Effect of rare earth metal Ce addition to Sn-Ag solder on interfacial reactions with Cu substrate

The effect of adding a small amount of rare earth cerium (Ce) element to low Ag containing Sn-1wt%Ag Pb-free solder on its interfacial reactions with Cu substrate was investigated. The growth of intermetallic compounds (IMCs) between three Sn-1Ag-xCe solders with different Ce contents and a Cu substrate was studied and the results were compared to those obtained for the Ce-free Sn-1Ag/Cu systems. In the solid-state reactions of the Sn-1Ag(-xCe)/Cu solder joints, the two IMC layers, Cu₆Sn₅ and Cu₃Sn, grew as aging time increased. Compared to the Sn-1Ag/Cu joint, the growth of the Cu₆Sn₅ and Cu₃Sn layers was depressed for the Ce-containing Sn-1Ag-xCe/Cu joint. The addition of Ce to the Sn-Ag solder reduced the growth of the interfacial Cu-Sn IMCs and prevented the IMCs from spalling from the interface. The evenly-distributed Ce elements in the solder region blocked the diffusion of Sn atoms to the interface and retarded the growth of the interfacial IMC layer.     

Effects of in situ nickel particle addition on the microstructure and microhardness of Sn–Ag solder

Abstract

In this study, various amounts of Ni particles were added in situ to Sn–3·5 wt-%Ag lead free solder to form new composite solders. Copper substrates were then dipped into these solders and aged at 150°C for 0, 25, 225, or 1000 h. The microstructure and microhardness of the as solidified solder and the aged solder/copper couples were investigated. Experimental results revealed that the addition of Ni particles increased the microhardness of the composite solder. Ni additions of less than 3 wt-% yielded a microstructure of β-Sn grains surrounded by a eutectic mixture of Ag₃Sn and a Sn rich matrix. An intermetallic compound of Ni₃Sn₄ particles was dispersed throughout the eutectic. For 5 wt-%Ni addition, the Ni₃Sn₄ phase and the remaining Ni particles were agglomerated. In the case of copper substrate dipped with a thick layer of composite solder, water quenched and then aged at 150°C, the induced (Ni, Cu)₃Sn₄ particles coarsened and agglomerated. Additionally, the intermetallic (Cu, Ni)₆Sn₅ compound layer formed at the solder/Cu interface thickened with increasing Ni content.     

Influence of Cu addition on intermetallic compound formation and microstructure of Sn–3Ag–1˙5Sb–xCu solder joints

Abstract

This study investigates the influence of 0–1˙5 wt-%Cu addition on the microstructure and the intermetallic compound (IMC) formation of the as soldered Sn–3Ag–1˙5Sb–xCu (wt-%) solders and following thermal storage at 150°C for 0, 25, 200 and 600 h, with the intention of identifying the optimum Cu addition for industrial applications. The experimental results show that the melting point of Sn–3Ag–1˙5Sb–xCu solder decreases with Cu addition. For Cu additions of 1˙0 wt-% or higher, an IMC of Cu₆Sn₅ particles is dispersed throughout the matrix, resulting in a dispersion strengthening effect, and its size increases with the levels of Cu addition increasing. The coarsened long strip like Cu₆Sn₅ with a length of more than 100 μm growing from the upper interface of IMC layer into the solder matrix is observed in the solder with 1˙5 wt-%Cu addition after thermal storage. Cu₆Sn₅ grains in the IMC layer develop the ripening grains with a more hexagonal or polygonal shape and smooth edged flat surfaces instead of scallop shape. Additionally, the microhardness of each solder increases with Cu addition and decreases with increasing time of thermal storage at 150°C.     

不同Ag元素含量Sn-Ag-Cu无铅钎料性能分析

分别研究了Ag元素含量(质量分数,%)为0.3,1.0,2.0,3.0,3.8的Sn-Ag-Cu(SAC)无铅钎料合金的显微结构、熔化行为、力学性能、润湿性和界面金属间化合物(IMC)形貌.结果表明,随着Ag元素含量的增加,钎料内部金属间化合物晶粒越小,晶粒在钎料中的密度越高,晶粒间距越小,金属间化合物产生的弥散强化越明显,钎料的抗拉强度越高,但韧性越低.随Ag元素含量的提高,钎料越接近共晶成分,熔化温度范围越小.Ag元素含量对润湿性影响不明显,SAC0307/Cu焊点界面IMC晶粒比其它钎料细小.     

