Degradation of Sn37Pb and Sn3.5Ag0.5Cu solder joints between Au/Ni (P)/Cu pads stressed with moderate current density

Sn37Pb (SP) and Sn3.5Ag0.5Cu (SAC) ball grid array (BGA) solder joints between Au/Ni (P)/Cu pads were stressed with a moderate current density of 6.0 × 10² A/cm² at an ambient temperature of 125°C up to 600 h. The solder joint reliability was evaluated in terms of temperature measurement, microstructural analysis and mechanical strength test. It was confirmed that no obvious electromigration occurred with this moderate current density. However, the local temperature of solder joints rose considerably due to massive Joule heating, which degraded the solder joint reliability seriously. Phase coarsening was observed for both solders and it was particularly apparent in the SP solder joints. Compared to the SP, the SAC was found to be more reactive and hence a thicker intermetallic compound (IMC) was developed during the current stressing. Nevertheless, the IMC thickening was not as remarkable as expected with current stressing at high temperature. It exhibited a sub-parabolic growth manner that was mainly controlled by grain boundary diffusion. However, a sufficiently thick IMC layer initially formed during reflow soldering and the low diffusivity of the Ni atoms retarded the growth. The shear strength of the solder joints was found to decrease severely with the current stressing time. This degradation was attributed to the large stresses arising from localized thermal mismatch, phase coarsening, volume shrinkage of IMC evolution, Ni–P layer crystallization and the pad cracking during current stressing.     

Correlation of intermetallic compound growth behavior and melt state of Sn–3.5Ag–3.5Bi/Cu joint during soldering and isothermal aging

The intermetallic compound (IMC) growth behavior of Sn–3.5Ag–3.5Bi/Cu joint was investigated with a change in the solder melt structure during soldering and 180 °C isothermal aging. The results show that when the solders undergo liqiud–liquid structure transition (LLST), the IMC of the joint is thinner and more evenly distributed during soldering. The interface IMC is also thinner, and the quantity of Ag₃Sn as well as Cu₆Sn₅ in the solder is relatively lower. However, the IMCs are more bulky after long-time aging at 180 °C. When the solders do not undergo LLST, microcracks form in the solder. Kirkendall voids are more abundant and interconnected after long, high-temperature aging. This finding indicates better joint reliability after than before LLST. The growth rate constants of the interface IMC for the two kinds of joints are calculated to be 1.94 × 10^?12 and 9.71 × 10^?13. The correlation of IMC growth behavior and melt state is analyzed from the viewpoints of LLST and atom diffusion.     

Thermomigration and electromigration in Sn8Zn3Bi solder joints

Individual effect of thermomigration (TM) and combined effect of TM and electromigration (EM) on the microstructural variation in Sn8Zn3Bi was investigated by stressing line-type Au/Ni–P/Cu-Sn8Zn3Bi-Au/Ni–P/Ni solder joints with a 5 × 10³ A/cm² alternating current (AC) or direct current (DC) at 110°C. Due to the different thermoelectric characteristics of Cu and Ni wires, a thermal gradient of 196°C/cm could be established across the solder joints according to the finite element simulation. In AC current stressing, there is no EM effect and only TM dominates the migration. Microstructural study shows that Zn atoms migrate towards the lower temperature side during TM. In DC current stressing, it is found that both EM and TM play important roles depending various experimental conditions. And the energy change during the EM and the TM is estimated to be ∆ω_em 3.2 × 10⁻²⁸ Joule and ∆ω_em 2.2 × 10⁻²⁸ Joule, respectively. Upon different current directions in DC current stressing, there is a counteractive or accelerated effect between TM and EM on Zn migration, resulting different microstructures at the cathode side in the solder joints.     

