Au Surface Finish Research Articles

An explanation for the effect of Au surface finish on the quality of sintered Ag-Au joints

As an environmentally friendly interconnection material in power electronics, silver paste is widely used in gold surface finish. To explain the delamination at the Ag-Au interface and clarify the mechanism of Ag-Au interconnection, the bonding properties of sintered silver joints on different gold-plated substrates are studied. It was concluded that the effect of grain size on Ag-Au diffusion is greater than that of grain boundary type. The gold-plating layer with larger grains provides fewer grain boundaries, which inhibits the rapid diffusion of Ag atoms due to high angle grain boundaries. In addition, to investigate the effects of Au grain boundary misorientation on Ag-Au interdiffusion, molecular dynamics simulation is carried out on the sintering process. It is not found that high angle grain boundary of Au layer promotes the diffusion of Ag atoms during pre-drying. However, in the sintering stage, the MSD (mean square displacement) of Ag atoms in the substitute-model with high angle grain boundaries is about 4 times that of with low angle grain boundaries. It is proved that owing to lots of high angle grain boundaries in the gold-plated substrate, the diffusion of Ag atoms is faster in the sintering stage.

An Explanation on the Effect of AU Surface Finish on the Quality of Sintered Ag-Au Joints

An Explanation on the Effect of AU Surface Finish on the Quality of Sintered Ag-Au Joints

Vertical Interconnections by Electroless Au Deposition on Electroless Ni Immersion Au Surface Finish

Electroless Au is used to vertically connect copper pillars with electroless Ni immersion Au surface finish on two stacked chips by a microfluidic electroless interconnection process. A very low bonding temperature of 50°C and a pressure-less bonding process can be achieved. The vertical interconnections are formed by a forced flow of electroless Au plating solution through a microchannel so that the reduced Au atoms self-assemble between the gaps of facing copper pillars to complete the interconnections. The deposited Au grains span the entire gap across the two copper pillars, suggesting that the rate-limiting step is more likely to be the nucleation rather than the growth of the Au grains. Four-point probe measurements show that the average resistance of the electroless-Au-bonded interconnections is low. Mechanical shear testing reveals that the bonding of the interconnections is strong. Furthermore, it is demonstrated that this process is capable of accommodating a high degree of pillar misalignment.

Effect of Surface Finish on the Shear Properties of SnAgCu-Based Solder Alloys

The reliability of an electronic assembly is typically limited by the failure of one of the solder interconnections. One of the key factors that define the quality of solder interconnections is the strength of the solder joint attachment to the printed circuit board (PCB). The ball shear testing is commonly used as a quantitative approach to evaluate the integrity of the solder. Several factors control the shear strength and failure mechanism of solder joints, including the surface finish, solder alloy composition, and speed of shearing (shear strain rate). In this paper, the effect of surface finish on the shear properties of various “microalloyed” SnAgCu-based solder materials was investigated. Individual solder joints were fabricated on a PCB with a solder mask defined configuration. A series of shear testing was conducted at four different strain rates of 0.001, 0.01, 0.1, and 1 s−1. Shear stress–strain curves were recorded for each test, and both shear strength and shear energy were measured. Following the shear testing, fractured top surfaces and cross sections were inspected using SEM/EDS microscopy to characterize the failure mechanism. The results showed that both shear strength and shear energy increase with increasing shear strain rate due to the viscoplasticity of the solder materials. Failure mode analysis indicated the existence of three failure mechanisms, including ductile failure, brittle failure, and mixed failure. Higher occurrence of brittle failure was observed when the shear strain rate is high regardless of surface finish. It was also found that the combination of solder alloy with high silver (Ag) and bismuth (Bi) content and electroless Ni/immersion Au (ENIG) surface finish is more susceptible to brittle failure compared to the rest of solder alloy-surface finish combinations.