Microstructure, kinetic analysis and hardness of Sn-Ag-Cu-1 wt% nano-ZrO2 composite solder on OSP-Cu pads

Nano-sized, nonreacting, noncoarsening ZrO₂ particle-reinforced Sn-Ag-Cu composite solders were prepared by mechanically dispersing ZrO₂ nano-particles into Sn-Ag-Cu solder and the interfacial morphology between the solder and organic solderability preservative (OSP)-Cu pads were characterized metallographically. At their interfaces, island-shaped Cu₆Sn₅ and Cu₃Sn intermetallic compound (IMC) layers were found in solder joints with and without the ZrO₂ particles and the IMC layer thickness was substantially increased with reaction time and temperature. In the solder ball region, needle-shaped Ag₃Sn and spherically-shaped Cu₆Sn₅ IMC particles were found to be uniformly distributed in the β-Sn matrix. However, after the addition of ZrO₂ nano-particles, Ag₃Sn and Cu₆Sn₅ IMC particles appeared with a fine microstructure and retarded the growth rate of the IMC layers at their interfaces. From a kinetic analysis, the calculated activation energies for the total (Cu₆Sn₅ + Cu₃Sn) IMC layers for Sn-Ag-Cu and Sn-Ag-Cu-1 wt% ZrO₂ composite solder joints on OSP-Cu pads were about 53.2 and 59.5 kJ/mol, respectively. In addition, solder joints containing ZrO₂ nano-particles displayed higher hardness due to the uniform distribution of ZrO₂ nano-particles as well as the refined IMC particles. The hardness values of the plain Sn-Ag-Cu solder joint and solder joints containing 1 wt% of ZrO₂ nano-particles after 5 min reaction at 250 °C were about 15.0 Hv and 17.1 Hv, respectively. On the other hand, their hardness values after 30 min reaction were about 13.7 Hv and 15.5 Hv, respectively.     

Influence of copper addition on adhesive strength of Sn–3Ag–1·5Sb solder joints

Abstract

The present work investigates the effects of adding a small amount of Cu to Sn–3Ag–1·5Sb solders. The present results indicate that adding 0·5 and 1·0 wt-%Cu to Sn–3Ag–1·5Sb solders causes the liquidus temperature to decrease from its original value of 233·4°C to 231·6°C and to 231·4°C, respectively. Furthermore, it is noted that the addition of 1·0 wt-%Cu reduces the difference between the liquidus and solidus temperatures. It is shown that the added Cu reacts with the Sn content of the solder to form Cu₆Sn₅ particles in the β-Sn matrix, which are distributed non-uniformly since the Cu content is low. The experimental results also reveal that the growth rate of the solder joint interfacial intermetallic compound layers increases at higher levels of Cu addition. Finally, it is established that adding Cu to the Sn–3Ag–1·5Sb solder not only improves the adhesive strength of the solder joints, but also reduces the rate of degradation of the adhesive strength of the joints during thermal storage.     

Influence of bismuth on microstructure,thermal properties,mechanical performance,and interfacial behavior of SAC305-xBi/Cu solder joints

This research sought to improve the properties of SAC305 solder joints by the addition of 1 and 2 wt.% Bi. The effects of bismuth doping on the microstructure, thermal properties, and mechanical performance of the SAC305-xBi/Cu solder joints were investigated. Bi-doping modified the microstructure of the solder joints by refining the primary β-Sn and eutectic phases. Bi-doping below 2 wt.% dissolved in the β-Sn matrix and formed a solid solution, whereas Bi additions equal to or greater than 2 wt.% formed Bi precipitates in the β-Sn matrix. Solid solution strengthening and precipitation strengthening mechanisms in the β-Sn matrix increased the ultimate tensile strength and microhardness of the alloy from 35.7 MPa and 12.6 HV to 55.3 MPa and 20.8 HV, respectively, but elongation decreased from 24.6% to 16.1%. The fracture surface of a solder joint containing 2 wt.% Bi was typical of a brittle failure rather than a ductile failure. The interfacial layer of all solder joints comprised two parallel IMC layers: a layer of Cu₆Sn₅ and a layer of Cu₃Sn. The interfacial layer was thinner and the shear strength was greater in SAC305-xBi/Cu joints than in SAC305/Cu solder joints. Therefore, small addition of Bi refined microstructure, reduced melting temperature and improved the mechanical performance of SAC305/Cu solder joints.     