Interfacial evolution in Sn–58Bi solder joints during liquid electromigration

For Sn–58Bi low temperature solder alloy, local molten induced from electromigration Joule heating might change the atomic diffusion and interfacial behavior. In this paper, the diffusion behavior and interfacial evolution of Cu/Sn–58Bi/Cu joints were studied under liquid–solid (L–S) electromigration in molten solder and were compared with the interfacial behaviors in solid–solid (S–S) electromigration in solid solder. L–S or S–S electromigration was realized by applying a current density of 1.0?×?10⁴ A/cm² to molten solder at 150 °C or solid solder at 25 °C, respectively. During S–S electromigration, Bi atoms were driven towards anode side under electromigration induced flux and then accumulated to form Bi-rich layer near anode interface with current stressing time increasing. During L–S electromigration, Bi atoms were reversely migrated from anode to cathode to produce Bi segregation at cathode interface, while Cu atoms were rapidly dissolved into molten solder from cathode and migrated to form large amounts of Cu₆Sn₅ rod-like phases near anode interface. The reversal in the direction of Bi atoms may be attributed to the reversal in the direction of electromigration induced flux and correspondingly the change on effective charge number of Bi atoms from negative to positive.     

Effect of Cu diffusion through Ni on the interfacial reactions of Sn3.5Ag0.75Cu and SnPb solders with Au/Ni/Cu substrate during aging

The effect of Cu diffusion through Ni on the interfacial reactions of Sn3.5Ag0.75Cu (SAC) and SnPb solders with Au/Ni/Cu substrate during aging was investigated in this study. AuSn₄ needle-like intermetallic compound (IMC) formed at the interfaces of both SAC and SnPb solder joints after laser reflow. In SnPb solder joints, a layer-type ternary IMC, (Au,Ni)Sn₄, formed over a thin layer quaternary IMC (Au,Ni,Cu)₃Sn₄ after aging at 150 °C for 4 days, in which Cu came from the Cu pad below the Ni layer through diffusion. This (Au,Ni,Cu)₃Sn₄ IMC evolved into a new kind of quaternary IMC (Au,Ni,Cu)₆Sn₅ with further aging at 150 °C for 8 days because more Cu diffused through Ni and was involved into the IMC formation. For SAC solder joints, needle-like binary AuSn₄ IMC in as-soldered state changed into a layer-type quaternary IMC (Au,Ni,Cu)Sn₄. The quaternary IMC (Au,Ni,Cu)₆Sn₅, which has the same composition as those found in SnPb solder joints, was developed below the (Au,Ni,Cu)Sn₄ layer after aging at 150 °C for 8 days. It was found that Cu diffusion played a key role in the interfacial reaction and compound formation.     

Effect of thermal cycles on interface and mechanical property of low-Ag Sn1.0Ag0.5Cu(nano-Al)/Cu solder joints

Low-Ag content SnAgCu solder has drawn more and more researchers’ attention due to the low cost. In this paper, the effect of 0.1 wt% nano-Al particles on interface reaction between Sn1.0Ag0.5Cu and Cu substrate was investigated, and the growth of intermetallic compounds (IMC) and mechanical property of solder joints during ??55 to 125 °C thermal cycling were also analyzed. The results show that the Cu₆Sn₅ IMC formed at the as-soldered interface and grow obviously with the increase of thermal cycling. The growth rate of IMC in the SnAgCu–0.1Al/Cu is lower than that of SnAgCu/Cu, which indicates that the nano-Al particles can inhibit the diffusion coefficient of IMC layers. Moreover, the shear force of two kinds of solder joints decrease during thermal cycling, but the shear force of SnAgCu–0.1Al is higher than that of SnAgCu.     

Influence of cerium oxide (CeO2) nanoparticles on the microstructure and hardness of tin–silver–copper (Sn–Ag–Cu) solders on silver (Ag) surface-finished copper (Cu) substrates

