Interfacial Reactions of Sn-3.5Ag-<Emphasis Type="Italic">x</Emphasis>Zn Solders and Cu Substrate During Liquid-State Aging

The effects of Zn (1 wt.%, 3 wt.%, and 7 wt.%) additions to Sn-3.5Ag solder and various reaction times on the interfacial reactions between Sn-3.5Ag-xZn solders and Cu substrates a during liquid-state aging were investigated in this study. The composition and morphological evolution of interfacial intermetallic compounds (IMCs) changed significantly with the Zn concentration and reaction time. For the Sn-3.5Ag-1Zn/Cu couple, CuZn and Cu₆Sn₅ phases formed at the interface. With increasing aging time, the Cu₆Sn₅ IMC layer grew thicker, while the CuZn IMC layer drifted into the solder and decomposed gradually. Cu₅Zn₈ and Ag₅Zn₈ phases formed at the interfaces of Sn-3.5Ag-3Zn/Cu and Sn-3.5Ag-7Zn/Cu couples. With increasing reaction time, the Cu₅Zn₈ layer grew and Cu atoms diffused from the substrate to the solder, which transformed the Ag₅Zn₈ to (Cu,Ag)₅Zn₈. The Cu₆Sn₅ layer that formed between the Cu₅Zn₈ layer and Cu was much thinner at the Sn-3.5Ag-7Zn/Cu interface than at the Sn-3.5Ag-3Zn/Cu interface. Additionally, we measured the thickness of interfacial IMC layers and found that 3 wt.% Zn addition to the solder was the most effective for suppressing IMC growth at the interfaces.     

Effects of Co Addition on Bulk Properties of Sn-3.5Ag Solder and Interfacial Reactions with Ni-P UBM

The effects of Co addition on the undercooling, microstructure, and microhardness of Sn-3.5Ag solder (all in wt.% unless specified otherwise) and interfacial reactions with Ni-P under bump metallurgy (UBM) are investigated when the Co content varies from 0.01 wt.% to 0.7 wt.%. When more than 0.02 wt.% Co was added to Sn-3.5Ag solder, the undercooling of the Sn-3.5Ag solder was significantly reduced and the microstructures coarsened with the increased eutectic region. In addition, the hardness value increased as the Co content in Sn-3.5Ag increased. In the interfacial reactions with Ni-P UBM, a spalling phenomenon of intermetallic compounds (IMCs) during reflow was prevented in the Sn-3.5Ag-xCo (x ≥ 0.02 wt.%). However, when more than 0.05 wt.% Co was added to Sn-3.5Ag, the IMC morphology changed from a bulky shape to a plate-like shape. The bulky IMCs were Ni₃Sn₄ and the plate-like IMCs were Sn-Ni-Co ternary compounds. The main issues discussed include the relations between the morphological changes and the IMC phases, the effects of Co addition on the prevention of IMC spalling, and the optimum level of Co addition.     

Investigation of Dissolution Behavior of Metallic Substrates and Intermetallic Compound in Molten Lead-free Solders

This study investigates the dissolution behavior of the metallic substrates Cu and Ag and the intermetallic compound (IMC)-Ag₃Sn in molten Sn, Sn-3.0Ag-0.5Cu, Sn-58Bi and Sn-9Zn (in wt.%) at 300, 270 and 240°C. The dissolution rates of both Cu and Ag in molten solder follow the order Sn > Sn-3.0Ag-0.5Cu >Sn-58Bi > Sn-9Zn. Planar Cu₃Sn and scalloped Cu₆Sn₅ phases in Cu/solders and the scalloped Ag₃Sn phase in Ag/solders are observed at the metallic substrate/solder interface. The dissolution mechanism is controlled by grain boundary diffusion. The planar Cu₅Zn₈ layer formed in the Sn-9Zn/Cu systems. AgZn₃, Ag5Zn₈ and AgZn phases are found in the Sn-9Zn/Ag system and the dissolution mechanism is controlled by lattice diffusion. Massive Ag₃Sn phases dissolved into the solders and formed during solidification processes in the Ag₃Sn/Sn or Sn-3.0Ag-0.5Cu systems. AgZn₃ and Ag₅Zn₈ phases are formed at the Sn-9Zn/Ag₃Sn interface. Zn atoms diffuse through Ag-Zn IMCs to form (Ag, Zn)Sn₄ and Sn-rich regions between Ag₅Zn₈ and Ag₃Sn.     

