Cu6Sn5 Intermetallic Compound Layer Research Articles

Interfacial reaction of Sn-1.5Ag-2.0Zn low-silver lead-free solder with oriented copper

High density packaging technology reduces the pad size and the number of grains contained in the pad. When the polycrystalline pad turns into a single crystal pad, the grain orientation has an important impact on the formation of the intermetallic compound (IMC) at the interface. The growth of IMC at the interface between the solder and the single-crystal copper substrate is investigated by selecting the prospective Sn-1.5Ag–2Zn as the solder alloy. Sn-1.5Ag-2.0Zn lead-free solder joints soldered with single crystal (111) copper substrate and polycrystalline red copper substrate are reflowed at 250 °C for 5 min. Samples are subsequently aged at 160 °C. The uneven scallop like Cu6Sn5 IMC layer grows rapidly when the alloy solder contacts with the copper substrate. The Cu6Sn5 grain size formed on the surface of single crystal copper is larger than that of polycrystalline copper. Single crystal copper has no grain boundary to block atomic diffusion, which affects grain nucleation and growth. The growth rate of Cu6Sn5 formed by alloy solder and the single crystal (111) copper solder joint after aging treatment at 160 °C for 20 h is about twice that of the polycrystalline copper solder joint. Then it grows slowly with the increase of aging treatment time. The thick layer Cu6Sn5 breaks due to crack diffusion after 600 h of aging treatment, and the thickness of IMC remains at 3.5 μm. Cu5Zn8 generated at the solder and polycrystalline copper solder joint during aging treatment acts as a barrier layer, preventing the solder from contacting the copper substrate and inhibiting the formation of Cu6Sn5. Cu5Zn8 is broken and decomposed after 300 h of aging treatment, and Cu6Sn5 grows rapidly after the barrier layer disappeared. The thickness of Cu6Sn5 is about 2.8 μm. The thickness of IMC of solder joint on single crystal copper is 0.7 μm more than that on polycrystalline copper. After aging treatment, the IMC formed at the interface between alloy solder and single crystal copper has better compactness and basically no pores, while there are obvious pores between IMC grains at the interface between alloy solder and polycrystalline copper, which can predict that the soldering quality of the interface between alloy solder and single crystal copper is better. This will provide application prospects for solder joint interface reliability research.

Intermetallic compound growth behavior and mechanical performance in Sn58Bi/Cu solder joint bearing Mg particles incorporation during thermal aging

The microstructure and intermetallic compound (IMC) growth in Sn58Bi/Cu and Sn58Bi-0.4 Mg/Cu solder joints were examined in this study under varied thermal aging time (6, 12, 18, 24, and 30 h) at 120 °C. The Shear test was conducted to assess the solder joint mechanical performance. The results indicated that the precipitation of the Bi phase in Sn58Bi/Cu and Sn58Bi-0.4 Mg/Cu solder joints was not significantly inhibited by adding Mg as aging time increased. The IMC layer thickness of both solder joints increased as the aging time increased, but the Cu3Sn IMC thickness in Sn58Bi-0.4 Mg/Cu solder joint was consistently thinner than that in Sn58Bi/Cu joint. The temporal evolution of the Cu3Sn IMC layer was investigated, and it was found that the inclusion of Mg particles reduced the diffusion coefficient of Cu3Sn IMC. Furthermore, the shear test revealed that throughout the initial phases of thermal aging, fractures occurred within the solder matrix for both joints. As thermal aging time increased, fractures extended into the IMC layer, accompanied by transgranular and intergranular fractures. Simultaneously, adding Mg particles delayed brittle fracture in the solder joints, enhancing their mechanical properties.

