Reactions of Sn-3.5Ag-Based Solders Containing Zn and Al Additions on Cu and Ni(P) Substrates

In this study we consider the effect of separately adding 0.5 wt.% to 1.5 wt.% Zn or 0.5 wt.% to 2 wt.% Al to the eutectic Sn-3.5Ag lead-free solder alloy to limit intermetallic compound (IMC) growth between a limited volume of solder and the contact metallization. The resultant solder joint microstructure after reflow and high-temperature storage at 150°C for up to 1000 h was investigated. Experimental results confirmed that the addition of 1.0 wt.% to 1.5 wt.% Zn leads to the formation of Cu-Zn on the Cu substrate, followed by massive spalling of the Cu-Zn IMC from the Cu substrate. Growth of the Cu₆Sn₅ IMC layer is significantly suppressed. The addition of 0.5 wt.% Zn does not result in the formation of a Cu-Zn layer. On Ni substrates, the Zn segregates to the Ni₃Sn₄ IMC layer and suppresses its growth. The addition of Al to Sn-3.5Ag solder results in the formation of Al-Cu IMC particles in the solder matrix when reflowed on the Cu substrate, while on Ni substrates Al-Ni IMCs spall into the solder matrix. The formation of a continuous barrier layer in the presence of Al and Zn, as reported when using solder baths, is not observed because of the limited solder volumes used, which are more typical of reflow soldering.     

微量Ni对Sn-3.0Ag-0.5Cu钎料及焊点界面的影响

研究了Ni的含量对无铅钎料Sn-3.0Ag-0.5Cu润湿性、熔点、重熔及老化条件下界面化合物(IMC)的影响。结果表明:微量Ni的加入使SnAgCu润湿力增加6%;使合金熔点略升高约3℃;重熔时在界面形成了(Cu,Ni)6Sn5IMC层,且IMC厚度远高于SnAgCu/Cu的Cu6Sn5IMC厚度。在150℃老化过程中,SnAgCuNi/Cu重熔焊点IMC随着时间的增加,其增幅小于SnAgCu/Cu的增幅,此时Ni对IMC的增长有一定抑制作用。     

Depressing effect of 0.2wt.%Zn addition into Sn-3.0Ag-0.5Cu solder alloy on the intermetallic growth with Cu substrate during isothermal aging

The formation and growth of intermetallic compounds (IMCs) in lead-free solder joints, during soldering or subsequent aging, have a significant effect on the thermal and mechanical behavior of solder joints. In this study, the effects of a 0.2wt.%Zn addition into Sn-3.0Ag-0.5Cu (SAC) lead-free solder alloys on the growth of IMCs with Cu substrates during soldering and subsequent isothermal aging were investigated. During soldering, it was found that a 0.2wt.%Zn addition did not contribute to forming the IMC, which was verified as the same phase structure as the IMC for Sn-3.0Ag-0.5Cu/Cu. However, during solid-state isothermal aging, the IMC growth was remarkably depressed by the 0.2 wt.% Zn addition in the SAC solder matrix, and this effect tended to be more prominent at higher aging temperature. The activation energy for the overall IMC growth was determined as 61.460 and 106.903 kJ/mol for Sn-Ag-Cu/Cu and Sn-Ag-Cu-0.2Zn/Cu, respectively. The reduced diffusion coefficient was confirmed for the 0.2Zn-containing solder/Cu system. Also, thermodynamic analysis showed the reduced driving force for the Cu₆Sn₅ IMC with the addition of Zn. These may provide the evidence to demonstrate the depressing effect of IMC growth due to the 0.2wt.%Zn addition in the Sn-Ag-Cu solder matrix.     

Comparison of Sn-Ag-Cu Solder Alloy Intermetallic Compound Growth Under Different Thermal Excursions for Fine-Pitch Flip-Chip Assemblies

The intermetallic compound (IMC) evolution in Cu pad/Sn-Ag-Cu solder interface and Sn-Ag-Cu solder/Ni pad interface was investigated using thermal shock experiments with 100-μm-pitch flip-chip assemblies. The experiments show that low standoff height of solder joints and high thermomechanical stress play a great role in the interfacial IMC microstructure evolution under thermal shock, and strong cross-reaction of pad metallurgies is evident in the intermetallic growth. Furthermore, by comparing the IMC growth during thermal aging and thermal shock, it was found that thermal shock accelerates IMC growth and that kinetic models based on thermal aging experiments underpredict IMC growth in thermal shock experiments. Therefore, new diffusion kinetic parameters were determined for the growth of (Cu,Ni)₆Sn₅ using thermal shock experiments, and the Cu diffusion coefficient through the IMC layer was calculated to be 0.2028 μm²/h under thermal shock. Finite-element models also show that the solder stresses are higher under thermal shock, which could explain why the IMC growth is faster and greater under thermal shock cycling as opposed to thermal aging.     