Nano-sized, non-reacting, non-coarsening CeO₂ particles with a density close to that of solder alloy were incorporated into Sn–3.0 wt%Ag–0.5 wt%Cu solder paste. The interfacial microstructure and hardness of Ag surface-finished Cu substrates were investigated, as a function of reaction time, at various temperatures. After the initial reaction, an island-shaped Cu₆Sn₅ intermetallic compound (IMC) layer was clearly observed at the interfaces of the Sn–Ag–Cu based solders/immersion Ag plated Cu substrates. However, after a prolonged reaction, a very thin, firmly adhering Cu₃Sn IMC layer was observed between the Cu₆Sn₅ IMC layer and the substrates. Rod-like Ag₃Sn IMC particles were also clearly observed at the interfaces. At the interfaces of the Sn–Ag–Cu based solder-Ag/Ni metallized Cu substrates, a (Cu, Ni)–Sn IMC layer was found. Rod-like Ag₃Sn and needle-shaped Cu₆Sn₅ IMC particles were also observed on the top surface of the (Cu, Ni)–Sn IMC layer. As the temperature and reaction time increased, so did the thickness of the IMC layers. In the solder ball region of both systems, a fine microstructure of Ag₃Sn, Cu₆Sn₅ IMC particles appeared in the β-Sn matrix. However, the growth behavior of the IMC layers of composite solder doped with CeO₂ nanoparticles was inhibited, due to an accumulation of surface-active CeO₂ nanoparticles at the grain boundary or in the IMC layers. In addition, the composite solder joint doped with CeO₂ nanoparticles had a higher hardness value than the plain Sn–Ag–Cu solder joints, due to a well-controlled fine microstructure and uniformly distributed CeO₂ nanoparticles. After 5 min of reaction on immersion Ag-plated Cu substrates at 250 °C, the micro-hardness values of the plain Sn–Ag–Cu solder joint and the composite solder joints containing 1 wt% of CeO₂ nanoparticles were approximately 16.6 and 18.6 Hv, respectively. However after 30 min of reaction, the hardness values were approximately 14.4 and 16.6 Hv, while the micro-hardness values of the plain Sn–Ag–Cu solder joints and the composite solder joints on Ag/Ni metallized Cu substrates after 5 min of reaction at 250 °C were approximately 15.9 and 17.4 Hv, respectively. After 30 min of reaction, values of approximately 14.4 and 15.5 Hv were recorded.     

Suppression effect of Cu and Ag on Cu3Sn layer in solder joints

Cu₃Sn intermetallic compound (IMC) layer is usually formed in solder joints. Since the formation of Cu₃Sn could induce large volume shrinkage, and further cause a lot of reliability issues, many works focused on suppressing the formation or growth of the Cu₃Sn layer. This work explored that Cu and Ag alloying elements also have benefit in suppressing the Cu₃Sn growth during isothermal aging stage. The Cu₆Sn₅ IMC layer seems to be much stable in the Sn/Cu solder joint during aged at 150 and 180 °C, its thickness changed little, while the Cu₃Sn IMC layer grew much quickly. After about 300 h, the thickness of Cu₃Sn layer exceeds that of Cu₆Sn₅ layer. For the Sn-3.5Ag/Cu and Sn-0.7Cu/Cu solder joints, the thickness of Cu₃Sn layer is near half of that of Cu₆Sn₅ layer. According to the relation between interface location and aging time, the reaction generated at the Cu₆Sn₅/Cu₃Sn interface, which is governed by atom fluxes, controls the growth of Cu₃Sn IMC layer. Since Ag and Cu alloying elements suppress the coarsening of Cu₆Sn₅ IMC grains, the diffusion paths for Cu atoms toward the solder are more for Ag or Cu containing solder joints. Therefore, the growth of the Cu₃Sn layer by consuming Cu₆Sn₅ layer is slower in the SnAg/Cu and SnCu/Cu solder joints than that in the Sn/Cu joints.     

Interfacial microstructure and mechanical properties of In–Bi–Sn lead-free solder