Interfacial Reaction between SAC3807 Lead-Free Solders and Different Copper Substrate via Reflow Soldering Process

The different composition material of copper substrate significantly affects the intermetallic compound (IMC) formation and the solder joints durability. This study was conducted on the interfacial reaction between lead-free solder and the different copper substrates via reflow soldering. The selected substrate is copper (Cu) and copper-beryllium (Cu-Be). The lead-free solder involved is Sn-3.8Ag-0.7Cu (SAC3807) solder ball with a diameter of 700 μm. All the samples were subjected to the isothermal aging process. The material characterization and analysis on the IMC formation were examined by scanning electron microscopy (SEM), optical microscope (OM), and energy dispersive X-ray analysis (EDX). After the reflow process, the result revealed that Cu3Sn, Cu6Sn5 IMC layer formed at SAC3870/Cu and SAC3870/Cu-Be interface. The changes to a rod-like shape Cu6Sn5 and irregular needle-shaped Cu3Sn4 occur after the aging treatment on SAC3870/ Cu. Meanwhile, the IMC layer for SAC3870/Cu-Be shows a rod-like shape transformed into a blocky-like shape Cu6Sn5 and Cu3Sn4 diamond-shape. This result indicates that Ag3Sn nanosized was formed on the intermetallic surface during the aging process for both SAC3807/Cu and SAC3807/Cu-Be. The Ag3Sn nanosized element at SAC3807/Cu-Be is many compared to SAC3807/Cu. In addition, IMC thickness for SAC3807/Cu-Be shows a thicker layer than SAC3807/Cu. Lastly, in this research, the element of Be in SAC3807/Cu-Be cannot be defined because the beryllium element is not easily detected as the percentage was very low.

Sn-Cu intermetallic compound laminated film with excellent friction coefficient, contact resistance, and solder wettability

ABSTRACT The connector used for electronic parts of automobiles is a type with Sn plating on the copper surface. In recent years, as the number of connector pins has increased and the insertion force has increased, problems have arisen in the insertion and removal operations. In addition, the reliability of the connection in high temperature environments needs to be improved. Various Sn plating alternative processes have been developed to meet these demands. Tha authors have previously reported on Sn-Cu plating processes that can provide more stable properties. 1 Using Sn-Cu plating and Sn plating, a Cu6Sn5 intermetallic compound layer with low roughness formed just below the surface. A pure Sn layer is formed on it. The authors have previously reported that the formation of a pure Sn layer with high connection reliability and a stable hard intermetallic compound layer can be expected to improve various properties. In this study, as a result of investigating the solder wettability of this plating film, which has excellent low friction coefficient and low contact resistance, it was clarified that good connector characteristics can be obtained.

Impact of different aging conditions on the IMC layer growth and shear strength of Ni-modified MWCNTs reinforced Sn-Ag-Cu composite solder

PurposeThe purpose of this paper is to develop a new composite solder to improve the reliability of composite solder joints. Nano-particles modified multi-walled carbon nanotubes (Ni-MWCNTs) can indeed improve the microstructure of composite solder joints and improve the reliability of solder joints. Although many people have conducted in-depth research on the composite solder of Ni-MWCNTs. However, no one has studied the performance of Ni-MWCNTs composite solder under different aging conditions. In this article, Ni-MWCNTs was added to Sn-Ag-Cu (SAC) solder, and the physical properties of composite solder, the microstructure and mechanical properties were evaluated.Design/methodology/approachIn this study, the effect of different aging conditions on the intermetallic compound (IMC) layer growth and shear strength of Ni-modified MWCNTs reinforced SAC composite solder was studied. Compared with SAC307 solder alloy, the influence of Ni-MWCNTs with different contents (0, 0.1 and 0.2 Wt.%) on composite solder was examined. To study the aging characteristics of composite solder joints, the solder joints were aged at 80°C, 120°C and 150°C.FindingsThe experimental results show that the content of Ni-MWCNTs affects the morphology and growth of the IMC layer at the interface. The microhardness of the solder increases and the wetting angle decreases. After aging at moderate (120°C) and high temperature (150°C), the morphology of the Cu6Sn5 IMC layer changed from scallop to lamellar and the grain size became coarser. The following two different phase compositions were observed in the solder joints with Ni-MWCNTs reinforcement: Cu3Sn and (Cu, Ni)6Sn5. The fracture surface of the solder joints all appeared ductile dents, and the size of the pits increased significantly with the increase of the aging temperature. Through growth kinetic analysis, Ni-modified MWCNTs in composite solder joints can effectively inhibit the diffusion of atoms in solder joints. In short, when the addition amount of Ni-MWCNTs is 0.1 Wt.%, the solder joints exhibit the best wettability and the highest shear strength.Originality/valueIn this study, the effects of aging conditions on the growth and shear strength of the IMC layer of Ni modified MWCNTs reinforced SAC307 composite solder were studied. The effects of Ni MWCNTs with different contents (0, 0.1 and 0.2 Wt.%) on the composite solder were examined.