(Invited) Packaging Material Technology for Wide Band Gap Power Devices and Its Performance/Reliability Evaluation

Wide band gap semiconductors such as SiC and GaN have been expected to be established as the next generation power semiconductors, which are capable to reduce huge energy-loss in electric power conversion. The application of them will increase power density resulting in operation temperature rise beyond 200 °C. In addition, both increase of current and of frequency will also increase stress on devices. In general, power devices must possess robustness and high reliability for safe control of power conversion. Heat dissipation from the hot die in operation is one of the essential key factors for establishing high performance WBG devices. Here we report the current status of high temperature packaging materials including die-attach, wiring, substrates and molding compounds. In addition, reliability issues including thermal shock, power cycling and transient thermal properties will be summarized. A series of DBC and DBA substrates, composed of Al and Cu as metal layers and of Al2O3, AlN and Si3N4, were examined by thermal shock test between -50 °C and 250 °C. DBC substrates with Al2O3 and AlN fractured only after 10 cycles due to severe thermal stress while all DBA and DBC with Si3N4 substrates survived. Al and Cu layer of all DBC and DBA substrates severely deformed by thermal shock. By microstructural observation, grain boundary sliding during thermal shock has a key to cause deformation of metal layers on ceramics. Ni and Ni-P plating on metal layer also deformed followed by Al and Cu layer deformation. Ni-P plating showed severe cracking which can be attributed to brittleness of Ni-P plating while Ni plating did not. Ag sinter joining can be successfully applied for die attach by low temperature and low pressure bonding in air. For better bonding quality, Ag coating is the best metallization due to the specific reaction of Ag in air. By modifying solvent, Ag particles paste was modified to form a good bonding with Au surface finish. It can provide not only high temperature structural stability but also excellent thermal dissipation capability. High die-shear strength beyond 30 MPa was maintained by thermal shock between -50 °C and 250 °C up to 1000 cycles for a DBC die-attach structure. The thermal resistance of Ag sinter joined die attach is much lower than that of solder. Figure shows the example of a GaN die-attached on DBC substrate with a new Ag sinter materials at 200 °C in air without any pressure. Even on the Au metallization, the GaN die was bonded with high strength. The interface shows a good bonding between Au and Ag. This work was supported both by JST ALCA project on GaN packaging and by METI standardization project for evaluation methods of thermal properties of ceramics substrates. Figure 1

Transmission Electron Microscopy Investigation on the Oxidation Behavior of Electroless Ni/Immersion Au Surface Finish at 250 °C

Transmission Electron Microscopy Investigation on the Oxidation Behavior of Electroless Ni/Immersion Au Surface Finish at 250 °C

A Comparison of Immersion Gold and Tin Surface Finishes on Sensing Electrodes for PCB Environmental Saltwater Concentration Sensors

Abstract Printed circuit board (PCB) sensors using low-cost commercial printed circuit board fabrication processes have been demonstrated for environmental sensing applications. One configuration of these sensors uses exposed electrodes to measure saltwater concentration in freshwater/seawater mixtures, through monitoring the resistance between the electrodes when they are immersed in the saltwater/freshwater solution. The lowest cost commercial PCB processes use an immersion Sn HASL surface finish on exposed copper cladding, including the sensing electrodes. This commercial PCB process has been demonstrated to make an effective, low-cost, short-lifetime sensor for saltwater concentration testing. The Sn finish, however, may not be optimal for this application. Sn oxidizes, which can interfere with sensor performance. Additionally, Sn and Sn oxides are potentially reactive with chemical constituents in seawater and seawater/freshwater solutions. An immersion Au (ENIG) surface finish is certainly less reactive with the atmosphere and chemicals likely present in the testing environment. However, an immersion Au finish increases the cost of the sensors by 30% to 40%. To investigate if the possible benefits of the more expensive Au surface finish are worth the extra expense, a study was performed where identical PCB sensors were procured from a commercial vendor with their standard low-cost Sn HASL finish and with their standard ENIG surface finish. Both sets of sensors were then evaluated in concentrations of seawater and freshwater, from 0% to 100% seawater concentration, using freshwater samples from a natural freshwater source near the coast where the seawater was obtained. Testing demonstrated an insignificant difference in sensor performance between the Sn HASL and the ENIG coated sensing electrodes. The results of this investigation indicated that for applications where the sensors will not be used for long periods of time, the added expense of an immersion Au surface finish is not worth the added cost.