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.     

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.     

Interfacial Reactions and Joint Strengths of Sn-<Emphasis Type="Italic">x</Emphasis>Zn Solders with Immersion Ag UBM

The solder joint microstructures of immersion Ag with Sn-xZn (x = 0 wt.%, 1 wt.%, 5 wt.%, and 9 wt.%) solders were analyzed and correlated with their drop impact reliability. Addition of 1 wt.% Zn to Sn did not change the interface microstructure and was only marginally effective. In comparison, the addition of 5 wt.% or 9 wt.% Zn formed layers of AgZn₃/Ag₅Zn₈ at the solder joint interface, which increased drop reliability significantly. Under extensive aging, Ag-Zn intermetallic compounds (IMCs) transformed into Cu₅Zn₈ and Ag₃Sn, and the drop impact resistance at the solder joints deteriorated up to a point. The beneficial role of Zn on immersion Ag pads was ascribed to the formation of Ag-Zn IMC layers, which were fairly resistant to the drop impact, and to the suppression of the brittle Cu₆Sn₅ phase at the joint interface.     

Interfacial microstructure and bonding strength of Sn-3Ag-0.5Cu and Sn-3Ag-0.5Cu-0.5Ce-xZn solder BGA packages with immersion Ag surface finish

Although it has been verified that tin whiskers can be prevented by the addition of 0.5 wt.% Zn into a Sn-3Ag-0.5Cu-0.5Ce solder, no detailed studies have been conducted on interfacial reactions and mechanical properties of Sn-3Ag-0.5Cu-0.5Ce-xZn solder joints with an immersion Ag surface finish. The intermetallic compounds formed during the reflow and aging of Sn-3Ag-0.5Cu and Sn-3Ag-0.5Cu-0.5Ce-xZn solder ball grid array (BGA) packages were investigated. Because more heterogeneous nucleation sites, provided by CeSn₃ intermetallics and Zn atoms, formed in the Sn-3Ag-0.5Cu-0.5Ce-xZn solder matrix, and Cu and Zn have a stronger affinity than Cu and Sn, the Cu-Sn intermetallics growth in Sn-3Ag-0.5Cu-0.5Ce-xZn solder joints with Ag/Cu pads was suppressed. The 0.2% Zn addition for inhibiting rapid whisker growth in RE-doped Sn-Ag-Cu solder joints is more appropriate than 0.5 wt.% additions, as excess Zn addition causes poor oxidation resistance and inferior bonding strength.     

The influence of solder composition on the impact strength of lead-free solder ball grid array joints

This study aims to investigate the shear and tensile impact strength of solder ball attachments. Tests were conducted on Ni-doped and non-Ni-doped Sn-0.7wt.% Cu, Sn-37wt.% Pb and Sn-3.0wt.% Ag-0.7wt.% Cu solder ball grid arrays (BGAs) placed on Cu substrates, which were as-reflowed and aged, over a wide range of displacement rates from 10 to 4000 mm/s in shear and from 1 to 400 mm/s in tensile tests. Ni additions to the Sn-0.7wt.% Cu solders has slowed the growth of the interface intermetallic compounds (IMCs) and made the IMC layer morphology smooth. As-reflowed Ni-doped Sn-0.7wt.% Cu BGA joints show superior properties at high speed shear and tensile impacts compared to the non-Ni-doped Sn-0.7wt.% Cu and Sn-3.0wt.% Ag-0.7wt.% Cu BGAs. Sn-3.0wt.% Ag-0.7wt.% Cu BGAs exhibit the least resistance in both shear and tensile tests among the four compositions of solders, which may result from the cracks in the IMC layers introduced during the reflow processes.     