The interfacial microstructure and mechanical properties of a low melting temperature lead-free solder of In-18.75Bi-22.15Sn (in at.%) (In–Bi–Sn) were investigated. The microstructure analysis of bulk In–Bi–Sn revealed that irregular lamellar γ-Sn phases distributed in the In₂Bi matrix. There was only a single endothermic peak with an onset temperature of 62 °C on the DSC curve, indicating that In–Bi–Sn is close to a ternary eutectic solder. The ultimate tensile strength of the bulk In–Bi–Sn was 21.76 MP at a strain rate of 10^?2s^?1 at 25 °C. The elongation of the bulk In–Bi–Sn solder reached 87 %, indicating an excellent ductility of the In–Bi–Sn solder. Two intermetallic compounds (IMCs), needle-like Cu(In,Sn)₂ and laminar Cu₆(In,Sn)₅, formed at the In–Bi–Sn/Cu interface. An IMC layer of polyhedral crystallites of InNi formed at the In–Bi–Sn/Ni interface. The shear strength of Cu/In–Bi–Sn/Cu solder joints was 21.15 MP, and the shear fractograph showed that the ductile fracture with dimples appearance occurred in the solder.     

Very long-term aging of 52In–48Sn (at.%) solder joints on Cu-plated stainless steel substrates

Long-term metallurgical aging was studied in thermal switches comprised of 52In–48Sn (at.%) alloy solder plugs contained in Cu-plated stainless steel cylinders. These switches are locking devices designed so that, if overheated, a “fusible” alloy melts and allows the activation of a spring-loaded mechanism. The soldered assemblies studied ranged in age from about 24 to 28 years old at the time of this analysis. A concern has been the buildup of intermetallic compound (IMC) within the solder or at the solder/substrate interface, which could raise the switch operating temperature. In this work, the melting temperature of the aged solder alloy was slightly lower (116.3 ± 0.3 °C) than the expected value, 118.4 °C (245 °F), based on differential scanning calorimetry (DSC). The slight decrease in melting temperature range was caused by the diffusion of a small amount of Cu into the solder during processing and possibly during long-term service. The interfacial IMC layer was primarily Cu₂In₃Sn. The IMC thickness agreed with that predicted by growth kinetics determined in a previous study, assuming aging temperatures in the vicinity of room temperature. Differences in the IMC phase chemistries were found between earlier research, which employed bulk Cu substrates, and the present analyses with thin electroplated Cu substrates. Evidence was found for depletion of the thin Cu plating layer over time, as well as incorporation of Fe and Ni from the stainless steel into the IMC layer.     

Tensile properties of Cu/Sn–58Bi/Cu soldered joints subjected to isothermal aging

The effects of isothermal aging on the tensile properties of Cu/Sn–58Bi/Cu soldered joints were investigated. Experimental results show that the scallop-shaped Cu₆Sn₅ and planar Cu₃Sn formed at the interface between solder and Cu substrate during reflowing and aging. The thickness of the intermetallic compounds (IMCs) increased almost linearly with the square root of aging time, and aging at 120 °C yielded a much faster growth of the IMCs layer than that of samples aged at 100 °C. The IMCs growth rate constants were 6.02 × 10^?18 and 1.85 × 10^?18 m² s^?1 for solder joints aged at 120 and 100 °C, respectively. The tensile strength of the Sn–58Bi/Cu soldered joints decreased slightly with the increasing aging time and temperature. The failure was dominated by the mixed fracturing in both the solder and the Cu₆Sn₅ grains irrespective of their thermal aging conditions. However, the fracture pattern tended to transform from ductile to brittle with increasing aging time and temperature. The Bi segregation and voids were observed around the Cu/Cu₃Sn interface as the long term aging at high aging temperature was carried out, which resulted in reduction of tensile strength of solder joints.     

Electromigration-induced Bi-rich whisker growth in Cu/Sn–58Bi/Cu solder joints

Effect of current stressing on whisker growth in Cu/Sn–58Bi/Cu solder joints was investigated with current densities of 5 × 10³ and 10⁴ A/cm² in oven at different temperatures. Two types of whiskers, columnar-type and filament-type, were observed on the solder film propagating along the surface of the Cu substrate and at the cathode interface, respectively, accompanied with many hillocks formation. Typically, these whiskers were 5–15 μm in length and 0.06–2 μm in diameter. EDX revealed that these whiskers and hillocks were mixtures of Sn and Bi rather than single crystal. It should be noted that the sprouted whiskers would not grow any more even if the current-stressing time increased again when the solder joint was stressed under lower current density. Nevertheless, when the current density was up to 10⁴ A/cm², the whiskers would melt along with the increasing current-stressing time. Results indicated that the compressive stress generated by precipitation of Cu₆Sn₅ intermetallics provides a driving force for whisker growth on the solder film, and the Joule heating accumulation should be responsible for whisker growth at the cathode interface.     