Investigation of microstructure, thermal properties, and mechanical performances of Ni-added Sn-5.0Sb-0.5Cu/Cu solder joints

Small amounts of Ni were added to Sn-5.0Sb-0.5Cu (SSC505) solder alloy. The added Ni slightly increased the melting temperature and slightly reduced the melting range of the alloy. The microstructure of the SSC505 solder included a β-Sn phase, and SbSn and Cu6Sn5 intermetallic compound (IMC) phases distributed in the solder matrix. The addition of Ni produced a new (Cu0.7,Ni0.3)6Sn5 IMC phase with a mixed morphology comprising hexagonal and flake-shaped phases. The addition of 0.1wt.%Ni increased the ultimate tensile strength (UTS) and hardness of the SSC505 solder to 43.31 MPa and 17.52 HV, respectively, but the addition of 0.2wt.%Ni increased the size and number of the IMC phases, which weakened both properties. The interfacial layers of SSC505/Cu solder joints were composed of Cu6Sn5 and Cu3Sn IMC layers. The addition of Ni inhibited the growth of the Cu3Sn IMC layers, but produced a new, thicker, dendritic IMC layer identified by energy dispersive X-ray spectroscopy (EDS) as (Cu0.96,Ni0.04)6Sn5. As a result, the shear strength of the SSC505-0.2Ni/Cu solder joint was low. This work showed that the microstructure and mechanical performances of both a SSC505 solder alloy and SSC505/Cu solder joint were improved by the addition of 0.1wt.%Ni.

Influences of silicon carbide nanowires’ addition on IMC growth behavior of pure Sn solder during solid–liquid diffusion

In this paper, various mass fractions (0, 0.2, 0.4, 0.6, 0.8, 1.0 wt%) of silicon carbide nanowires (SiC NWs) were incorporated into pure Sn solder to enhance the performances of Sn solder joint. The wetting behavior, shear strength, and intermetallic compound (IMC) growth mechanism of Sn–xSiC/Cu solder during solid–liquid diffusion at 250 °C were investigated systematically. The experimental results demonstrated that the wettability of Sn–xSiC/Cu solder had a significant improvement when the addition of SiC NWs was up to 0.6 wt%, and excessive additives would degrade the wettability of the composite solder. The formation of the Cu6Sn5 IMC layer was observed at the Sn–xSiC solder/Cu interface. Meanwhile, SiC NWs as additives were conducive to restraining the growth of interfacial IMC during soldering process and the IMC thickness overtly fell down after doping 0.8 wt% SiC NWs into Sn solder. Moreover, SiC NW addition would contribute to enhancing the mechanical performance of Sn solder joint. The fracture mechanism of solder joint changed from a mix of brittle and ductile fracture to a characteristic of typical ductile fracture.

Effect of Graphene Nanosheet Addition on the Wettability and Mechanical Properties of Sn-20Bi-xGNS/Cu Solder Joints.

