binary system [17]. Based on the simple model above, suppose the reactivity of the Ni plate with regard to the Sn atoms remains constant, the flux of Sn atoms through the Ni 3 Sn 4 layer continuously decreases as the thickness of the Ni 3 Sn 4 layer increases with the passage of aging time [27]. This will result in the formation of some areas lacking in Sn atoms near the Ni plate surface and multiphase layers may form following the chemical reaction equations: 2Sn + 3Ni = Ni 3 Sn 2 and Sn + 3Ni = - Ni 3 Sn. However, although many tests have been carried out on the Sn/Ni interfacial reaction, very few reports have been made of the existence of the Ni 3 Sn 2 and/or Ni 3 Sn phases (in our test, no Ni 3 Sn 2 and/or Ni 3 Sn were detected by X-ray analysis, see Fig. 4). The variation between the theoretical predictions and experimental results is likely to be attributed to the kinetics of the actual growth of the IMC layer at the Sn/Ni interface being different from that described in the simple model. Microstructural observations show that the IMC layers formed in the Sn/Ni interface are not whole single crystals but are in a polycrystalline form with grain boundaries (see Figs. 2 and 3). The effect of the anisotropic growth of grains and grain coarsening led to the IMC layers showing scallop-like morphologies, where the grain boundaries and grain boundary grooves acted as rapid diffusion paths for the Sn and Ni atoms to react with each other. Hence, the areas supposedly depleted in Sn atoms in the simple model hardly existed in the actual Sn/Ni reaction interface to form the Ni 3 Sn 2 or Ni 3 Sn phases. So these phases could not be resolved by our analytical techniques (including the use of EDX) or perhaps were not formed because of kinetic constraints. Since the actual Sn/Ni interfacial reaction involves several processes, the time exponent n of IMC growth kinetics would not be expected to be a simple 0.5 following parabolic growth kinetics. Although many researchers have reported that both the thickening and grain growth kinetics of Ni 3 Sn 4 in the Sn/Ni interfacial reaction followed a parabolic law in their tests [14,15,25,26], this only means the relationship between the values of thickness and time can be fitted to a parabolic law, but does not necessarily mean that the growth of Ni 3 Sn 4 layer is totally controlled by volume diffusion. In fact, more precise tests results showed that different values of n were achieved in the growth of the Ni 3 Sn 4 layer. Kang and Ramachandran [12] reported that in the temperature range K, in the initial stage of thermal aging, the value of n was 0.54; at an intermediate stage of thermal aging, the value of n was 0.12; and in the final stage of thermal aging, the value of n was Ghosh [9] calculated the value of n by multivariant linear regression analysis of experimental data and obtained values of n which were always slightly smaller than 1/3. Although he stressed that, due to the limited amount of experimental data, the confidence limit of his kinetic parameters could not be given, a time exponent n 6 1/3 is in good agreement with the growth model of Sn Cu IMC based on the effect of grain boundary diffusion, grain boundary grooving and grain coarsening [29]. Tao et al. have reported that the growth of the Ni 3 Sn 4 layer at lower thermal aging temperatures (453 K, 513 K and 573 K) gave parabolic growth kinetics (n = 0.5), while at a high temperature (693 K), due to the two-phase mixture of Ni 3 Sn 4 + solder at the interface, the growth of the Ni 3 Sn 4 layer gave linear growth kinetics (n = 1)[22]. It is meaningless to judge the precise values of the time exponent n in the growth of Ni 3 Sn 4 in different reports since it is dependent on the test conditions, measuring methods and, most importantly, it is a dynamically changing value. But it is worth clarifying the relationship between the influencing factors and time exponent n in