Mechanisms and solutions to the brittle solder joint in electroless Ni plating

Brittle solder joints in Electroless Ni electroless Pd immersion Au (ENEPIG) surface finishes are one of the key reliability issues in electronics assembly. Previous characterization of the reflow process has indicated that interfacial voids formed after solder reflow are responsible for the decreases in solder joint strength. However, the mechanisms behind the formation of these voids in the ENEPIG process remain unclear. In this paper, the interaction between various aspects of the ENEPIG process and solder joint strength were investigated. Surface roughness, morphology, and nano-pitting at the interface between electroless Pd and Ni-P were characterized. The size and density of nano voids inside Ni2SnP were measured after the specimens were reflowed with Sn4Ag0·5Cu solder ball. Additionally, high speed shear solder joint strength measurements were made. The results indicated that anion adhesion induced nano-pitting at the interface between the Ni2SnP intermetallic and Pd, resulting in the formation of a nano void layer during reflow. These interfacial voids lead to lower solder joint strength. Based on the results, a solution to prevent the brittle solder joint failures is suggested.

On the Modeling, Characterization, and Analysis of the Current Distribution in PCB Transmission Lines With Surface Finishes

Due to manufacturing requirements, surface finishes have become a necessity in printed circuit board design. These finishes have significant effects on the RF performance of the transmission lines. In this paper, a filament modeling approach is used to model skin, proximity, and surface roughness effects in transmission lines with surface finishes up to 70 GHz. The approach shows a high accuracy compared with measurements. The model also gives an insight into how the current distributes itself by showing the frequency dependent proportion of the current that flows in each surface finish layer. In the case of NiP–Au or Ni–Au surface finishes, current migrates increasingly into gold at high frequencies and reaches a maximum in the Ni or NiP at around 3.5 GHz, and then declines. The distribution of the current in different materials can also be explained as the decay of an electromagnetic wave at the surface of the conductor. This approach shows that the evanescent wave in the cross section of the conductor can be analyzed as analog to a transmission–reflection problem, what we will call the surface finish effect. This effect brings into question the accuracy of the traditional skin-depth value, $\delta $ , and the models that depend on it, such as most surface roughness correction factors, for structures where different metals are layered in thicknesses that are not much larger than $\delta $ .

Effect of Au-Sn IMCs’ formation and morphologies on shear properties of laser reflowed micro-solder joints

Purpose – The purpose of this paper is to investigate the effect of typical morphologies of Au-Sn IMCs (intermetallic compounds) at the interfaces of solder and pads on shear properties of laser reflowed micro-solder joints. Design/methodology/approach – Sn-2.0Ag-0.75Cu-3.0Bi (SnAgCuBi) solder balls (120 μm in diameter), pads with 0.1, 0.5, 0.9 or 4.0 μm thickness of Au surface finish, and different laser input energies were utilized to fabricate micro-solder joints with Au-Sn IMCs having different typical morphologies. The joints were performed by a shear test through a DAGE bond test system. Fracture surfaces of the joints were analyzed by scanning electron microscopy and energy-dispersive X-ray spectrometry to identify fracture modes and locations. Findings – Morphologies of Au-Sn IMCs would affect shear properties of the joints remarkably. When needle-like AuSn4 IMCs formed at the interfaces of solder and pads, almost entire surfaces presented the manner of ductile fracture. Moreover, shear forces of this kind of solder joints were higher than those of joints without Au-Sn IMCs or with a nearly continuous/continuous Au-Sn IMCs layer. The reason was that the shear performance of the solder joints with needle-like AuSn4 IMCs was enhanced by an interlocking effect between solder and needle-like AuSn4 IMCs. As a nearly continuous or continuous Au-Sn IMCs layer formed, the fracture surfaces presented more character of brittle than ductile fracture. However, if an Au layer still remained under Au-Sn IMCs, the shear performance of the joints would be enhanced. Originality/value – The results in this study can be used to optimize microstructures and shear properties of laser reflowed micro-solder joints.

Effects of Ni3Sn4 and (Cu,Ni)6Sn5 intermetallic layers on cross-interaction between Pd and Ni in solder joints