Graphene nanosheets (GNSs) have an extensive application in materials modification. In this study, the effects of graphene nanosheets on the wettability of Sn-20Bi lead-free solder on copper (Cu) substrate and the growth behavior of intermetallic compound (IMC) layers at Sn-20Bi-xGNS/Cu solder joints were investigated. The experimental results indicate that the wettability of Sn-20Bi solder firstly diminished and then increased by the addition of GNSs. Meanwhile, a prism-shaped and scallop-shaped Cu6Sn5 IMC layer was clearly observed at the interface of the solder/substrate system. Moreover, it was found that a small amount of GNS addition can significantly inhibit the growth of the IMC layer at the interface as well as refine the microstructure. Additionally, by nano-indentation apparatus, it can be concluded that the hardness and elastic module of IMCs show the same variation trend, which firstly decreased and then increased. Besides, to intuitively demonstrate the reliability of IMCs, the relationship between the hardness and elastic module was established, and the ratio of hardness/elastic module (H/E) was adopted to characterize the reliability of IMCs. The results show that when the addition of GNSs was 0.02 wt%, the value of H/E is the minimum and the solder joint has the highest reliability.

Effect of Ni(P) thickness in Au/Pd/Ni(P) surface finish on the electrical reliability of Sn–3.0Ag–0.5Cu solder joints during current-stressing

The effects of Ni(P) layer thickness (5 μm and 0.7 μm) on the microstructural behavior and electrical reliability of electroless-nickel electroless-palladium immersion gold (ENEPIG) (substrate-side) surface-finished printed circuit boards (PCBs) with Sn–3.0Ag–0.5Cu (SAC305) solder joints under current stressing of 9000 A/cm2 have been investigated. An organic solderability preservative (OSP) surface finish was applied on the chip-side. (Cu,Ni)6Sn5 and Cu6Sn5 intermetallic compound (IMC) layers were formed on the chip-side (the interface between SAC305 and the OSP surface finish) and substrate-side (the interface between the ENEPIG surface finish and SAC305) solder joints after reflow. The thicknesses of the (Cu,Ni)6Sn5 and Cu6Sn5 IMC layers of the chip- and substrate-side of the normal- and thin-ENEPIG/SAC305/OSP solder joints increased with increasing current stressing time, regardless of the nickel phosphorous (Ni(P)) layer thickness. The total IMC thicknesses of normal-ENEPIG/SAC305/OSP solder joints were relatively thinner than those of thin-ENEPIG/SAC305/OSP solder joints under current stressing for 50–120 h. This is the reason why only a P-rich Ni layer was formed at the interface between SAC305 solder and the Cu pad in the thin-ENEPIG/SAC305 solder joints under current stressing. Otherwise, the P-rich Ni and Ni(P) layers remained at the interface of the normal-ENEPIG/SAC305 solder joint under current stressing. The Ni(P) layer of the ENEPIG surface finish played an important diffusion barrier role by suppressing IMC growth and movement toward the SAC305 solder under current stressing. In the electrical evaluation, the time to failure at the normal-ENEPIG solder joint was relatively longer (approximately 2.2 times) than that of the thin-ENEPIG solder joint. Therefore, the relatively thick Ni(P) layer contained in the ENEPIG/SAC305/OSP solder joint is expected to attain higher electrical reliability under the electromigration test.

Effects of Joint Height on the Interfacial Microstructure and Mechanical Properties of Cu-Cored SAC305 Solder Joints