石墨烯纳米片增强的Sn-58Bi/Cu焊点可靠性

将不同含量（0%,0.025%,0.05%,0.075%,0.1%,0.2%,质量分数）的石墨烯纳米片（GNSs）添加到Sn-58Bi低温钎料中,研究了GNSs对钎料熔化温度、润湿性能、剪切强度、显微组织和界面反应的影响。结果表明：添加GNSs可以改善Sn-58Bi钎料焊点的润湿性能和抗剪切强度,但对其熔化温度的影响较小。随着GNSs的添加,钎料得到了相对细化的显微组织,界面金属间化合物（IMC）的厚度明显降低,并逐渐趋于平整。另外,随着GNSs的加入,Sn-58Bi钎料的剪切断裂模式从脆性断裂转变为脆性和韧性混合的断裂模式,这与其抗剪切强度的变化是一致的。因此,添加微量的GNSs是增强Sn-58Bi/Cu焊点可靠性的有效途径。     

Intermetallic compound growth suppression at high temperature in SAC solders with Zn addition on Cu and Ni-P substrates

The effect of adding 0.5-1.5 wt.% Zn to Sn-3.8Ag-0.7Cu (SAC) solder alloy during reflow and solid state ageing has been investigated. In particular, the role of the Zn addition in suppressing interfacial Intermetallic Compound (IMC) growth on Cu and Ni-P substrates has been determined. Solder-substrate couples were aged at 150 °C and 185 °C for 1000 h. In the case of 0.5-1.0 wt.% Zn on Cu substrate, Cu₃Sn IMC was significantly suppressed and the morphology of Cu₆Sn₅ grains was changed, leading to suppressed Cu₆Sn₅ growth. In the SAC-1.5Zn/Cu substrate system a Cu₅Zn₈ IMC layer nucleated at the interface followed by massive spalling of the layer into the solder, forming a barrier layer limiting Cu₆Sn₅ growth. On Ni-P substrates the (Cu,Ni)₆Sn₅ IMC growth rate was suppressed, the lowest growth rate being found in the SAC-1.5Zn/Ni-P system. In all cases the added Zn segregated to the interfacial IMCs so that Cu₆Sn₅ became (Cu,Zn)₆Sn₅ and (Cu,Ni)₆Sn₅ became (Ni,Cu,Zn)₆Sn₅. The effect of Zn concentration on undercooling, wetting angles and IMC composition changes during ageing are also tabulated, and a method of incorporating Zn into the solder during reflow without compromising solder paste reflow described.     

Microstructural evolution of intermetallic compounds in Sn–3.5Ag–X (X = 0, 0.75Ni, 1.0Zn and 1.5In)/Cu solder joints during liquid aging

In this paper, the microstructural evolution of IMCs in Sn–3.5Ag–X (X = 0, 0.75Ni, 1.0Zn, 1.5In)/Cu solder joints and their growth mechanisms during liquid aging were investigated by microstructural observations and phase analysis. The results show that two-phase (Ni₃Sn₄ and Cu₆Sn) IMC layers formed in Sn–3.5Ag–0.75Ni/Cu solder joints during their initial liquid aging stage (in the first 8 min). While after a long period of liquid aging, due to the phase transformation of the IMC layer (from Ni₃Sn₄ and Cu₆Sn phases to a (Cu, Ni)₆Sn₅ phase), the rate of growth of the IMC layer in Sn–3.5Ag–0.75Ni/Cu solder joints decreased. The two Cu₆Sn₅ and Cu₅Zn₈ phases formed in Sn–3.5Ag–1.0Zn/Cu solder joints during the initial liquid aging stage and the rate of growth of the IMC layers is close to that of the IMC layer in Sn–3.5Ag/Cu solder joints. However, the phase transformation of the two phases into a Cu–Zn–Sn phase speeded up the growth of the IMC layer. The addition of In to Sn–3.5Ag solder alloy resulted in Cu₆(Sn_x,In_1?x)₅ phase which speeded up the growth of the IMC layer in Sn–3.5Ag–1.5In/Cu solder joint.     