Compound formation for electroplated Ni and electroless Ni in the under-bump metallurgy with Sn-58Bi solder during aging

The Ni-based under-bump metallurgies (UBMs) are of interest because they have a slower reaction rate with Sn-rich solders compared to Cu-based UBMs. In this study, several UBM schemes using Ni as the diffusion barrier are investigated. Joints of Sn-58Bi/Au/electroless nickel (EN)/Cu/Al₂O₃ and Sn-58Bi/Au/electroplated nickel/Cu/Al₂O₃ were aged at 110°C and 130°C for 1–25 days to study the interfacial reaction and microstructural evolution. The Sn-Bi solder reacts with the Ni-based multimetallization and forms the ternary Sn-Ni-Bi intermetallic compound (IMC) during aging at 110°C. Compositions of ternary IMC were (78–80)at.%Sn-(12–16)at.%Ni-(5–8)at.%Bi in joints of Sn-58Bi/Au/Ni-5.5wt.%P/Cu, Sn-58Bi/Au/Ni-12wt.%P/Cu, and Sn-58Bi/Au/Ni/Cu. Elevated aging at 130°C accelerates the IMC growth rate and results in the formation of (Ni,Cu)₃Sn₄ and (Cu,Ni)₆Sn₅ adjacent to the ternary Sn-Ni-Bi IMC for the Sn-58Bi/Au/Ni-12wt.%P/Cu and Sn-58Bi/Au/Ni/Cu joints, respectively. The Cu content in the (Cu,Ni)₆Sn₅ IMC is six times that in (Ni,Cu)₃Sn₄. Electroplated Ni fails to prevent Cu diffusion toward the Ni/solder interface as compared to EN-based joints. Cracks are observed in the Sn-58Bi/Au/Ni-5.5wt.%P/Cu/Al₂O₃ joint aged at 130°C for 25 days. It is more favorable to employ Ni-12wt.%P for the Sn-58Bi/Au/EN/Cu joint. Electroless nickel, with the higher P content of 12 wt.%, is a more effective diffusion barrier during aging. In addition, P enrichment occurs near the interface of the EN/solder, and the degree of P enrichment is enhanced with aging time. The Au(Sn,Bi)₄, with pyramidal and cubic shape, is observed in the Sn-58Bi/Au/Ni/Cu/Al₂O₃ joint.     

Optimal phosphorous content selection for the soldering reaction of Ni-P under bump metallization with Sn-Ag-Cu solder

Nickel plating has been used as the under bump metallization (UBM) in the microelectronics industry. The electroplated Ni-P UBM with different phosphorous contents (7 wt.%, 10 wt.%, and 13 wt.%) was used to evaluate the interfacial reaction between Ni-P UBM and Sn-3Ag-0.5Cu solder paste during multiple reflow. (Cu,Ni)₆Sn₅ intermetallic compounds (IMC) formed in the SnAgCu solder/Ni-P UBM interface after the first reflow. For three times reflow, (Ni,Cu)₃Sn₄ IMC formed, while (Cu,Ni)₆Sn₅ IMC spalled into the solder matrix. With further increasing cycles of reflow, the Ni-Sn-P layer formed between (Ni,Cu)₃Sn₄ IMC and Ni-P UBM for Ni-10wt.%P and Ni-13wt.%P UBM. However, almost no Ni-Sn-P layer was revealed for the Ni-7wt.%P UBM even after ten cycles of reflow. In consideration of the wettability of Ni-P UBM, the interfacial reaction of SnAgCu/Ni-P, and dissolution of Ni-P UBM, the optimal phosphorous selection in Ni-P UBM was proposed and also discussed.     

