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.     

Growth Behavior of Intermetallic Compounds at SnAgCu/Ni and Cu Interfaces

The growth behavior of reaction-formed intermetallic compounds (IMCs) at Sn3.5Ag0.5Cu/Ni and Cu interfaces under thermal-shear cycling conditions was investigated. The results show that the morphology of (Cu _x Ni_1–x )₆Sn₅ and Cu₆Sn₅ IMCs formed both at Sn3.5Ag0.5Cu/Ni and Cu interfaces gradually changed from scallop-like to chunk-like, and different IMC thicknesses developed with increasing thermal-shear cycling time. Furthermore, Cu₆Sn₅ IMC growth rate at the Sn3.5Ag0.5Cu/Cu interface was higher than that of (Cu _x Ni_1–x )₆Sn₅ IMC under thermal-shear cycling. Compared to isothermal aging, thermal-shear cycling led to only one Cu₆Sn₅ layer at the interface between SnAgCu solder and Cu substrate after 720 cycles. Moreover, Ag₃Sn IMC was dispersed uniformly in the solder after reflow. The planar Ag₃Sn formed near the interface changed remarkably and merged together to large platelets with increasing cycles. The mechanism of formation of Cu₆Sn₅, (Cu _x Ni_1–x )₆Sn₅ and Ag₃Sn IMCs during thermal-shear cycling process was investigated.     

Suppressing effect of 0.5 wt.% nano-TiO2 addition into Sn-3.5Ag-0.5Cu solder alloy on the intermetallic growth with Cu substrate during isothermal aging

This study investigated the effects of adding 0.5 wt.% nano-TiO₂ particles into Sn3.5Ag0.5Cu (SAC) lead-free solder alloys on the growth of intermetallic compounds (IMC) with Cu substrates during solid-state isothermal aging at temperatures of 100, 125, 150, and 175 °C for up to 7 days. The results indicate that the morphology of the Cu₆Sn₅ phase transformed from scallop-type to layer-type in both SAC solder/Cu joints and Sn3.5Ag0.5Cu-0.5 wt.% TiO₂ (SAC) composite solder/Cu joints. In the SAC solder/Cu joints, a few coarse Ag₃Sn particles were embedded in the Cu₆Sn₅ surface and grew with prolonged aging time. However, in the SAC composite solder/Cu aging, a great number of nano-Ag₃Sn particles were absorbed in the Cu₆Sn₅ surface. The morphology of adsorption of nano-Ag₃Sn particles changed dramatically from adsorption-type to moss-type, and the size of the particles increased.The apparent activation energies for the growth of overall IMC layers were calculated as 42.48 kJ/mol for SAC solder and 60.31 kJ/mol for SAC composite solder. The reduced diffusion coefficient was confirmed for the SAC composite solder/Cu joints.     

Ni segregation in the interfacial (Cu,Ni)6Sn5 intermetallic layer of Sn-0.7Cu-0.05Ni/Cu ball grid array (BGA) joints

Ni segregation in the interfacial (Cu,Ni)₆Sn₅ intermetallic layer of Sn-0.7Cu-0.05Ni/Cu BGA solder joints was investigated by using synchrotron micro X-ray fluorescence (XRF) analysis and synchrotron X-ray diffraction (XRD). Compared to Sn-0.7Cu/Cu BGA joints, Ni containing solder show suppressed Cu₃Sn growth in both reflow and annealed conditions. In as-reflowed Sn-0.7Cu-0.05Ni/Cu BGA joints, Ni was relatively homogenously distributed within interfacial (Cu,Ni)₆Sn₅. During subsequent annealing, the diffusion of Ni in Cu₆Sn₅ was limited and it remained concentrated adjacent the Cu substrate where it contributes to the suppression of Cu₃Sn formation at the interface between the Cu substrate and Cu₆Sn₅ intermetallics.     