In order to discuss the effects of Cu-cored solder with pure Ni coating and joint height on the interfacial microstructure and mechanical properties of joints, Cu/Sn-3.0Ag0.5Cu (SAC305)/Cu and Cu/Cu-cored + SAC305/Cu sandwich solder joints were prepared using a reflow process. The interfacial microstructure of the solder joints was investigated by scanning electron microscope with energy-dispersive x-ray spectroscopy. The results showed that, at as-reflowed joints, an intermetallic compound (IMC) of scallop-like Cu6Sn5 was formed at the Cu wire/solder interface, and a plane-like (Cux, &!nbsp;Ni1−x)6Sn5 IMC was formed at the Cu core/solder interface. Compared with SAC305 solder joints, Cu-cored solder joints showed more interface layers, a thicker Cu6Sn5 IMC layer, and higher tensile strength. With increasing joint height, the thickness of the IMC at the two interfaces decreased, and the tensile strength also decreased for Cu-cored solder joints. Mixed brittle/ductile fracture appeared in SAC305 and Cu-cored solder joints with height of 600 μm, but were suppressed and transformed into ductile fractures with increasing height of the solder joints.

Influence of Substrate Surface Finish Metallurgy on Lead-Free Solder Joint Microstructure with Implications for Board-Level Reliability

Solder joints can experience fatigue cracking at the interface between the solder and substrate during thermal cycling due to the difference in thermal expansion between joined components. In order to strengthen the interfacial region and prevent cracking, two Cu pad surface treatments were studied on Sn-3Ag-0.5Cu solder, and on matte Sn plate and Ni plate coatings. The resulting intermetallic microstructures were characterized using scanning electron microscopy and energy-dispersive spectroscopy. Mechanical testing was performed using nano-indentation on the solder joints. The matte Sn plate on Cu sample formed an uneven distribution of Ag3Sn particles and a planar Cu6Sn5 interfacial intermetallic. The Ni-plated sample formed a needle-like Ni-rich interfacial intermetallic and uniform dispersion of Ag3Sn particles. The rough intermetallic compound (IMC)/solder interface and even IMC particle distribution cause the Ni-plated sample to experience reduced damage under board-level thermomechanical cycling because the interfacial structure reduces the Sn matrix recrystallization that contributes to fatigue cracking. In contrast, the matte Sn-plated sample exhibited an Ag3Sn particle-free zone adjacent to a planar Cu6Sn5 IMC layer which allows for rapid Sn recrystallization and fatigue crack propagation.

Microstructural analysis of Sn-3.0Ag-0.5Cu-TiO2 composite solder alloy after selective electrochemical etching

This work aims to provide deep morphological observation on the incorporated TiO2 nanoparticles within the SAC305 by selective electrochemical etching. Cyclic voltammetry and chronoamperometry were used to investigate the selective etching performances. The removal of β-Sn matrix was conducted at a fixed potential of −350 mV. Average performances of 2.19 and 2.30 mA were attained from the chronoamperometry. The efficiency of β-Sn removal was approved according to the reduction of the intensities on the phase analysis. Successful observation of the TiO2 near the Cu6Sn5 layer was attained for an optimum duration of 120 s. Clusters of TiO2 nanoparticles were entrapped by Cu6Sn5 and Ag3Sn intermetallic compound (IMC) layer network and at the solder/substrate interface. The presence of TiO2 nanoparticles at the solder interface suppresses the growth of the Cu6Sn5 IMC layer. The absence of a β-Sn matrix also allowed in-depth morphological observations to be made of the shape and size of the Cu6Sn5 and Ag3Sn. It was found that TiO2 content facilitates the β-Sn removal, which allows better observation of the IMC phases as well as the TiO2 reinforcement particles.

Impact of different isothermal aging conditions on the IMC layer growth and shear strength of MWCNT-reinforced Sn–5Sb solder composites on Cu substrate