Effect of substrate metallization on interfacial reactions and reliability of Sn-Zn-Bi solder joints

The scope of this paper covers a comprehensive study of the lead-free Sn-Zn-Bi solder system, on Cu, electrolytic Ni/Au and electroless Ni(P)/Au surface finishes. This includes a study of the shear properties, intermetallic compounds at the substrate-ball interface and dissolution of the under bump metallization. The Sn-8Zn-3Bi (wt.%) solder/Cu system exhibited a low shear load with thick IMCs formation at the interface. The dissolution of the Cu layer in the Sn-Zn-3Bi solder is higher than that of the other two Ni metallizations. It was found that the formation of a thick Ni-Zn intermetallic compound (IMC) layer at the solder interface of the electrolytic Ni bond pad reduced the mechanical strength of the joints during high temperature long time liquid state annealing. The solder ball shear-load for the Ni(P) system during extended reflow increased with an increase of reflow time. No spalling was noticed at the interface of the Sn-Zn-3Bi solder/Ni(P) system. Sn-8Zn-3Bi solder with electroless Ni(P) metallization appeared as a good combination in soldering technology.     

Thermal Cycling Aging Effect on the Reliability and Morphological Evolution of SnAgCu Solder Joints

In solder ball grid array (BGA) technology, solder joint reliability is one of the critical issues in microelectronics manufacturing industries. In this reliability aging study, Sn3.5AgO.7Cu solder joints were subjected to accelerated temperature cycling (ATC) test in TBGA assembly. Fatigue fracture occurred, very close to the solder/intermetallic compound (IMC) interface, at the TBGA component side due to the larger coefficient of thermal expansion (CTE) mismatch compared to the PCB side. During reflow, needle-type and scallop-type morphologies of (Cu,Ni)₆Sn₅ IMCs were formed at the TBGA component and PCB interfaces. In the process of thermal cycling, a layer of (Ni,Cu)₃Sn₄ IMC grew beneath the (Cu,Ni)₆Sn₅ IMC due to the out diffusion of Ni from the under bump metallization (UBM). After extended thermal cycling aging, Ni-Sn-P IMC was found between the (Ni,Cu)₃Sn₄ IMC and the In₃P layer at the printed circuit board (PCB) interface. Grain ripening and spalling of (Cu,Ni)₆Sn₅ IMC grains into the solder joint was also observed in the process of thermal cycling. The spalling phenomena of (Cu,Ni)₆Sn₅ IMCs was caused by interface structure change and cyclic shear stresses and strains incurred during temperature cycling.     

Effect of reaction time on mechanical strength of the interface formed between the Sn-Zn(-Bi) solder and the Au/Ni/Cu bond pad

In this work, the shear strengths and the interfacial reactions of Sn-9Zn, Sn-8Zn-1Bi, and Sn-8Zn-3Bi (wt.%) solders with Au/Ni/Cu ball grid array (BGA) pad metallization were systematically investigated after extended reflows. Zn-containing Pb-free solder alloys were kept in molten condition (240°C) on the Au/electrolytic Ni/Cu bond pads for different time periods ranging from 1 min. to 60 min. to render the ultimate interfacial reaction and to observe the consecutive shear strength. After the shear test, fracture surfaces were investigated by scanning electron microscopy equipped with an energy dispersive x-ray spectrometer. Cross-sectional studies of the interfaces were also conducted to correlate with the fracture surfaces. The solder ball shear load for all the solders during extended reflow increased with the increase of reflow time up to a certain stage and then decreased. It was found that the formation of thick Ni-Zn intermetallic compound (IMC) layers at the solder interface of the Au/electrolytic Ni/Cu bond pad with Sn-Zn(-Bi) alloys deteriorated the mechanical strength of the joints. It was also noticed that the Ni-Zn IMC layer was larger in the Sn-Zn solder system than that in the other two Bi-containing solder systems.     