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。     

Interfacial reactions and mechanical properties between Sn-4.0Ag-0.5Cu and Sn-4.0Ag-0.5Cu-0.05Ni-0.01Ge lead-free solders with the Au/Ni/Cu substrate

Sn-4.0Ag-0.5Cu (SAC) and Sn-4.0Ag-0.5Cu-0.05Ni-0.01Ge (SACNG) lead-free solders reacting with the Au/Ni/Cu multi-layer substrate were investigated in this study. All reaction couples were reflowed at 240 and 255 °C for a few minutes and then aged at 150 °C for 100-500 h. The (Cu, Ni, Au)₆Sn₅ phase was formed by reflowing for 3 min at the interface. If the reflowing time was increased to 10 min, both (Cu, Ni, Au)₆Sn₅ and (Ni, Cu, Au)₃Sn₄ phases formed at the interface. The AuSn₄ phase was found in the solder for all reaction couples. An addition of Ni and Ge to the solder does not significantly affect the IMC formation. After a long period of heat-treatment, the thickness of the (Cu, Ni, Au)₆Sn₅ and (Ni, Cu, Au)₃Sn₄ phases increased and the intermetallic compounds (IMCs) growth mechanism obeyed the parabolic law and the IMC growth mechanism was diffusion-controlled. The mechanical strengths for both the soldered joints decreased with increasing thermal aging time. The SACNG/Au/Ni/Cu couple had better mechanical strength than that in the SAC/Au/Ni/Cu couple.     

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.     

TiO_2纳米颗粒掺杂对Sn-3.0Ag-0.5Cu-xTiO_2焊点界面Cu_6Sn_5 IMC晶粒生长的影响机理

研究了Ti O2纳米颗粒掺杂影响回流焊过程中Sn-3.0Ag-0.5Cu-x Ti O2焊点界面Cu6Sn5金属间化合物(intermetallic compound,IMC)晶粒生长机理.基于Cu原子扩散通量驱动晶粒成熟生长(flux driven ripening,FDR)理论模型分析了Cu6Sn5IMC晶粒生长机理.结果表明,Ti O2纳米颗粒掺杂改变了焊点界面Cu6Sn5IMC晶粒形貌和尺寸.含Ti O2纳米颗粒的焊点Cu6Sn5IMC晶粒尺寸要小于不含Ti O2纳米颗粒的焊点,且晶粒分布要更加均匀.试验数据与FDR理论模型基本吻合.Cu6Sn5IMC晶粒生长指数分别为0.346,0.338,0.332和0.342,这说明Cu6Sn5IMC晶粒生长是由原子互扩散和晶粒成熟共同控制.     

Reliability studies of Sn–9Zn/Cu solder joints with aging treatment

Sn–9Zn (in wt.%) solder ball was bonded to Cu pad, and the effect of aging on shear reliability was investigated. After reflow, the intermetallic compound (IMC) phase formed at the interface was Cu₅Zn₈, and the as-reflowed Sn–9Zn/Cu joint had sufficient shear strength. In the isothermal aging test, only Cu₅Zn₈ IMC was observed in the samples aged at temperatures between 70 and 120 °C. On the other hand, after aging at 150 °C for 250 h, Cu₆Sn₅ phase was observed at the interface between the interfacial Cu₅Zn₈ IMC layer and the Cu substrate. And, the layer-type Cu₅Zn₈ IMC layer was disrupted locally at the interface. In the ball shear test conducted after aging treatment, the shear strength significantly decreased after aging at all temperatures for initial 100 h, and then remained constant by further prolonged aging. The fracture mainly occurred at the interface between the solder and Cu₅Zn₈ IMC layer. The aged Sn–9Zn/Cu solder joint had an inferior joint reliability.     

Effects of number of reflows on the mechanical and electrical properties of BGA package

Mechanical and electrical properties of the Sn–37Pb/Cu joints were investigated in terms of the effect of intermetallic compound (IMC) layer growth. A layer of continuous scallop-shaped Cu₆Sn₅ IMC was formed at the interface between the solder and substrate after 1 reflow, while Cu₃Sn IMC layer was formed after 3 reflows. The thickness of the total IMC layer increased as a function of cubic root of reflow time. The shear force of the solder joints did not vary much with the number of reflows. Only ductile failure mode was observed regardless of the number of reflows, and explaining well the shear force variations. The electrical resistivity of the BGA package was measured to investigate the relation between the microstructural variation and electrical properties of the solder joints. The electrical resistivity increased with the number of reflows.     