In this study, the effect of multi-walled carbon nanotubes (MWCNTs) reinforcement on the intermetallic compound (IMC) layer growth and shear strength of Sn–5Sb-xCNT/Cu composite solder joints subjected to different isothermal aging conditions has been investigated. A series of plain and composite lead-free solder systems (Sn–5Sb-xCNT; x = 0, 0.01, 0.05 and 0.1 wt%) was successfully developed through the powder metallurgy method. Isothermal aging process was performed on the solder/Cu joints at 120 °C, 150 °C and 170 °C temperatures in order to investigate the evolution of the interfacial IMC layers and the shear strength property. Experimental results showed that the thickness of the total IMC layer increased with rising aging temperature. While the Cu3Sn IMC maintained a layer-type morphology for all aging conditions, morphology of the Cu6Sn5 IMC transformed from a scallop-type to a layer-type after aging at intermediate (150 °C) and high (170 °C) temperatures. Given the potential of MWCNTs as a reinforcement material, significant suppression in IMC layer growth was demonstrated by the composite solder joints relative to the plain counterpart. Comprehensive investigation on the growth kinetics showed that the presence of MWCNTs in the composite solder joints was effective in slowing down the diffusion mechanism responsible for IMC growth. Overall, the Sn–5Sb-0.05CNT composite solder joint exhibited the lowest diffusion coefficient within the range of 0.09 × 10−14- 1.03 × 10−14 cm2/s and 0.95 × 10−14- 9.8 × 10−14 cm2/s for Cu6Sn5 and Cu3Sn IMC layers respectively. Moreover, the strengthening effect of the MWCNTs reinforcement was well marked in the composite solder joints as the maximum shear strength within a range of 24.2–15.2 MPa was exhibited by the Sn-0.01CNT/Cu composite solder joint subjected to reflow soldering and isothermal aging conditions.

Significant Inhibition of IMCs Growth between an Electroless Ni-W-P Metallization and SAC305 Solder During Soldering and Aging

The formation and growth behavior of intermetallic compound (IMC) layers after introducing an electroless Ni-W-P metallization into the Sn-3.0Ag-0.5Cu (SAC305) solder joint during soldering and aging were investigated. The soldering was performed at 250 °C for 10 min, followed by air cooling and aging treatment at 150 °C up to 15 days. The results show that the scallop-like Cu6Sn5 IMC layer and planar-like Cu3Sn formed between solder and Cu substrate during soldering and aging. The Ni3P and (Ni,Cu)3Sn4 compounds were formed between electroless Ni-W-P layer and solder, and Cu substrate was not damaged and kept a smooth interface. When the isothermal aging treatment was applied, the total thickness of IMCs which formed at the SAC305/Cu and SAC305/Ni-W-P/Cu interface increased with increasing aging time. Kirkendall voids emerged at the Cu3Sn and the Ni3P layers, but the voids emerged at the Ni3P layer in the form of crack. The amount of Kirkendall voids increased and the crack elongated with increasing aging time. The Cu6Sn5 and (Ni,Cu)3Sn4 grains grew by merging adjacent grains. In the process of growth, the growing interfacial compounds filled the free space, and new columnar dendrite grain of (Ni,Cu)3Sn4 constantly generated during aging treatment. After 15 days aging, the Ni-W-P barrier layer was still remained, which indicated that the Ni-W-P layer can be a good barrier layer between the solder alloys and Cu substrate.

Effects of Ni nanoparticles addition on the microstructure, electrical and mechanical properties of Sn-Ag-Cu alloy

The effects of Ni nanoparticles addition on the microstructure, electrical resistivity and mechanical properties such as elastic/shear moduli, microhardness and creep of Sn-3.0Ag-0.5Cu (SAC305; in wt.%) electronic interconnect materials have been investigated. Microstructure analysis of Sn-3.0Ag-0.5Cu-0.5 Ni nanoparticles (in wt.%) solder revealed that adding Ni nanoparticles has promoted the formation and growth of (Cu, Ni)-Sn intermetallic compound (IMC) in the bulk phase of the samples and the structure of composite material resulted refined. This finer structure and (Cu, Ni)-Sn IMCs in a composite solder resulted in improved mechanical and electrical properties with respect to the Sn-3Ag-0.5Cu reference alloy. Indeed, a comparison between the reference alloy and composite material doped with 0.5 wt.% Ni nanoparticles indicates that the improvement in elastic and shear modulus was 8% and 11.2%, respectively, whereas the microhardness value was increased about 16.7%. Moreover, in the electronic interconnect systems on organic solderability preservative (OSP)-Cu pad, a scallop-shaped Cu6Sn5 IMC was detected at their interfaces at early stage reaction. Increasing the reaction time, the growth of the Cu6Sn5 and the formation of a very thin Cu3Sn IMC layer was observed. However, in Sn-3.0Ag-0.5Cu-0.5 Ni nanoparticles composite material, a very stable (Cu, Ni)-Sn IMC grew at the interface between composite solder and (OSP)-Cu pad in absence of Cu3Sn IMC. In addition, a relatively fine Ag3Sn and (Cu, Ni)-Sn IMC appeared to be evenly dispersed in β-Sn matrix and the fine microstructure surely affect the overall properties of solder joints.