9 5204 J. Shen et al. / Acta Materialia 57 (2009) the growth of Ni 3 Sn 4. In our study, the time exponent n of growth of Ni 3 Sn 4 in the Sn 3.5Ag/Ni sample is 0.32 ± at 523 K and 0.43 ± at 553 K. The value of n of 0.32 ± at 523 K is in good agreement with a mathematical model based on grain boundary diffusion controlling the mass transport and grain coarsening reducing the availability of grain boundaries as diffusion paths as the IMC layer grows thicker [29]. While the value of n increased to 0.43 ± at 553 K, this is because during long-time high-temperature thermal aging, due to the driving force of the Gibbs Thomson effect [30], some Ni 3 Sn 4 particles coarsened and became separated from the main Ni 3 Sn 4 layer and were dispersed into the solder matrices (see Figs. 2 and 3). This led to the situation that the main Ni 3 Sn 4 layer thinned out considerably and in some areas of the Ni 3 Sn 4 /Ni surface, the Ni 3 Sn 4 layer was too thin to provide protection to the surface of the Ni plate. Hence, Sn and Ni atoms passed through the very thin layer to react with each other, and therefore, this increased the rate of growth of the Ni 3 Sn 4 layer markedly. Tao et al. have reported a linear rate of growth of the Ni 3 Sn 4 layer (n = 1) at a high temperature (693 K) by this mechanism [22]. Still, it should be stressed that if the growth kinetics of the Ni 3 Sn 4 layer follow a linear law this does not mean that the growth of Ni 3 Sn 4 is totally controlled by the chemical reaction without an influence from diffusion after longtime high-temperature thermal aging. On the contrary, this only means that the relationship between the values of the thickness and the time fit a linear law (with a value of n close to 1), and that the chemical reaction dominated the growth process (in fact, the value of n cannot be equal to 1 except in the very initial stages of thermal aging, theoretically). In our study, the experimental results of nonparabolic growth kinetics of the Ni 3 Sn 4 layer are in good agreement with the model of the growth of the IMC layer based on grain boundary diffusion, grain boundary grooving, grain coarsening and dissolution into the molten solder [29]. However, the effects of these factors on the growth of the Ni 3 Sn 4 layer could not be observed separately, directly and dynamically. So the addition of minor amounts of Ni into the solder alloy was used to assist in verifying the controlling mechanisms. When Ni was introduced into the solder alloy, Ni 3 Sn 4 particles formed by the direct reaction nucleated and adhered to the Ni 3 Sn 4 layer and experienced growth to thicken the whole Ni 3 Sn 4 layer due to the match in their crystallographic orientation relationships. Hence, when measured by method II, the thickness of the whole Ni 3 Sn 4 layer in the Sn 3.5Ag 0.75Ni/Ni sample was larger than that in the Sn 3.5AgiNi sample (see Fig. 5b). However, when method III was used to measure the thickness of Ni 3 Sn 4 layers formed only by the diffusion reaction, it was found that the thickness of the Ni 3 Sn 4 layer in the Sn 3.5Ag 0.75Ni/Ni sample was slightly smaller than that in the Sn 3.5Ag/Ni sample (see Fig. 5c). This means the addition of minor amounts of Ni into the Sn 3.5Ag solder suppressed the growth of the Ni 3 Sn 4 layer which formed by the diffusion reaction. The reason for this result is that the Ni 3 Sn 4 formed by the direct reaction accelerated the grain coarsening of Ni 3 Sn 4 (see Figs. 2 and 3: relatively large Ni 3 Sn 4 gains appeared in the Sn 3.5Ag 0.75Ni/Ni sample after long-time thermal aging), and thus reduced the availability of grain boundaries and grain grooves as diffusion paths so as to decrease the growth rate of the Ni 3 Sn 4 layer formed by the diffusion reaction. Hence, in this way, the effect of the influencing factors on the growth of the Ni 3 Sn 4 layer was proved indirectly. Obviously, during the growth of Ni 3 Sn 4 by the Sn/Ni liquid/solid interfacial reaction, some of the Ni 3 Sn 4 layer also dissolved into the molten solder. Fig. 7 gives a TEM bright-field image and a selected area diffraction pattern of a Ni 3 Sn 4 particle, which formed in the solder matrix by solidification, showing a faceted crystal structure. The faceted Ni 3 Sn 4 particles formed both in the Sn 3.5Ag 0.75Ni solder matrix and in the Sn 3.5Ag 0.75Ni/Ni interfaces. Hence, because the microstructures of Sn 3.5Ag/Ni samples after long-time high-temperature thermal aging show both faceted and round edged Ni 3 Sn 4 particles (see Figs. 2c and 3c), it is easy to understand that the Ni 3 Sn 4 particles with faceted surfaces originated from the those Ni atoms which were dissolved from the Ni 3 Sn 4 layer and re-reacted with Sn to solidify as particles. However, it is difficult to analyze the effect of the dissolution of Ni 3 Sn 4 on the growth of the Ni 3 Sn 4 layer quantitatively, although the dissolution of Ni 3 Sn 4 into molten solder was proved by microstructural observations. According to Dybkov s research regarding the kinetics of dissolution of a solid in a liquid by a solid/liquid interfacial reaction, Fig. 7. Bright-field TEM image and selected area diffraction pattern of a Ni 3 Sn 4 particle formed in the Sn 3.5Ag 0.75Ni solder matrix showing a faceted structure.