Abstract We examined the effects of layers of intermetallic compound (IMC) Ni 3 Sn 4 and (Cu,Ni) 6 Sn 5 formed at the solder/Ni interface, on the cross-interactions between Pd and Ni during solid-state aging and reflow soldering. Two types of diffusion couples, Pd/Sn/Ni and Pd/Sn–Cu/Ni, were aged at 150 °C to study the solid-state interactions. In contrast to the Pd/Sn/Ni couples in which a Ni 3 Sn 4 layer formed at the Ni interface, the Pd/Sn–Cu/Ni couple where a (Cu,Ni) 6 Sn 5 layer formed at the Ni interface exhibited no significant interaction between Pd and Ni. The (Cu,Ni) 6 Sn 5 layer acted as an effective barrier against Ni diffusion and thus inhibited the resettlement of (Pd,Ni)Sn 4 onto the Ni interface. For the interaction during reflow, Sn–3.5Ag and Sn–3.0Ag–0.5Cu solder balls were isothermally reflowed on an electroless Ni(P)/electroless Pd/immersion Au (ENEPIG) surface finish at 250 °C, and the dissolution kinetics of the (Pd,Ni)Sn 4 particles converted from the 0.2-μm-thick Pd-finish layer were examined. The spalled (Pd,Ni)Sn 4 particles very quickly dissolved into the molten solder when the IMC layer formed on the Ni substrate was (Cu,Ni) 6 Sn 5 rather than Ni 3 Sn 4 . The dependence of the dissolution kinetics of the spalled (Pd,Ni)Sn 4 particles on the IMC layers was rationalized on the basis of a Sn–Ni–Pd isotherm at 250 °C. The present study suggests that the formation of a dense (Cu,Ni) 6 Sn 5 layer at the solder/Ni interface can effectively alleviate the (Pd,Ni)Sn 4 -related degradation of solder joint reliability.

Long-term Electromigration Study of Lead-Free Flip-Chips with Solder Bumps with 50 μm or 60 μm Diameter Employing ENIG Surface Finish on Both Chip and Substrate Side

Electromigration behaviour of Pb-free solder joints in flip-chip interconnects is usually studied in highly accelerated, short-term experiments using high current density and temperature. Failures typically occur in bumps which are in cathode contact at the chip side. There are only a few published studies in which Electroless Ni-P/Immersion Au (ENIG) surface finish was used as under-bump-metallization (UBM) structure, e.g. [5]. This paper deals with the long-term electromigration behaviour of Pb-free SAC305 flip-chip solder joints with a pitch of 100 μm and solder bump diameters of 50 μm or 60 μm, respectively. The ENIG surface finish was used on both the substrate and chip side. Test specimens were subjected to several levels of temperature and current density and tested up to 16,000 hours. The life time data is summarized using Weibull and lognormal distribution. The microstructure changes of interconnects in failed samples were subsequently investigated by SEM and EDX. Interconnects had failed due to consumption of Nickel, voids caused by electromigration, and Kirkendall void formation in the Ni-P-layer. The damage was asymmetric in respect to the current flow direction through the solder bumps and was most pronounced at the cathode side. Unexpectedly, however, the most severe damage occurred at the substrate and not at the chip side. We could show that - allowing for a few guidelines - lead-free flip-chip solder joints with 50 μm or 60 μm diameter have a sufficient electromigration life time for most applications.

Effects of Pd(P) Thickness on the Microstructural Evolution Between Sn-3Ag-0.5Cu and Ni(P)/Pd(P)/Au Surface Finish During the Reflow Process

The microstructural evolution between Sn-3Ag-0.5Cu (SAC305) solder and Ni(P)/Pd(P)/Au finish during the reflow process was investigated for various Pd(P) thicknesses (0 μm to 0.6 μm). The reflow process was carried out in a belt-conveying reflow oven with peak temperature of 260°C. In the early stages of the reflow process, the Pd(P) layer either dissolved or spalled in the form of (Pd,Ni)Sn4 into the molten solder, leaving behind an Ni2SnP/Ni3P bilayer on the Ni(P) layer. From the dissolution of the spalled (Pd,Ni)Sn4 particles during the reflow process, the solubility of Pd in the molten SAC305 solder in the reflow process was estimated to be 0.18 wt.% to 0.25 wt.%. Regardless of the ratio of solder volume to pad opening size, the Ni2SnP layer that formed in the early stage of reflow had a significant influence on the subsequent formation and growth of (Cu,Ni)6Sn5 at the solder interface. As the Ni2SnP layer became thicker with increasing Pd(P) thickness, the formation of (Cu,Ni)6Sn5 became increasingly sluggish and occurred only at locations where the Ni2SnP layer was locally thin or discontinuous, leading to a discontinuous morphology of (Cu,Ni)6Sn5. This was attributed to the Ni2SnP layer that became an increasingly effective barrier to Ni diffusion with increasing thickness. Based on the experimental results, this study suggests detailed mechanisms underlying the effects of the Pd(P) thickness on the morphology and growth of the (Cu,Ni)6Sn5 formed during the reflow process.