外能辅助下Sn2.5Ag0.7Cu0.1RE0.05Ni/Cu钎焊接头组织与性能

采用SEM、EDS、XRD等方法研究了超声、电场外能辅助下Sn2.5Ag0.7Cu0.1RE0.05Ni/Cu钎焊接头的组织与性能。结果表明,借助于超声、超声-电场外能辅助能细化Sn2.5Ag0.7Cu0.1RE0.05Ni/Cu钎焊接头钎缝组织并使共晶组织比例增加,界面区金属间化合物(IMC)平均厚度、粗糙度和界面IMC颗粒尺寸减小。超声和电场外能辅助下Sn2.5Ag0.7Cu0.1RE0.05Ni/Cu钎焊接头强度与其界面IMC层粗糙度密切相关,超声的作用更为显著,在超声-电场外能辅助钎焊接头界面IMC层粗糙度降低中占主导作用,施加超声-电场外能辅助下钎焊接头剪切强度与传统钎焊相比提高24.1%;施加超声、超声-电场外能辅助使Sn2.5Ag0.7Cu0.1RE0.05Ni/Cu钎焊接头断裂途径由钎缝和界面IMC层组成的界面过渡区向钎缝侧迁移,呈界面(Cu,Ni)_6Sn_5 IMC解理和钎缝解理+韧窝的脆-韧混合型断裂机制,使接头剪切断口塑性区比例增加,从而提高接头剪切强度。     

Sn-Cu基多组元无铅钎料组织、性能及界面反应研究

研究了多组元无铅钎料Sn-2Cu-0.5Ni和Sn-2Cu-0.5Ni-0.5Au的熔化行为、微观组织、力学性能及与Ni基板的钎焊反应。结果表明:Au的引入降低了钎料合金的熔化温度;2种钎料基体中的金属间化合物(IMC)均基于Cu6Sn5结构,且在260℃下与Ni基板钎焊60s后在接头界面处均生成棒状的(Cu,Ni)6Sn5;通过拉伸试验得到了2种钎料的抗拉强度和延伸率。     

Interfacial Reactions and Bonding Strength of Sn-xAg-0.5Cu/Ni BGA Solder Joints

The present study details the microstructure evolution of the interfacial intermetallic compounds (IMCs) layer formed between the Sn-xAg-0.5Cu (x = 1, 3, and 4 wt.%) solder balls and electroless Ni-P layer, and their bond strength variation during aging. The interfacial IMCs layer in the as-reflowed specimens was only (Cu,Ni)₆Sn₅ for Sn-xAg-0.5Cu solders. The (Ni,Cu)₃Sn₄ IMCs layer formed when Sn-4Ag-0.5Cu and Sn-3Ag-0.5Cu solders were used as aging time increased. However, only (Cu,Ni)₆Sn₅ IMCs formed in Sn-1Ag-0.5Cu solders, when the aging time was extended beyond 1500 h. Two factors are expected to influence bond strength and fracture modes. One of the factors is that the interfacial (Ni,Cu)₃Sn₄ IMCs formed at the interface and the fact that fracture occurs along the interface. The other factor is Ag₃Sn IMCs coarsening in the solder matrix, and fracture reveals the ductility of the solder balls. The above analysis indicates that during aging, the formation of interfacial (Ni,Cu)₃Sn₄ IMCs layers strongly influences the pull strength and the fracture behavior of a solder joint. This fact demonstrates that interfacial layers are key to understanding the changes in bonding strength. Additionally, comparison of the bond strength with various Sn-Ag-Cu lead-free solders for various Ag contents show that the Sn-1Ag-0.5Cu solder joint is not sensitive to extended aging time.     