10 J. Shen et al. / Acta Materialia 57 (2009) the rate of dissolution, dc dt ; of the Ni 3Sn 4 layer in the molten solders during aging may be expressed by the equation [27]: dc dt ¼ k S V ðc s cþ where C is the concentration of Ni in the molten solders measured at time t, C s is the saturation concentration of Ni in the molten solder at the aging temperature, k is the dissolution rate constant of Ni 3 Sn 4, S is the interfacial area of the Ni 3 Sn 4 layer in contact with the molten solders, and V is the volume of the molten solder. In our tests, the rate of dissolution, dc,ofni dt 3Sn 4 in the molten solders could not be calculated to evaluate the effect of dissolution on the growth of the Ni 3 Sn 4 layer quantitatively unless the unknown dissolution rate constant, k, was given. However, ð6þ at least from the microstructural observations that no faceted Ni 3 Sn 4 was formed in the Sn 3.5Ag solder matrix (see Figs. 2 and 3), we can assume that during a long period of time of thermal aging, the dissolution of the Ni 3 Sn 4 layer into molten solders is slow and the concentration of Ni in the molten solders is far from the saturation concentration of Ni in these molten solders at the aging temperature. Notwithstanding this, according to the above Eq. (6), compared with the Sn 3.5Ag solder, the higher concentration of Ni in the Sn 3.5Ag 0.75Ni solder should decrease the rate of dissolution of the Ni 3 Sn 4 layer into the molten solder and increase the rate of growth of the Ni 3 Sn 4 layer in the Sn 3.5Ag 0.75Ni/Ni sample during aging. However, using method III, the thickness of the Ni 3 Sn 4 layer in the Sn 3.5Ag 0.75Ni/Ni sample was slightly smaller than that Fig. 8. Schematic diagrams of the growth behavior of the Ni 3 Sn 4 layer with different values of the time exponent n with different conditions.