The influence of Pd on growth behavior of a quaternary (Cu,Ni,Pd)6Sn5 compound in Sn–3.0Ag–0.5Cu/Au/Pd/Ni–P solder joint during a liquid state reaction

This study investigated the liquid state reaction of a Sn–3.0Ag–0.5Cu solder jointed with electroless Ni–P/immersion Au (ENIG) and electroless Ni–P/electroless Pd/immersion Au (ENEPIG) surface finishes. Treatments with various soldering temperatures (240, 250, and 260 °C) and times (60, 180, 300, and 600 s) were performed to study the microstructure evolution. Detailed interfacial images revealed that the morphology of (Cu,Ni)6Sn5 affects the formation of Ni3P and the curvature of the interface between them. In addition, the growth kinetics of (Cu,Ni)6Sn5 and (Cu,Ni,Pd)6Sn5 were studied and compared. The effect of grain coarsening during extended reflow modified the diffusion transport mechanism. However, because of the refinement of Pd on the grain structure, reduced IMC growth and a lower degree of transition from grain boundary diffusion to volume diffusion could be observed in the growth kinetics of (Cu,Ni,Pd)6Sn5. Moreover, the activation energy of IMC growth was evaluated using the Arrhenius equation. Pd may act as heterogeneous nucleation sites in the initial stage of soldering and lower the activation energy of (Cu,Ni,Pd)6Sn5, compared to (Cu,Ni)6Sn5. The lower activation energy of (Cu,Ni,Pd)6Sn5 growth ensured that no phase transformation occurred in the SAC305/ENEPIG joints, which may benefit the solder joint reliability. Finally, the detailed influence of Pd on the growth kinetics of IMC formation was investigated and discussed.

Oxidation and Au‐Sn reaction of laser reflowed micro‐solder joints protected by N2 or exposed to air atmosphere

PurposeThe purpose of this paper is to investigate oxidation and the Au‐Sn reaction of laser reflowed (LR) micro‐solder joints when different protective atmospheres were applied.Design/methodology/approachA N2 atmosphere at room temperature, 60°C, 100°C and 130°C, or an air atmosphere at room temperature were utilized in this study. The solder balls were composed of Sn‐2.0Ag‐0.75Cu‐3.0Bi, and 120 μm in diameter. The surface finish of one pad of the joints was 4.0 μm Au/0.1 μm NiFe/0.01 μm Ta, another pad was made of Cu plated with 3.0 μm Au. The laser reflow process time was controlled to within 10 ms. Auger Electron Spectroscopy (AES) was used to identify the oxidation condition of LR solder joints with or without protection from a N2 atmosphere at room temperature. The appearance and cross‐sections of the joints protected by a N2 atmosphere at different temperatures were evaluated using SEM analysis.FindingsOxidation of LR solder joints from an air atmosphere was extremely severe, and the surfaces of solder were rough as compared with joints protected by a N2 atmosphere. Au‐rich phases and needle‐like AuSn4 intermetallic compounds (IMCs) formed at the interfaces of the solder and the pads. As the temperature of the N2 atmosphere was increased above 100°C, almost all of the Au‐rich phases disappeared. More needle‐like AuSn4 IMCs formed at the interfaces, as compared with that in joints protected by a N2 atmosphere at room temperature and 60°C. In addition, the orientation of the IMCs had clearly changed.Originality/valueThe results may provide a guide for controlling oxidation and the Au‐Sn reaction of micro‐solder joints during the LR process, and improving the properties of joints between solder and pads with Au surface finishes, by regulating the protective atmosphere.

Formation of AuSnx IMCs in Sn3.5Ag0.75Cu micro-solder joints fabricated by laser and hot air reflow processes

The present study aims to investigate morphology and distribution of AuSnx IMCs (Intermetallic Compounds) in laser and hot air reflowed micro-solder joints. Diameter of Sn3.5Ag0.75Cu solder balls used in the experiments was 120 μm. Thickness of Au surface finish on pads of the joints was 0.1, 3.0 or 4.0 μm. In solder joints fabricated by the laser reflow (LR) process, most AuSnx IMCs were in needle-like or dendritic shape, and located near interfaces of solder and pads. Whereas, the AuSnx IMCs within the solder joints fabricated by the hot air reflow (HR) process presented in fibrous or faceted shape, and distributed all over the solder joints. Diversity of morphology and distribution of AuSnx IMCs in the LR and HR solder joints is caused by different characteristics of heat source, different solidification rate of solder joints, and different reaction time between Au and Sn in the two reflow processes.