Effect of Ag on the Microstructure of Sn-8.5Zn-xAg-0.01Al-0.1Ga Solders Under High-Temperature and High-Humidity Conditions

The effect of Ag on the microstructure and thermal behavior of Sn-Zn and Sn-8.5Zn-xAg-0.01Al-0.1Ga solders (x from 0.1 wt.% to 1 wt.%) under high-temperature/relative humidity conditions (85°C/85% RH) for various exposure times was investigated. Scanning electron microscopy (SEM) studies revealed that, in all the investigated solders, the primary α-Zn phases were surrounded by eutectic β-Sn/α-Zn phases, in which fine Zn platelets were dispersed in the β-Sn matrix. SEM micrographs revealed that increase of the Ag content to 1 wt.% resulted in coarsening of the dendritic plates and diminished the Sn-9Zn eutectic phase in the microstructure. Differential scanning calorimetry (DSC) studies revealed that the melting temperature of Sn-8.5Zn-xAg-0.01Al-0.1Ga solder decreased from 199.6°C to 199.2°C with increase of the Ag content in the solder alloy. Both ZnO and SnO₂ along with Ag-Zn intermetallic compound (IMC) were formed on the surface when Sn-8.5Zn-0.5Ag-0.01Al-0.1Ga solder was exposed to high-temperature/high-humidity conditions (85°C/85% RH) for 100 h. The thickness of the ZnO phase increased as the Ag content and exposure time were increased. Sn whiskers of various shapes and lengths varying from 2 μm to 5 μm were extruded from the surface when the investigated five-element solder with Ag content varying from 0.5 wt.% to 1 wt.% was exposed to similar temperature/humidity conditions for 250 h. The length and density of the whiskers increased with further increase of the exposure time to 500 h and the Ag content in the solder to 1 wt.%. The Sn whisker growth was driven by the compressive stress in the solder, which was generated due to the volume expansion caused by ZnO and Ag-Zn intermetallic compound formation at the grain boundaries of Sn.     

Morphology and growth kinetics of intermetallic compounds in solid-state interfacial reaction of electroless Ni-P with Sn-based lead-free solders

A comparative study of solid/solid interfacial reactions of electroless Ni-P (15 at.% P) with lead-free solders, Sn-0.7Cu, Sn-3.5Ag, Sn-3.8Ag-0.7Cu, and pure Sn, was carried out by performing thermal aging at 150°C up to 1000 h. For pure Sn and Sn-3.5Ag solder, three distinctive layers, Ni₃Sn₄, SnNiP, and Ni₃P, were observed in between the solder and electroless Ni-P; while for Sn-0.7Cu and Sn-3.8Ag-0.7Cu solders, two distinctive layers, (CuNi)₆Sn₅ and Ni₃P, were observed. The differences in morphology and growth kinetics of the intermetallic compounds (IMCs) at the interfaces between electroless Ni-P and lead-free solders were investigated, as well as the growth kinetics of the P-enriched layers underneath the interfacial IMC layers. With increasing aging time, the coarsening of interfacial Ni₃Sn₄ IMC grains for pure Sn and Sn-3.5Ag solder was significantly greater than that of the interfacial (CuNi)₆Sn₅ IMC grains for Sn-0.7Cu and Sn-3.8Ag-0.7Cu solders. Furthermore, the Ni content in interfacial (CuNi)₆Sn₅ phase slightly increased during aging. A small addition of Cu (0.7 wt.%) resulted in differences in the type, morphology, and growth kinetics of interfacial IMCs. By comparing the metallurgical aspects and growth kinetics of the interfacial IMCs and the underneath P-enriched layers, the role of initial Cu and Ag in lead-free solders is better understood.     