Interfacial reaction between Sn-0.7Cu (-Ni) solder and Cu substrate

The interfacial reaction between Sn-0.7mass%Cu-(Ni) solders and a Cu substrate was investigated to reveal the effect of the addition of Ni to Sn-Cu solder on the formation of intermetallic compounds (IMCs). Sn-0.7Cu-xNi solders (x=0, 0.05, 0.1, 0.2 mass%) were prepared. For the reflow process, specimens were heated in a radiation furnace at 523 K for 60 sec, 300 sec, and 720 sec to estimate the interfacial reaction between the molten solder and Cu substrate. Then, for the aging process, some specimens were heat-treated in an oil bath at 423 K for 168 h and 504 h. The cross sections of soldered specimens were observed to measure the dissolution thickness of the Cu substrate and the thickness of the IMC and to investigate the microstructures of IMC. The results showed that, just after the reflow process, the dissolution thickness of the Cu substrate increased with the increase of Ni content in the Sn-0.7Cu-xNi solder and the thickness of the IMC between the solder and Cu substrate was the minimum in the Sn-0.7Cu-0.05Ni solder. After the aging process, the IMC grew with the increase of aging time. In the case of 0.05% Ni, the IMC thickness was the thinnest regardless of aging time. It is clear that 0.05% Ni addition to Sn-0.7Cu solder very effectively inhibits the formation and growth of the IMC between solder and Cu substrate. Electron probe microanalysis of the IMC showed that the IMC layer in the Sn-0.7Cu-Ni solder contained Ni, and the IMC was expressed as (Cu_1−y ,Ni _y )₆Sn₅.     

Suppressing Sn-Patch Growth in Ti/Ni(V)/Cu Under Bump Metallization with Sn-Ag-Cu Solder After Reflow and Aging

Sputtered Ti/Ni(V)/Cu under bump metallization (UBM) is widely used in flip chip technology because the metals are nonmagnetic and the consumption of the Ni(V) layer is low. It is noted that V does not react with solders and intermetallic compounds (IMC) during reflow and aging; however, a Sn-patch forms in the Ni(V) layer, and the Sn-patch growth may cause the IMCs to detach from the interface of solder joints. In this study, Sn-3.0Ag-0.5Cu solder was reflowed on Ti/Ni(V)/Cu UBM with different Cu thicknesses at 250°C for 60 s, and then aged at 150°C for various periods of time. It was revealed that the Sn-patch growth could be controlled by increasing the Cu thickness in the Ti/Ni(V)/Cu UBM. A feasible approach to suppress Sn-patch formation after reflow and aging is discussed.     

The influence of solder volume and pad area on Sn–3.8Ag–0.7Cu and Ni UBM reaction in reflow soldering and isothermal aging

This paper examines various aspects of SAC (Sn–3.8Ag–0.7Cu wt.%) solder and UBM interactions which may impact interconnection reliability as it scales down. With different solder joint sizes, the dissolution rate of UBM and IMC growth kinetics will be different. Solder bumps on 250, 80 and 40 μm diameter UBM pads were investigated. The effect of solder volume/pad metallization area (V/A) ratio on IMC growth and Ni dissolution was investigated during reflow soldering and solid state isothermal aging. Higher V/A ratio produced thinner and more fragmented IMC morphology in SAC solder/Ni UBM reflow soldering interfacial reaction. Lower V/A ratio produced better defined IMC layer at the Ni UBM interface. When the ratio of V/A is constant, the IMC morphology and growth trend was found to be similar. After 250 h of isothermal aging, the IMC growth rate of the different bump sizes leveled off. No degradation in shear strength was observed in these solder bump after 500 h of isothermal aging.     

Reaction characteristics of the In-15Pb-5Ag solder with a Au/Ni/Cu pad and their effects on mechanical properties

Reaction characteristics of the In-15Pb-5Ag (wt.%) solder with a Au/Ni/Cu pad during reflow soldering and aging treatment were examined. Interfacial reaction during reflow resulted in either an AuIn₂ or Ni₂₈In₇₂ layer, depending on reflow time. The AuIn₂ layer became thinner and disappeared from the interface, and only the Ni₂₈In₇₂ layer grew with the progress of aging treatment at 130°C. Based on those observations, the dissolution rate of the Au top layer was estimated, and the behavior of the AuIn₂ layer during reflow and aging treatment was discussed. In addition, peak shear load and fracture energy of the solder bump were measured as a function of reflow time and aging treatment. The results were compared with those measured with the Sn-37Pb solder bump.     