A novel high-speed shear test for lead-free flip chip packages

Despite the importance of lead-free solders in modern environmentally friendly packaging, few studies have been conducted on their mechanical reliability at the wafer level. In the present study, high-speed die shear tests were conducted to investigate the effects of strain rate on the shearing resistance and fracture mode of Sn-3wt%Ag-0.5wt%Cu solder joints on electroless Ni-P/immersion Au surface finish pads. The results indicated that the solder joints underwent ductile and mixed ductile-brittle fracture at low ( 25,385 s−1) strain rates, respectively. Thus, the overall shear stress-strain curve can be divided into three areas according to Hollomon’s law, starting from low strain rates: area I, 100% ductile fracture of the solder itself; area II, mixed ductile-brittle fracture resulting in a ductile-brittle transition region; and area III, 100% brittle fracture at the interface between the intermetallic compound and the Ni-P layer.

Effect of laser input energy on AuSn x intermetallic compounds formation in solder joints with different thickness of Au surface finish on pads

Formation of AuSnx intermetallic compounds (IMCs) in laser reflowed solder joints was investigated. The results showed that few IMCs formed at the solder/0.1 μm Au interface. Needlelike AuSn4 IMCs were observed at the solder/0.5 μm Au interface. In Sn-2.0Ag-0.75Cu-3.0Bi and Sn-3.5Ag-0.75Cu solder joints, when the laser input energy was increased, AuSn4 IMCs changed from a layer to needlelike or dendritic distribution at the solder/0.9 μm Au interface. As for the solder joints with 4.0 μm thickness of Au surface finish on pads, AuSn4, AuSn2, AuSn IMCs, and Au2Sn phases formed at the interface. Moreover, the content of AuSnx IMCs, such as, AuSn4 and AuSn2, which contained high Sn concentration, would become larger as the laser input energy increased. In the Sn-37Pb solder joints with 0.9 μm or 4.0 μm thickness of the Au surface finish on pads, AuSn4 IMCs were in netlike distribution. The interspaces between them were filled with Pb-rich phases.

Additive Effect of Kirkendall Void Formation in Sn-3.5Ag Solder Joints on Common Substrates

The Sn-3.5Ag and Sn-3.5Ag-0.2Co-0.1Ni lead-free solders were investigated on common electronics substrates, namely, organic solderability preservative (OSP) and electroless Ni/immersion Au (ENIG) surface finishes. The formation of Kirkendall voids at the interfacial region during isothermal solid aging was explored. For the Sn-3.5Ag-0.2Co-0.1Ni/OSP solder joint, the Kirkendall voids were present after isothermal solid-state aging at higher temperature (e.g., 150°C); however, the size of voids did not change remarkably with prolonged aging time due to the depressed Cu 3 Sn layer growth. For ENIG surface finishes, the 0.2Co-0.1Ni additions seemed to enhance the longitudinal groove-shaped voids at the Ni 3 P layer; however, void formation at the solder/Ni 3 Sn 4 interface was effectively reduced. This might be attributed to the reduced Sn activity in the solder matrix and the suppressed Ni-P-Sn layer formation.

Cross-Interaction of Interfacial Reactions in Ni (Au/Ni/Cu)-SnAg-Cu Solder Joints during Reflow Soldering and Thermal Aging

Interfacial reactions in Ni-SnAg-Cu and Au/Ni/Cu-SnAg-Cu solder joints were investigated to understand the coupling effect between different pads during soldering and thermal aging processes. Scanning electron microscopy (SEM) was used to characterize the microstructures and phases. The element distributions in the joints were identified using the x-ray mapping technique. The thickness variation of intermetallic compounds (IMCs) with aging time was also measured. The results showed that interfacial reactions were not only affected by the compositions of solders and the local metallizations but the remote pads as well. The Au surface finish had an effect on the growth of IMCs at the interfaces. No redeposition of (Au, Ni)Sn4 was found in the Au/Ni/Cu-SnAg-Cu solder joint. The effect of Cu on the formation of IMCs and redeposition of (Au, Ni)Sn4 was also discussed.