11 5206 J. Shen et al. / Acta Materialia 57 (2009) in the Sn 3.5Ag/Ni sample. Hence, one can suppose that without the effect of dissolution of the Ni 3 Sn 4 layer on the growth of the Ni 3 Sn 4 layer, the thickness of the Ni 3 Sn 4 layer in the Sn 3.5Ag 0.75Ni/Ni sample should be much smaller than that in the Sn 3.5Ag/Ni sample. This proves that grain coarsening again suppressed the growth of Ni 3 Sn 4. Based on the discussion above, a clear image of the growth mechanism of the Ni 3 Sn 4 layer in the Sn/Ni reaction interface is given in the schematic diagram of Fig. 8. In the very initial period of interaction of Sn and Ni, because the Ni 3 Sn 4 layer formed was very thin, the Sn and Ni atoms reacted with each other instantaneously without long range diffusion. This growth behavior of the Ni 3 Sn 4 layer gave linear kinetics and a time exponent n = 1 (see Fig. 8a). After several reflow cycles or thermal aging for several minutes (these conditions are close to the procedures used in the microelectronic packaging in industry), if a relatively continuous and even Ni 3 Sn 4 layer formed at the interface of the Sn/Ni couple, the growth behavior of the Ni 3 Sn 4 layer gave parabolic kinetics and a time exponent n close to 0.5 due to volume diffusion dominating the diffusion process of Sn and Ni atoms (see Fig. 8b). However, usually, this is not the case in the Sn/ Ni reaction interface since grain boundary diffusion, grain boundary grooving, grain coarsening and dissolution into the molten solder also occurred during the growth of Ni 3 Sn 4. When grain boundary diffusion and grain boundary grooving dominated the diffusion process of Sn and Ni atoms, the growth rate of the Ni 3 Sn 4 layer increased and the time exponent n increased with a value between 0.5 and 1 (see Fig. 8c). By contrast, the grain coarsening of Ni 3 Sn 4 should reduce the availability of grain boundaries and grooves as diffusion paths so that it should slow down the growth of the Ni 3 Sn 4 layer (see Fig. 8d). With these conditions, according to the mathematic model [29], the time exponent n should decrease to be near 1/3 (the value of the time exponent achieved in our tests by thermal aging the Sn 3.5Ag/Ni sample at 523 K is 0.32 ± 0.013, which is good agreement with the model). After long-time high-temperature thermal aging, due to grain coarsening and the Gibbs Thomson effect, some Ni 3 Sn 4 particles became separated from the main Ni 3 Sn 4 layer, and therefore this resulted in the formation of some bare interfaces at the main Ni 3 Sn 4 layer. In this situation, Ni 3 Sn 4 grew rapidly in these bare areas and the overall thickness of the Ni 3 Sn 4 layer increased sharply. Then, the time exponent n increased to be close to 1. In practice, some researchers have called this linear growth kinetics. 5. Conclusions With the help of a classical simple model of the liquid/ solid interfacial reaction, the growth mechanisms of Ni 3 Sn 4 in actual Sn 3.5Ag/Ni couples were investigated and discussed. An introduction of a minor addition of Ni into the solder alloy changed the growth process of Ni 3 Sn 4 at the Sn 3.5Ag 0.75Ni/Ni reaction interface and proved the actual growth of Ni 3 Sn 4 involves the net effect of several interrelated phenomena, such as diffusion through the layer via bulk (volume) diffusion and grain boundary diffusion, grain boundary grooving, grain coarsening, and dissolution into the molten solder. The time exponent n and morphology of Ni 3 Sn 4 were dependent on the relative values of these factors, i.e., which factor/factors dominated the growth process. Notwithstanding this, reflow tests showed that the slow growth rate of Ni 3 Sn 4 in industrial soldering applications is desirable since a brittle and thick Ni 3 Sn 4 layer will severely weaken the solder joints. Acknowledgments The authors would like to acknowledge the financial support provided by an RGC Competitive Earmarked Research Grant (Project No , CityU ). Special thanks to Prof. B. Ralph in Brunel University for his cooperation in this study. References [1] Abtew M, Selvaduray G. Mater Sci Eng R 2000;27:95. [2] Laurila T, Vuorinen V, Kivilahti JK. Mater Sci Eng R 2005;49:1. [3] Tu KN, Gusak AM, Li M. J Appl Phys 2003;93:1335. [4] Zeng K, Tu KN. Mater Sci Eng R 2002;38:55. [5] Tu KN, Zeng K. Mater Sci Eng R 2001;34:1. [6] Tu KN, Lee TY, Jang JW, Li L, Frear DR, Zeng K, et al. J Appl Phys 2001;89:4843. [7] Zuruzi AS, Chiu C-h, Lahiri SK. J Appl Phys 1999;86:4916. [8] Suh JO, Tu KN, Tamura N. J Appl Phys 2007;102: [9] Ghosh G. J Appl Phys 2000;88:6887. 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