热循环对Sn58Bi(纳米Ti)/Cu焊点界面与性能影响

为了改善Sn-58Bi低温钎料的性能,通过在Sn-58Bi低温钎料中添加质量分数为0.1%的纳米Ti颗粒制备了Sn-58Bi-0.1Ti纳米增强复合钎料。在本文中,研究了纳米Ti颗粒的添加对-55～125 _^oC热循环过程中Sn-58Bi/Cu焊点的界面金属间化合物(IMC)生长行为的影响。研究结果表明：回流焊后,在Sn-58Bi/Cu焊点和Sn-58Bi-0.1Ti/Cu焊点的界面处都形成一层扇贝状的Cu₆Sn₅ IMC层。在热循环300次后,在Cu₆Sn₅/Cu界面处形成了一层Cu₃Sn IMC。Sn-58Bi/Cu焊点和Sn-58Bi-0.1Ti/Cu焊点的IMC层厚度均和热循环时间的平方根呈线性关系。但是,Sn-58Bi-0.1Ti/Cu焊点的IMC层厚度明显低于Sn-58B/Cu焊点,这表明纳米Ti颗粒的添加能有效抑制热循环过程中界面IMC的过度生长。另外计算了这两种焊点的IMC层扩散系数,结果发现Sn-58Bi-0.1Ti/Cu焊点的IMC层扩散系数(整体IMC、Cu₆Sn₅和Cu₃Sn IMC)明显比Sn-58Bi/Cu焊点小,这在一定程度上解释了Ti纳米颗粒对界面IMC层的抑制作用。     

Ni/P/Ce元素对SnAgCu无铅钎料性能和组织的影响

研究添加微量单一Ni、P或稀土Ce元素对Sn3.0Ag0.5Cu(质量分数, %)无铅钎料熔化温度、润湿性能、时效前后钎焊接头的剪切强度和显微组织的影响。结果表明：单一Ni、P和稀土Ce元素的添加,对Sn3.0Ag0.5Cu钎料合金的熔化温度影响很小;P元素的加入能够增大合金的润湿力,缩小润湿时间。加入单一的Ni或稀土Ce元素对合金的润湿性能和接头时效前的剪切强度影响不大,但能够很好地抑制高温时效引起的接头剪切强度的下降。此外,P 元素的添加明显地改善了合金的抗氧化性能,但稀土Ce元素的添加对其有所恶化。这些性能的改变与微量Ni、P或稀土Ce元素对其显微组织的影响有关     

Effect of adding 1 wt% Bi into the Sn–2.8Ag–0.5Cu solder alloy on the intermetallic formations with Cu-substrate during soldering and isothermal aging

Intermetallic compound (IMC) formations of Sn–2.8Ag–0.5Cu solder with additional 1 wt% Bi were studied for Cu-substrate during soldering at 255 °C and isothermal aging at 150 °C. It was found that addition of 1 wt% Bi into the Sn–2.8Ag–0.5Cu solder inhibits the excessive formation of intermetallic compounds during the soldering reaction and thereafter in aging condition. Though the intermetallic compound layer was Cu₆Sn₅, after 14 days of aging a thin Cu₃Sn layer was also observed for both solders. A significant increase of intermetallic layer thickness was observed for both solders where the increasing tendency was lower for Bi-containing solder. After various days of aging, Sn–2.8Ag–0.5Cu–1.0Bi solder gives comparatively planar intermetallic layer at the solder–substrate interface than that of the Sn–2.8Ag–0.5Cu solder. The formation of intermetallic compounds during aging for both solders follows the diffusion control mechanism and the diffusion of Cu is more pronounced for Sn–2.8Ag–0.5Cu solder. Intermetallic growth rate constants for Sn–2.8Ag–0.5Cu and Sn–2.8Ag–0.5Cu–1.0Bi solders were calculated as 2.21 × 10⁻¹⁷ and 1.91 × 10⁻¹⁷ m²/s, respectively, which had significant effect on the growth behavior of intermetallic compounds during aging.     

超声作用下Ni/Sn/Ni钎焊界面金属间化合物的演变

对比研究了超声作用和无超声作用下Ni/Sn/Ni钎焊界面金属间化合物的形成和演变规律。结果表明,无超声作用时,Ni/Ni_3Sn_4界面较为平直且致密,而Sn/Ni_3Sn_4界面被液态Sn钎料逐渐溶解而呈扇贝状,并且有少量Ni_3Sn_4分布在焊缝中。其次,界面金属间化合物(intermetallic compound, IMC)层厚度与时间呈抛物线关系,Ni_3Sn_4的生长受体扩散的控制。超声作用下,声空蚀作用使得界面Ni_3Sn_4发生溶解而形成很多沟槽,甚至在界面IMC的局部区域出现了"neck"状连接,重新为母材Ni原子向钎料的溶解打开了通道,在声流的辅助作用下促进母材的溶解。随着超声时间的增加,声空化作用将界面"neck"状连接的细长的Ni_3Sn_4晶粒打碎而进入焊缝,使得界面IMC逐渐减薄。进入焊缝的Ni_3Sn_4进一步在空化作用下溶解和破碎,最终大量细小的Ni_3Sn_4均匀分布在焊缝中。     