Effect of Aluminum Concentration on the Interfacial Reactions of Sn-3.0Ag-<Emphasis Type="Italic">x</Emphasis>Al Solders with Copper and ENIG Metallizations

Aluminum was added into Sn-3.0Ag (wt.%) solder to investigate the effect of aluminum concentration on the interfacial reaction of Sn-3.0Ag-xAl solders with copper or electroless nickel immersion gold (ENIG) metallizations. Four different Sn-3.0Ag-xAl solders (x = 0 wt.%, 0.1 wt.%, 0.5 wt.%, and 1.0 wt.%) were used for comparison. It was found that the composition, morphology, and thickness of interfacial reaction products were strongly dependent on aluminum concentration. At low aluminum concentration (0.1 wt.%), the typical Cu₆Sn₅ layer was formed at the interface. When the aluminum concentration was 0.5 wt.%, a continuous CuAl₂ layer spalled off from the interfacial Cu-Sn intermetallic compound (IMC) layer. Only a planar CuAl₂ layer was observed at the interface when the aluminum concentration was increased to 1.0 wt.%. In Sn-Ag-Al/ENIG reactions, Ni₃Sn₄ was formed and spallation occurred near the interface in the Sn-3.0Ag and Sn-3.0Ag-0.1Al solder joints. When the aluminum concentration was higher than 0.1 wt.%, a thin planar AuAl compound formed at the interface. There was no P-rich phase formation that retarded the spalling phenomenon. The aluminum additive in Sn-Ag solder inhibited the growth of IMCs in the reaction with copper or ENIG metallizations, which was favorable for the reliability of solder joints.     

Mechanical Properties and Electrochemical Corrosion Behavior of Al/Sn-9Zn-<Emphasis Type="Italic">x</Emphasis>Ag/Cu Joints

The effect of Ag content on the wetting behavior of Sn-9Zn-xAg on aluminum and copper substrates during soldering, as well as the mechanical properties and electrochemical corrosion behavior of Al/Sn-9Zn-xAg/Cu solder joints, were investigated in the present work. Tiny Zn and coarsened dendritic AgZn₃ regions were distributed in the Sn matrix in the bulk Sn-9Zn-xAg solders, and the amount of Zn decreased while that of AgZn₃ increased with increasing Ag content. The wettability of Sn-9Zn-1.5Ag solder on Cu substrate was better than those of the other Sn-9Zn-xAg solders but worse than that of Sn-9Zn solder. The wettability of Sn-9Zn-1.5Ag on the Al substrate was also better than those of the other Sn-9Zn-xAg solders, and even better than that of Sn-9Zn solder. The Al/Sn-9Zn/Cu joint had the highest shear strength, and the shear strength of the Al/Sn-9Zn-xAg/Cu (x = 0 wt.% to 3 wt.%) joints gradually decreased with increasing Ag content. The corrosion resistance of the Sn-9Zn-xAg solders in Al/Sn-9Zn-xAg/Cu joints in 5% NaCl solution was improved compared with that of Sn-9Zn. The corrosion potential of Sn-9Zn-xAg solders continuously increased with increasing Ag content from 0 wt.% to 2 wt.% but then decreased for Sn-9Zn-3Ag. The addition of Ag resulted in the formation of the AgZn₃ phase and in a reduction of the amount of the eutectic Zn phase in the solder matrix; therefore, the corrosion resistance of the Al/Sn-9Zn-xAg/Cu joints was improved.     

Interfacial behaviors of Sn-Pb,Sn-Ag-Cu Pb-free and mixed Sn-Ag-Cu/Sn-Pb solder joints during electromigration