Effect of aging on mechanical properties of high temperature Pb-rich solder joints

The effect of aging on the evolution of interfacial microstructure and mechanical properties of Pb-rich PbSnAg solder joints with different Sn content was investigated. Tensile samples were prepared by soldering two pieces of Ni or Cu strips resulting in joints with gap sizes of about 250 μm. Multi-layered structures composed of Ni coated Si-chips soldered onto Ni/Cu metallized ceramic substrates were used for fatigue testing. All samples were subjected to thermal aging at 250 °C up to 500 h. Microstructural investigations revealed that independent from the substrate material, increased Sn content of the solder alloy results in improvement of the tensile and fatigue properties of the joints and a higher growth rate of the interfacial intermetallic compound (IMC) layers. It was found that the thickness of the wettable substrate especially in the case of low-Sn alloys, also affects the interfacial properties of high temperature PbSnAg solder joints. In this case, a reduced Sn content in solder joints results in weakening of the interface during the reflow process and subsequent thermal aging. The dominant failure mode of the solder joints subjected to cyclic loading was delamination of the interfacial IMC layer from the substrate. Finite element simulations were conducted by using a strain rate and pressure dependent material model for PbSnAg solder in order to analyse the states of stress and strain during static and cyclic loading.     

Effect of Bi on the Interfacial Reaction between Sn-3.7Ag-xBi Solders and Cu

It was reported in previous studies that the addition of Bi could improve the wettability and reduce the melting temperature of Sn-Ag solders. This work investigates the effect of Bi on the interfacial reaction between Sn-Ag-xBi solders and the Cu substrate reflowed at 250°C for different times and thermally aged at 150°C for different durations. Five types of Sn-Ag-based solders, Sn-3.7Ag-xBi (x = 0 wt.% to 4 wt.%), were used in this study. The microstructure of the interfacial Cu-Sn intermetallic compound (IMC) layers between the solders and the Cu substrate was studied, and the thickness of the Cu-Sn IMCs in different solder/Cu systems has been measured. It was found that the thickness of the Cu-Sn IMC layer decreased with increasing amount of Bi in both the reflow and thermally aged condition. The effect of Bi addition on the interfacial reaction between the solder and the Cu substrate was discussed based on the experimental results.     

Effects of microstructural evolution and intermetallic layer growth on shear strength of ball-grid-array Sn-Cu solder joints

The shear strength of ball-grid-array (BGA) solder joints on Cu bond pads was studied for Sn-Cu solder containing 0, 1.5, and 2.5 wt.% Cu, focusing on the effect of the microstructural changes of the bulk solder and the growth of intermetallic (IMC) layers during soldering at 270°C and aging at 150°C. The Cu additions in Sn solder enhanced both the IMC layer growth and the solder/IMC interface roughness during soldering but had insignificant effects during aging. Rapid Cu dissolution from the pad during reflow soldering resulted in a fine dispersion of Cu₆Sn₅ particles throughout the bulk solder in as-soldered joints even for the case of pure Sn solder, giving rise to a precipitation hardening of the bulk solder. The increased strength of the bulk solder caused the fracture mode of as-soldered joints to shift from the bulk solder to the solder/IMC layer as the IMC layer grew over a critical thickness about 1.2 m for all solders. The bulk solder strength decreased rapidly as the fine Cu₆Sn₅ precipitates coarsened during aging. As a consequence, regardless of the IMC layer thickness and the Cu content of the solders, the shear strength of BGA solder joints degraded significantly after 1 day of aging at 150°C and the shear fracture of aged joints occurred in the bulk solder. This suggests that small additions of Cu in Sn-based solders have an insignificant effect on the shear strength of BGA solderjoints, especially during system use at high temperatures.     

Influence of Cu content on compound formation near the chip side for the flip-chip Sn-3.0Ag-(0.5 or 1.5)Cu solder bump during aging

Sn-Ag-Cu solder is a promising candidate to replace conventional Sn-Pb solder. Interfacial reactions for the flip-chip Sn-3.0Ag-(0.5 or 1.5)Cu solder joints were investigated after aging at 150°C. The under bump metallization (UBM) for the Sn-3.0Ag-(0.5 or 1.5)Cu solders on the chip side was an Al/Ni(V)/Cu thin film, while the bond pad for the Sn-3.0Ag-0.5Cu solder on the plastic substrate side was Cu/electroless Ni/immersion Au. In the Sn-3.0Ag-0.5Cu joint, the Cu layer at the chip side dissolved completely into the solder, and the Ni(V) layer dissolved and reacted with the solder to form a (Cu_1−y,Ni_y)₆Sn₅ intermetallic compound (IMC). For the Sn-3.0Ag-1.5Cu joint, only a portion of the Cu layer dissolved, and the remaining Cu layer reacted with solder to form Cu₆Sn₅ IMC. The Ni in Ni(V) layer was incorporated into the Cu₆Sn₅ IMC through slow solid-state diffusion, with most of the Ni(V) layer preserved. At the plastic substrate side, three interfacial products, (Cu_1−y,Ni_y)₆Sn₅, (Ni_1−x,Cu_x)₃Sn₄, and a P-rich layer, were observed between the solder and the EN layer in both Sn-Ag-Cu joints. The interfacial reaction near the chip side could be related to the Cu concentration in the solder joint. In addition, evolution of the diffusion path near the chip side in Sn-Ag-Cu joints during aging is also discussed herein.     