Ni对SnAgCuRE钎料及其钎焊接头性能的影响

以商用Sn3.8Ag0.7Cu为参照系,研究了Ni对Sn2.5Ag0.7Cu0.1 RE钎料合金及其钎焊接头性能的影响。研究结果表明,添加0.05%(质量分数)Ni能在不降低Sn2.5Ag0.7Cu0.1RE钎料合金抗拉强度的同时显著提高其伸长率,是商用Sn3.8Ag0.7Cu的1.4倍;相应地钎焊接头的蠕变断裂寿命最长,为未添加Ni时的13.3倍,远高于现行商用Sn3.8Ag0.7Cu的,满足微电子连接高强韧高可靠性无铅钎料合金系的需求。     

Improving the shear strength of Sn–Ag–Cu–Ni/Cu–Zn solder joints via modifying the microstructure and phase stability of Cu–Sn intermetallic compounds

This study focuses on the correlation between high-speed impact tests and the interfacial reaction in Sn-3.0Ag-0.5Cu-0.1Ni/Cu (wt%) and Sn-3.0Ag-0.5Cu-0.1Ni/Cu-15Zn solder joints. Adding Ni into the Sn–Ag–Cu solder alters the interfacial morphology from scallop type to layer type and exhibits high shear strength after reflow in both solder joints. However, the shear strength of Sn-3.0Ag-0.5Cu-0.1Ni/Cu solder joints degrades significantly after thermal aging at 150 °C for 500 h. It is notable that Sn-3.0Ag-0.5Cu-0.1Ni/Cu-15Zn solder joints still present higher shear strength after aging at 150 °C. The weakened shear strength in Sn-3.0Ag-0.5Cu-0.1Ni/Cu solder joints is due to stress accumulation in the interfacial (Cu,Ni)₆Sn₅ compound induced by the phase transformation from a high-temperature hexagonal structure (η-Cu₆Sn₅) to a low-temperature monoclinic structure (η'-Cu₆Sn₅). However, doping small amounts of Zn into (Cu,Ni)₆(Sn,Zn)₅ can inhibit the phase transformation during thermal aging and maintain strong shear strength. These experiments demonstrate that Sn-3.0Ag-0.5Cu-0.1Ni/Cu-15Zn solder joints can act as a stable connection in the micro-electronic packaging of most electronic products at their average working temperatures.     

Dissolution and interfacial reactions of thin-film Ti/Ni/Ag metallizations in solder joints

The dissolution and interfacial reactions involving thin-film Ti/Ni/Ag metallizations on two semiconductor devices, diode and metal-oxide-semiconductor field-effect transistor (MOSFET), a Sn–3.0Ag–0.7Cu solder, and a Au-layer on the substrates are studied. To simulate the dissolution kinetics of the Ag-layer in liquid solder during the reflow process, the computational thermodynamics (Thermo-Calc) and kinetics (DICTRA: DIffusion Controlled TRAnsformations) tools are employed in conjunction with the assessed thermochemical and mobility data. The simulated results are found to be consistent with the observed as-reflowed microstructures and the measured Ag contents in the solder. In the as-reflowed joints two different intermetallic compounds (IMC) are found near the diode/solder interface. Both are in the form of particles of different morphologies, not a continuous layer, and are referred to as IMC-I and IMC-II. The former corresponds to Ni₃Sn₄ with Cu atoms residing in the Ni sublattice. It is uncertain whether IMC-II is Cu₆Sn₅ phase with Ni atoms residing in the Cu sublattice or a Cu–Ni–Sn ternary phase. Near the as-reflowed MOSFET/solder interface, both particles and a skeleton-like layer of Ni₃Sn₄ are observed. The primary microstructural dynamics during solid state aging are the coarsening of IMC particles and the reactions involving the unconsumed (after reflow) Ni- and the Ti-layer with Sn and Au. While the reaction with the Ni-layer yields only Ni₃Sn₄ intermetallic, the reaction involving the Ti-layer suggests the formation of Ti–Sn and Au–Sn–Ti intermetallics. The latter is due to the diffusion of Au from the substrate side to the die side. It is postulated that the kinetics of Au–Sn–Ti layer is primarily governed by the diffusion of Au through the Ni₃Sn₄ layer by a grain boundary mechanism.