SnPb-SnAgCu mixed solder joints with Sn-Pb soldering Sn-Ag-Cu Pb-free components are inevitably occurred in the high reliability applications. In this study, the interfacial behaviors in Sn-37Pb and Sn-3.0Ag-0.5Cu mixed solder joints was addressed and compared with Sn-37Pb solder joints and Sn-3.0Ag-0.5Cu solder joints with the influence from isothermal aging and electromigration. Considering the difference on the melting point between Sn-3.0Ag-0.5Cu and Sn-37Pb solder, two mixed solder joints: partial mixing and full mixing between Sn-Pb and Sn-Ag-Cu solders were reached with the peak reflowing temperature of 190 and 250 °C, respectively. During isothermal aging, the intermetallic compound (IMC) layer increased with aging time and its growth was diffusion controlled. There was also no obvious affect from the solder composition on IMC growth. After electromigration with the current density of 2.0 × 10³ A/cm², Sn-37Pb solder joints showed the shortest lifetime with the cracks observed at the cathode for the stressing time < 250 h. In Sn-3.0Ag-0.5Cu Pb-free solder joints, current stressing promoted the growth of IMC layer at the interfaces, but the growing rate of IMC at the anode interface was far faster than that at the cathode interface. Therefore, there existed an obvious polarity effect on IMC growth in Sn-Ag-Cu Pb-free solder joints. After Sn-37Pb was mixed with Sn-3.0Ag-0.5Cu Pb-free solder, whether the partial mixing or the full mixing between Sn-Pb and Sn-Ag-Cu can obviously depress both the crack formation at the cathode side and the IMC growth at the anode.     

Influence of Ag micro-particle additions on the microstructure, hardness and tensile properties of Sn-9Zn binary eutectic solder alloy

In this study, an addition of Ag micro-particles (8-10 μm) with a content in the range between 0 and 1.5 wt.% to Sn-9Zn eutectic solder, were examined in order to understand the effect of Ag additions as the particulate reinforcement on the microstructural and mechanical properties as well as the thermal behavior of the newly developed composite solders. Here, an approach to prepare a micro-composite solder alloy by mixing Ag micro-particles with a molten Sn-Zn solder alloy was developed. The composite solder was prepared by mechanically mixing Ag micro-particles into the Sn-9Zn alloy melt to ensure a homogeneous distribution of the reinforcing particles. The distribution of the Ag micro-particles in the matrix was found to be fairly uniform. The Ag particles reacted with the Zn and formed ε-AgZn₃ intermetallic compounds (IMC) in the β-Sn matrix. It was found that the more Ag particles added to the Sn-9Zn solder, the more Ag-Zn compound formed. In the Sn-9Zn/XAg composite solder, the microstructure was composed of AgZn₃ IMC and α-Zn phase in the β-Sn matrix. Interestingly, as the Ag particles in the composite solder increased, the α-Zn phase was found to be depleted from the matrix. The average tensile strength of the composite solders increased with the Ag micro-particles content up to a certain limit. Beyond this limit, the addition of Ag particles actually decreased the strength.     

Spreading Behavior and Evolution of IMCs During Reactive Wetting of SAC Solders on Smooth and Rough Copper Substrates

The effect of surface roughness of copper substrate on the reactive wetting of Sn-Ag-Cu solder alloys and morphology of intermetallic compounds (IMCs) was investigated. The spreading behavior of solder alloys on smooth and rough Cu substrates was categorized into capillary, diffusion/reaction, and contact angle stabilization zones. The increase in substrate surface roughness improved the wetting of solder alloys, being attributed to the presence of thick Cu₃Sn IMC at the interface. The morphology of IMCs transformed from long needle shaped to short protruded type with an increase in the substrate surface roughness for the Sn-0.3Ag-0.7Cu and Sn-3Ag-0.5Cu solder alloys. However, for the Sn-2.5Ag-0.5Cu solder alloy the needle-shaped IMCs transformed to the completely scallop type with increase in the substrate surface roughness. The effect of Ag content on wetting behavior was not significant.     