Interfacial microstructure and hardness of nickel (Ni) nanoparticle-doped tin–silver–copper (Sn–Ag–Cu) solders on immersion silver (Ag)-plated copper (Cu) substrates

Sn–Ag–Cu composite solder has been prepared by adding Ni nanoparticles. Interfacial reactions, the morphology of the intermetallic compounds (IMC) that were formed, the hardness between the solder joints and the plain Cu/immersion Ag-plated Cu pads depending on the number of the reflow cycles and the aging time have all been investigated. A scallop-shaped Cu₆Sn₅ IMC layer that adhered to the substrate surface was formed at the interfaces of the plain Sn–Ag–Cu solder joints during the early reflow cycles. A very thin Cu₃Sn IMC layer was found between the Cu₆Sn₅ IMC layer and the substrates after a lengthy reflow cycle and solid-state aging process. However, after adding Ni nanoparticles, a scallop-shaped (Cu, Ni)–Sn IMC layer was clearly observed at both of the substrate surfaces, without any Cu₃Sn IMC layer formation. Needle-shaped Ag₃Sn and sphere-shaped Cu₆Sn₅ IMC particles were clearly observed in the β-Sn matrix in the solder-ball region of the plain Sn–Ag–Cu solder joints. Additional fine (Cu, Ni)-Sn IMC particles were found to be homogeneously distributed in the β-Sn matrix of the solder joints containing the Ni nanoparticles. The Sn–Ag–Cu–0.5Ni composite solder joints consistently displayed higher hardness values than the plain Sn–Ag–Cu solder joints for any specific number of reflow cycles–on both substrates–due to their well-controlled, fine network-type microstructures and the homogeneous distribution of fine (Cu, Ni)–Sn IMC particles, which acted as second-phase strengthening mechanisms. The hardness values of Sn–Ag–Cu and Sn–Ag–Cu–0.5Ni on the Cu substrates after one reflow cycle were about 15.1 and 16.6 Hv, respectively–and about 12.2 and 14.4 Hv after sixteen reflow cycles, respectively. However, the hardness values of the plain Sn–Ag–Cu solder joint and solder joint containing 0.5 wt% Ni nanoparticles after one reflow cycle on the immersion Ag plated Cu substrates were about 17.7 and 18.7 Hv, respectively, and about 13.2 and 15.3 Hv after sixteen reflow cycles, respectively.     

Shear performance of microscale ball grid array structure Sn–3.0Ag–0.5Cu solder joints with different surface finish combinations under electro-thermo-mechanical coupled loads

The shear performance and fracture behavior of microscale ball grid array structure Sn–3.0Ag–0.5Cu solder joints with different substrate surface finishes (Cu with organic solderability preservatives and electroless Ni/immersion Au) combinations under electro-thermo-mechanical (ETM) coupled loads with increasing current density (from 1.0?×?10³ to 6.0?×?10³ A/cm²) were systematically investigated by experimental characterization, theoretical analysis, and finite element simulation. The results reveal that the shear strength varies slightly with different surface finish combinations, initially increasing and then decreasing as the current density is increased. Moreover, the increase in current density shifts the fracture location from the solder matrix to the interface between solder and intermetallic compound (IMC) layer, resulting in a ductile-to-brittle transition. The interfacial fracture is triggered by electric current crowding at the groove of the IMC layer and driven by the mismatch strain at the solder/IMC layer interface.