Isothermal and thermal cycling aging on IMC growth rate in lead-free and lead-based solder interface

The growth of interfacial intermetallic compounds (IMC) between Pb-free and Pb-based solders with different surface finish (Cu and Ni/Au) metallization is a major concern for long-term solder joint reliability performance in electronic assemblies. The growth rate of the IMC layer can affect the solder joint reliability. Analysis of solid-state diffusion mechanism for the growth of IMC between solder-to-substrate interface for Pb-free and Pb-based solders subject to isothermal and thermal cycling aging were conducted. Experimental study of IMC layer growth between Sn3.8Ag0.7Cu and Ni/Au surface finish by isothermal aging versus thermal cycling (TC) aging was investigated to develop a framework for correlating IMC layer growth behavior. An integrated model for IMC growth was derived to describe the Ni-Cu-Sn IMC growth behavior subject to TC aging. Comparison of modeling and test results showed that IMC layer growth rate under TC aging was accelerated. It is noted that IMC layer growth study from various references showed different experimental data and growth kinetic parameters for both liquid-state and solid-state reactions.     

Effect of reflow and thermal aging on the microstructure and microhardness of Sn-3.7Ag-xBi solder alloys

This work investigates the effect of reflow and the thermal aging process on the microstructural evolution and microhardness of five types of Sn-Ag based lead-free solder alloys: Sn-3.7Ag, Sn-3.7Ag-1Bi, Sn-3.7Ag-2Bi, Sn-3.7Ag-3Bi, and Sn-3.7Ag-4Bi. The microhardness and microstructure of the solders for different cooling rates after reflow at 250°C and different thermal aging durations at 150°C for air-cooled samples have been studied. The effect of Bi is discussed based on the experimental results. It was found that the microhardness increases with increasing Bi addition to Sn-3.7Ag solder regardless of reflow or thermal aging process. Scanning electron microscopy images show the formation of Ag₃Sn particles, Sn-rich phases, and precipitation of Bi-rich phases in different solders. The increase of microhardness with Bi addition is due to the solution strengthening and precipitation strengthening provided by Bi in the solder. The trend of decrease in microhardness with increasing duration of thermal aging was observed.     

Interfacial reactions and shear strengths between Sn-Ag-based Pb-free solder balls and Au/EN/Cu metallization

The morphological and compositional evolutions of intermetallic compounds (IMCs) formed at three Pb-free solder/electroless Ni-P interface were investigated with respect to the solder compositions and reflow times. The three Pb-free solder alloys were Sn3.5Ag, Sn3.5Ag0.75Cu, and Sn3Ag6Bi2In (in wt.%). After reflow reaction, three distinctive layers, Ni₃Sn₄ (or Ni-Cu-Sn for Sn3.5Ag0.75Cu solder), NiSnP, and Ni₃P, were formed on the electroless Ni-P layer in all the solder alloys. For the Sn3.5Ag0.75Cu solder, with increasing reflow time, the interfacial intermetallics switched from (Cu,Ni)₆Sn₅ to (Cu,Ni)₆Sn₅+(Ni,Cu)₃Sn₄, and then to (Ni,Cu)₃Sn₄ IMCs. The degree of IMC spalling for the Sn3.5Ag0.75Cu solder joint was more than that of other solders. In the cases of the Sn3.5Ag and Sn3Ag6Bi2In solder joints, the growth rate of the Ni₃P layer was similar because these two type solder joints had a similar interfacial reaction. On the other hand, for the Sn3.5Ag0.75Cu solder, the thickness of the Ni₃P and Ni-Sn-P layers depended on the degree of IMC spalling. Also, the shear strength showed various characteristics depending on the solder alloys and reflow times. The fractures mainly occurred at the interfaces of Ni₃Sn₄/Ni-Sn-P and solder/Ni₃Sn₄.