Atomic migration on Cu in Sn-58Bi solder from the interaction between electromigration and thermomigration

This paper mainly studied the atomic migration in molten Sn-58Bi solder joints induced by the thermo-electric coupling effect. Individual electromigration (EM), individual thermomigration (TM) and the coupling EM with TM were conducted to confirm the prominent diffusing species and its migrating behaviors in the solder, respectively. During EM, step loading with different current densities was designed to verify that there existed a transition on the prominent diffusing species when Sn-58Bi solder melted. Cu atoms from the cathode were driven towards the anode to produce thicker Cu6Sn5 intermetallic compound (IMC) layer at the anode interface under current stressing. During TM, it was found that the main migrating species was also Cu atoms which were driven towards the cold end to form thicker Cu6Sn5 IMC layer at the cold interface under temperature gradient. Finally, thermo-electric coupling was realized by superimposing TM on EM with current density of 1.0 × 104 A cm−2. TM played the assisting or counteracting effect on the Cu migration occurred in EM when the anode was connected with cold end or hot end. With anode on hot end, TM played the reverse effect on the migration of Cu atoms, which prevented Cu atoms to be accumulated at the anode/hot interface. With anode on cold end, TM played the accumulative effect on the migration of Cu atoms, which led to a thicker IMC layer at the anode/cold interface.

Investigation and Optimization of Ultrathin Buffer Layers Used in Cu/Sn Eutectic Bonding

Ultrathin buffer layers (UBLs) with varied thickness ranging from 10 to 100 nm and different materials were used in Cu/Sn eutectic bonding. A Cu/Sn film thinner than $2~\mu \text{m}$ could fully react and became stiff and rough Cu-Sn intermetallic compound layer, which leads to failure bonding. Four kinds of semiconductor compatible materials including Ti, Pd, Co, and Ni were inserted between Cu/Sn to delay interdiffusion prior to eutectic bonding. In addition to symmetric Cu/Sn bonding with UBL, asymmetric Cu/Sn-Cu bonding scheme with 50-nm Ni UBL was demonstrated. With good mechanical properties, bonding quality, and electrical characteristics, the application of submicrometer Cu/Sn wafer-level bonding by assistance of buffering layer gives a promising and flexible platform for future 3-D integration applications.

Effects of thermal aging on growth behavior of interfacial intermetallic compound of dip soldered Sn/Cu joints

Intermetallic compounds (IMCs) formations of Sn/Cu reaction couple prepared by dip soldering were studied with isothermal aging at four levels of temperatures (120, 150, 170 and 200 °C, respectively). It is found that an obvious scallop-type layer of Cu6Sn5 IMC formed between the liquid Sn solder and Cu substrates and a planar Cu3Sn IMC layer between Cu6Sn5 IMC layer and Cu substrates appears after solid-state aging treatment. A significant increment of IMC layer thickness is observed with increasing aging time as well as aging temperature. And it is further found that the thickness of IMC layer is linearly related to the square root of aging time during the aging process. The values of growth rate constants for the different solid state aging temperatures (120, 150, 170 and 200 °C) were calculated as 6.578 × 10−18, 1.595 × 10−17, 9.89 × 10−17 and 4.521 × 10−16 m2 s−1, respectively. The apparent activation energy calculated for the growth of the total IMC is 84.43 kJ mol−1. Moreover, The mean diameters of the Cu6Sn5 grains increased linearly with cubic root of aging time.