     

Formation and growth of interfacial intermetallic layers in eutectic Sn-Ag solder and its composite solder joints

Single shear lap joints were made by soldering two Cu substrates with eutectic Sn-Ag solder, and its composite solders containing FeSn/FeSn₂ or Ni₃Sn₂ intermetallic particles introduced by an in-situ method. Ageing of solder joints was performed at 70, 100, 120, 150, 180 °C for 1400 h. The growth of the interfacial intermetallic compound (IMC) layers was characterized assuming diffusion-controlled growth kinetics. Effects of such FeSn/FeSn₂ and Ni₃Sn₄ particulates on the IMC layer growth rate were extensively characterized. Composite solder joints in the fabricated condition formed thinner IMC layers compared to the corresponding non-composite solder joints. The Cu₆Sn₅ IMC layer grew faster at temperatures above 120 °C (T/T _m=0.8), while growing slower at temperatures below 120 °C in composite solder joints. In-situ introduced FeSn/FeSn₂ and Ni₃Sn₄ particle reinforcements in composite solder joints proved effective in reducing the overall growth of the interfacial Cu₆Sn₅ IMC layer only at lower temperatures. Composite solder joints exhibited slower growth of the Cu₃Sn layer during ageing at all temperatures used in this study. Two different regions having different activation energies depending on the temperature were identified for the growth of Cu₆Sn₅ and Cu₃Sn IMC layers. © 2000 Kluwer Academic Publishers     

Interfacial reaction of Sn–2.0Ag–2.5Zn solder on Cu and Ni–W substrates

The interfacial reactions of Sn–2.0Ag–2.5Zn solder on Cu and Ni–W substrates after soldering and subsequent aging have been investigated in this study. Ni–W alloy layers with tungsten content of 3.0 and 10.0 at.% were electrodeposited on copper substrate. The interfacial micrographs of solder joints prepared at 250 °C for 15 s and aged at 150 °C for 24, 96 and 216 h are shown. Double-layer IMC composed of Cu₅Zn₈ and Ag₃Sn was observed at the interface of Sn–2Ag–2.5Zn and Cu couple, which was compact and acted as a barrier layer to confine the further growth of Cu–Sn IMC. On Ni–W barrier layer, a thin Ni₃Sn₄ film appeared between the solder and Ni–W layer, whose thickness decreases with the increase of W content. During the aging process, a thin layer of the Ni–W substrate transforms into an amorphous bright layer, and the thickness of amorphous layer increased as aging time extended. Referring to the elemental line-distribution and the thickness of different layers at the interface, the formation of the bright layer is caused by the fast diffusion of Sn into Ni–W layer.     

Influence of TiO2 nanoparticles on IMC growth in Sn–3.0Ag–0.5Cu–xTiO2 solder joints during isothermal aging process

The influence of TiO₂ nanoparticles on the growth of intermetallic compound (IMC) between Sn–3.0 wt% Ag–0.5 wt% Cu–x wt% TiO₂ (x = 0, 0.02, 0.05, 0.1, 0.3, and 0.6) composite solder and the Cu substrate during isothermal aging process at temperatures of 120, 150, and 190 °C has been investigated in this study. Scanning electron microscopy was used to observe the microstructural evolution of the solder joints and measure the thickness of IMC layer. The IMC phases were identified by energy-dispersive X-ray spectroscopy and X-ray diffractometry. Results show that two intermetallic layers, Cu₆Sn₅ and Cu₃Sn phase are formed at the interface and the morphology of the Cu₆Sn₅ phase transforms from scallop-type to layer-type in Sn–3.0Ag–0.5Cu–xTiO₂ solder joints. The addition of nano-TiO₂ has a strong influence on the growth of overall IMC layers, and the thickness of overall IMC layers rapidly increase with aging temperature and time. The growth rates and activation energies of the IMC growth of six solder alloys were determined. Results reveal that, for three different ageing temperatures, the growth rates of overall IMC layers decrease with an increase in nano-TiO₂ proportion. The activation energies for the growth of overall IMC layers range from 48.34 to 63.61 kJ/mol. Adding nano-TiO₂ to Sn–3.0Ag–0.5Cu solder could evidently increase the activation energy of overall IMC layers, reduce the atomic interdiffusion rate, and thus inhibit excessive growth of overall IMC layers.     