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.     

Cu基板退火处理的Cu/Sn58Bi/Cu钎焊接头界面微结构

研究了铜基板退火处理对Cu/Sn58Bi界面微结构的影响. 结果表明,在回流以及时效24 h后Cu/Sn58Bi/Cu界面只观察到Cu₆Sn₅. 随着时效时间的增加,在界面形成了Cu₆Sn₅和Cu₃Sn的双金属间化合物(IMC)层,并且IMC层厚度也随之增加. 长时间时效过程中,在未退火处理的铜基板界面产生了较多铋偏析,而在退火处理的铜基板界面较少产生铋偏析. 比较退火处理以及未退火处理的铜基板与钎料界面IMC层生长速率常数,发现铜基板退火处理能减缓IMC层生长,主要归因于对铜基板进行退火处理能够有效的消除铜基板的内应力与组织缺陷,从而减缓Cu原子的扩散,起到减缓IMC生长的作用.     

Dissolution and precipitation kinetics of Cu6Sn5 intermetallics in Cu/Sn/Cu micro interconnects under temperature gradient

Multiple reflows are often required in 3D packaging. To elucidate the effect of temperature gradient during subsequent reflow on existing intermetallic compounds (IMCs), Cu₆Sn₅ IMC layers were initially formed in Cu/Sn/Cu micro interconnects. Upon subsequent reflow, synchrotron radiation real-time imaging technology was used to in situ study the dissolution and precipitation behavior of the pre-formed Cu₆Sn₅ under different temperature gradients. The pre-formed Cu₆Sn₅ IMC at the cold end continued to grow linearly with increasing aspect ratio, whereas that at the hot end dissolved linearly and then maintained a critical thin layer. The thick pre-formed Cu₆Sn₅ IMC at the hot end significantly hindered the dissolution of the neighboring Cu substrate until a dynamic equilibrium between chemical potential gradient and temperature gradient was satisfied. The thermomigration of Cu atoms from the hot end towards the cold end was responsible for the asymmetrical evolution of the interfacial Cu₆Sn₅ between the cold and hot ends. A theoretical model was proposed based on Cu diffusion flux to calculate the IMC thickness at the both ends as a function of reflow time and the equilibrium IMC thickness at the hot end under temperature gradient.     

First-principles investigation of the structural and electronic properties of Cu6−xNixSn5 (x = 0, 1, 2) intermetallic compounds

Cu_6−xNi_xSn₅ is an important intermetallic compound (IMC), which was known to greatly improve the reliability of the solder joints in integrated circuits. However, the improvement mechanisms and even the crystal structure of the IMC were not fully understood. In this paper, the first-principles calculations were performed to determine the stable structure of Cu_6−xNi_xSn₅ IMC. The structural and electronic properties of Cu_6−xNi_xSn₅ (x = 0, 1, 2) IMCs have been calculated. The results show that Ni atoms preferentially occupied 8f sites (Cu2) and formed Cu₄Ni₂Sn₅. The results of energy calculation and density of states demonstrate that this Cu₄Ni₂Sn₅ IMC had a more stable structure than Cu₆Sn₅. These results are also expected to account for the improvement in the reliability of the solder joint.     

Effect of bismuth on the growth kinetics of intermetallic compounds in Sn-3.5Ag solder joints: A growth kinetic model

The effect on the growth kinetics of the intermetallic compounds (IMCs) in solder/Cu joints, caused by adding Bi to eutectic Sn-3.5Ag solder alloy, was examined at the aging temperatures of 150°C and 180°C. The Cu₆Sn₅ layer growth was significantly enhanced, but the Cu₃Sn layer growth was slightly retarded by the addition of Bi, resulting in significant growth enhancement of the total (Cu₆Sn₅+Cu₃Sn) IMC layer with increasing Bi addition. The IMC layer growth in the Bi-containing solder joints was accompanied by the accumulation of Bi ahead of the Cu₆Sn₅ layer that resulted in the formation of a liquid layer at the Cu₆Sn₅/solder interface. A kinetic model was developed for the planar growth of the Cu₆Sn₅ and Cu₃Sn layers in the solder joints, accounting for the existence of interfacial reaction barriers. Predictions from the kinetic model showed that the experimental results could be well explained by the hypothesis that the formation of a Bi-rich liquid layer at the Cu₆Sn₅/solder interface reduces the interfacial reaction barrier at the interface.     