Effect of Ni addition to the Cu substrate on the interfacial reaction and IMC growth with Sn3.0Ag0.5Cu solder

The formation and growth of intermetallic compound (IMC) layer at the interface between Sn3.0Ag0.5Cu (SAC305) solder and Cu–xNi (x = 0, 0.5, 1.5, 5, 10 wt%) substrate during reflowing and aging were investigated. The soldering was conducted at 270 °C using reflowing method, following by aging treatment at 150 °C for up to 360 h. The experimental results indicated that the total thickness of IMC increased with increasing aging time. The scallop-like Cu6Sn5 and planar-like Cu3Sn IMC layer were observed between SAC305 solder and purely Cu substrate. As the content of Ni element in Cu substrate was 0.5% or 1.5%, the scallop-like Cu6Sn5 and planar-like Cu3Sn IMC layer were still found between solder and Cu–Ni substrate and the total thickness of IMC layer decreased with the increasing Ni content. Besides, when the Ni content was up to 5%, the long prismatic (Cu,Ni)6Sn5 phase was the only product between solder and substrate and the total thickness of IMC layer increased significantly. Interestingly, the total thickness of IMC decreased slightly as the Ni addition was up to 10%. In the end, the grains of interfacial IMC layer became coarser with aging time increasing while the addition of Ni in Cu substrate could refine IMC grains.

Interfacial Reaction and Mechanical Properties of Sn-Bi Solder joints

Sn-Bi solder with different Bi content can realize a low-to-medium-to-high soldering process. To obtain the effect of Bi content in Sn-Bi solder on the microstructure of solder, interfacial behaviors in solder joints with Cu and the joints strength, five Sn-Bi solders including Sn-5Bi and Sn-15Bi solid solution, Sn-30Bi and Sn-45Bi hypoeutectic and Sn-58Bi eutectic were selected in this work. The microstructure, interfacial reaction under soldering and subsequent aging and the shear properties of Sn-Bi solder joints were studied. Bi content in Sn-Bi solder had an obvious effect on the microstructure and the distribution of Bi phases. Solid solution Sn-Bi solder was composed of the β-Sn phases embedded with fine Bi particles, while hypoeutectic Sn-Bi solder was composed of the primary β-Sn phases and Sn-Bi eutectic structure from networked Sn and Bi phases, and eutectic Sn-Bi solder was mainly composed of a eutectic structure from short striped Sn and Bi phases. During soldering with Cu, the increase on Bi content in Sn-Bi solder slightly increased the interfacial Cu6Sn5 intermetallic compound (IMC)thickness, gradually flattened the IMC morphology, and promoted the accumulation of more Bi atoms to interfacial Cu6Sn5 IMC. During the subsequent aging, the growth rate of the IMC layer at the interface of Sn-Bi solder/Cu rapidly increased from solid solution Sn-Bi solder to hypoeutectic Sn-Bi solder, and then slightly decreased for Sn-58Bi solder joints. The accumulation of Bi atoms at the interface promoted the rapid growth of interfacial Cu6Sn5 IMC layer in hypoeutectic or eutectic Sn-Bi solder through blocking the formation of Cu6Sn5 in solder matrix and the transition from Cu6Sn5 to Cu3Sn. Ball shear tests on Sn-Bi as-soldered joints showed that the increase of Bi content in Sn-Bi deteriorated the shear strength of solder joints. The addition of Bi into Sn solder was also inclined to produce brittle morphology with interfacial fracture, which suggests that the addition of Bi increased the shear resistance strength of Sn-Bi solder.