Electromigration-enhanced intermetallic growth and phase evolution in Cu/Sn–58Bi/Cu solder joints

This paper presents some experimental observations relative to the influence of elevated current densities on the intermetallic growth and phase evolution in Cu/Sn–58Bi/Cu solder joints. Three samples were stressed with different current densities of 10⁴, 1.2 × 10⁴, and 1.4 × 10⁴ A/cm², respectively, for 80 h. The abnormal polarity effect of electromigration (EM) on chemical reactions at the cathode and the anode was investigated as well as the effect of EM on phase segregation in the two-phase eutectic microstructure. Results indicate that electric current enhances the growth of IMC layer at the cathode and retards it at the anode due to the Bi accumulation acting as a barrier layer with current density of 10⁴ A/cm². However, when current density increases, the electrical force dissolves the IMC at the cathode into the solder. More and more intermetallic precipitates formed due to the dissolution of Cu into the solder at the cathode side with increased current densities, leading to a very different morphology at the anode and the cathode interfaces, one being planar and the other being very irregular. It can be concluded that the chemical force and the electrical force are the main driving forces contributing to the IMC growth at both interfaces.     

Effects of Co additions on electromigration behaviors in Sn–3.0 Ag–0.5 Cu-based solder joint

As the miniaturization trend of electronic packing industry, electromigration (EM) has become a critical issue for fine pitch packaging. The EM effects on microstructure evolution of intermetallic compound layer (IMC) in Sn–3.0 Ag–0.5 Cu + XCo (X = 0, 0.05, 0.2 wt%) solder joint was investigated. Findings of this study indicated that current stressing of Sn–3.0 Ag–0.5 Cu–0.2 Co solder joint with 10⁴ A/cm² at 50 °C for 16 days, no remarkable EM damages exhibited in solder matrix. Whereas, after current stressing at 150 °C for 1 and 3 days, Sn–3.0 Ag–0.5 Cu specimens showed obvious polarity effect between cathode and anode. Different morphology changes were also observed at both sides. After current stressing for 1 day, two IMC layers, Cu₆Sn₅ and Cu₃Sn, with wave type morphology formed at cathode. Sn phases were also observed inside in the IMC layer. However, only Cu₆Sn₅ formed in anode. Three days later, Sn phases were found in anode. Besides, Co additions, aging treatment, Ag₃Sn, and other IMCs improved the resistance of EM by the evidence of retarding polarity effect.     

Electromigration statistics and damage evolution for Pb-free solder joints with Cu and Ni UBM in plastic flip-chip packages

A series of electromigration (EM) tests were performed as a function of temperature and current density to investigate lifetime statistics and damage evolution for Pb-free solder joints with Cu and Ni under-bump-metallizations (UBMs). The EM lifetime was found to depend on the failure criterion used, so the results were compared based on the first resistance jump and conventional open-failure criterion. Solder joints with Cu UBM had a longer lifetime than Ni UBM based on the open-failure criterion, but the lifetime with Ni UBM became comparable when the first resistance jump criterion was applied. To determine the temperature in solder joints, the Joule heating effect was investigated with experiments and finite element analysis. The temperature of solder joints was determined to be approximately 15°C higher than that at the Si die surface when 1 A of current was applied. With the appropriate temperature correction, the activation energies and the current density exponents were found to be Q = 1.11 eV, n = 3.75 and Q = 0.86 eV, n = 2.1 based on the open-failure criterion for solder joints with Cu and Ni UBM, respectively. Based on the first resistance jump criterion, Q = 1.05 eV, n = 1.45 for Cu UBM and Q = 0.94 eV, n = 2.2 for Ni UBM, respectively. For solder joints with Cu UBM, voids were formed initially at the Cu₆Sn₅/solder interface while the final open failure occurred at the Cu₃Sn/Cu₆Sn₅ interface. For Ni UBM, voids were formed initially at the Ni₃Sn₄/solder interface leading to failure at the same interface. The formation of intermetallic compounds (IMCs) was enhanced under current stressing, which followed linear growth kinetics with time. The IMC growth was accompanied by volume shrinkage, which accelerated damage evolution under EM.