Decomposition of Cu6Sn5 particles in solder for the growth of a ternary (Cu1−xNix)6Sn5 layer on a Ni substrate

The interaction between Cu₆Sn₅ particles in the bulk of a solder and a Ni substrate was examined during solid-state aging using Cu/Sn/Ni and Cu/Sn/Cu/Sn/Ni diffusion couples with initially thin Cu layers. The results clearly demonstrated that the (Cu,Ni)₆Sn₅ particles dispersed in the bulk solder decomposed in order for a ternary (Cu_1−xNi_x)₆Sn₅ layer to grow at the solder/Ni interface during solid-state aging. The interaction between the (Cu,Ni)₆Sn₅ particles and the (Cu_1−xNi_x)₆Sn₅ layer occurs owing to the driving force for the (Cu,Ni)₆Sn₅ compound to become saturated with Ni. A (Ni,Cu)₃Sn₄ layer forms at the (Cu_1−xNi_x)₆Sn₅/Ni interface only after the Ni composition of the (Cu,Ni)₆Sn₅ phase in the bulk solder approaches that of the (Cu_1−xNi_x)₆Sn₅ layer. Once the (Ni,Cu)₃Sn₄ layer has formed, it grows at an exceptionally rapid rate by consuming the (Cu_1−xNi_x)₆Sn₅ and Sn layers, which can be problematic in solder joint reliability.     

Sequential interfacial intermetallic compound formation of Cu6Sn5 and Ni3Sn4 between Sn-Ag-Cu solder and ENEPIG substrate during a reflow process

The interfacial reactions between Sn-3.0 wt.% Ag-0.5 wt.% Cu solder and an electroless nickel-electroless palladium-immersion gold (ENEPIG) substrate were investigated. After initial reflowing, discontinuous polygonal-shape (Cu,Ni)₆Sn₅ intermetallic compounds (IMCs) formed at the interface. During reflowing for up to 60 min, the interfacial IMCs were sequentially changed in the following order: discontinuous (Cu,Ni)₆Sn₅, (Cu,Ni)₆Sn₅ and (Ni,Cu)₃Sn₄, and embedded (Cu,Ni)₆Sn₅ in (Ni,Cu)₃Sn₄. The interfacial product variation resulted from the preferential consumption of Cu atoms within the solder and continuous Ni diffusion from the Ni(P) layer.     

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.     

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.     

Kinetics of intermetallic phase formation at the interface of Sn-Ag-Cu-X (X = Bi, In) solders with Cu substrate

The effects of Bi and In additions on intermetallic phase formation in lead-free solder joints of Sn-3.7Ag-0.7Cu; Sn-1.0Ag-0.5Cu-1.0Bi and Sn-1.5Ag-0.7Cu-9.5In (composition given in weight %) with copper substrate are studied. Soldering of copper plate was conducted at 250 °C for 5 s. The joints were subsequently aged at temperatures of 130-170 °C for 2-16 days in a convection oven. The aged interfaces were analyzed by optical microscopy and energy dispersive X-ray spectroscopy (EDX) microanalysis. Two intermetallic layers are observed at the interface - Cu₃Sn and Cu₆Sn₅. Cu₆Sn₅ is formed during soldering. Cu₃Sn is formed during solid state ageing. Bi and In decrease the growth rate of Cu₃Sn since they appear to inhibit tin diffusion through the grain boundaries. Furthermore, indium was found to produce a new phase - Cu₆(Sn,In)₅ instead of Cu₆Sn₅, with a higher rate constant. The mechanism of the Cu₆(Sn,In)₅ layer growth is discussed and the conclusions for the optimal solder chemical composition are presented.