The interfacial reaction between Sn-Zn-Ag-Ga-Al solders and metallized Cu substrates

The interfacial interaction between the Sn-8.55Zn-0.5Ag-0.5Ga-0.1Al solder and three kinds of metallized substrates (Cu, Cu/Au, and Cu/Ni-P/Au) does not form the Cu-Sn intermetallic compound (IMC). Continuous Cu-Zn and discontinuous Ag-Zn interfacial IMC layers formed between the Cu and Sn-Zn-Ag-Ga-Al solder, while Cu-Zn and Au-Al-Zn IMCs formed on the Cu/Au substrate. Only the Au-Al-Zn IMC formed at the interface when the electroless Ni-P deposit was the diffusion barrier between Cu and the Au surface layer.     

Effect of Ag Content on Solidification Cracking Susceptibility of Sn-Ag-Cu Solder Joints

In the present work, the effect of Ag content on solidification cracking susceptibility of Sn-Ag-Cu solder joints has been investigated. Solders containing 1.0 wt.% to 3.8 wt.% Ag were used in the experiment. Solidification cracks were created using a copper self-restraint specimen, which could simulate the process of solidification cracking. Meanwhile, solidification cracking susceptibility was evaluated by comparing the total crack length of the solder joint. The results indicate that solidification cracks exist in solder joints with 1.0 wt.% to 3.0 wt.% Ag content, whereas there are no cracks in Sn-3.8Ag-0.7Cu solder joints. When the Ag content increases from 1.0 wt.% to 3.0 wt.%, the total crack length of Sn-Ag-Cu solder joints increases to a maximum and then drops to zero when the Ag content reaches 3.8 wt.%. In addition, the susceptibility to solidification cracking is observed as follows: SAC207 > SAC305 > SAC107 > SAC387.     

Effect of Ag micro-particles content on the mechanical strength of the interface formed between Sn–Zn binary solder and Au/Ni/Cu bond pads

In this study, addition of Ag micro-particles with a content in the range between 0 and 4 wt.% to a Sn–Zn eutectic solder, were examined in order to understand the effect of Ag additions on the microstructural and mechanical properties as well as the thermal behavior of the composite solder formed. The shear strengths and the interfacial reactions of Sn–Zn micro-composite eutectic solders with Au/Ni/Cu ball grid array (BGA) pad metallizations were systematically investigated. Three distinct intermetallic compound (IMC) layers were formed at the solder interface of the Au/electrolytic Ni/Cu bond pads with the Sn–Zn composite alloys. The more Ag particles that were added to the Sn–Zn solder, the more Ag–Zn compound formed to thicken the uppermost IMC layer. The dissolved Ag–Zn IMCs formed in the bulk solder redeposited over the initially formed interfacial Au–Zn IMC layer, which prevented the whole IMC layer lifting-off from the pad surface. Cross-sectional studies of the interfaces were also conducted to correlate with the fracture surfaces.     

Morphology and Growth of Intermetallics at the Interface of Sn-based Solders and Cu with Different Surface Finishes

Several types of surface finishes have been applied on Cu substrates in an effort to facilitate bonding and improve the reliability of lead-free solder joints. In the current research, the effects of printed circuit board surface finishes on the reliability of the solder joints were investigated by examining the morphology and growth behavior of the intermetallic compounds (IMCs) between Sn-based solders and different surface finishes on Cu. Three types of Cu substrates with different surface finishes were fabricated in this study: organic solderability preservative (OSP)/Cu, Ni/Cu, and electroless nickel immersion gold (ENIG)/Cu. Sn-3.5Ag and Sn-3.0Ag-0.5Cu were used as the solders. In the experiment, the solder joint specimens were aged isothermally at 150°C for up to 1000 h. Experimental results revealed that the OSP surface finish promoted the interdiffusion between Cu and Sn during soldering. The composition and morphology of the IMC layer at the solder/Ni/Cu interface were sensitive to the Cu concentration in the solder. Meanwhile, the solder joints with different morphological features of the IMCs exhibited significant differences in shear strengths. The Au-containing ENIG surface finish affected the shear strength of the solder joint significantly at the initial stage of isothermal aging.     

Studies of the mechanical and electrical properties of lead-free solder joints

The mechanical and electrical properties of several Pb-free solder joints have been investigated including the interfacial reactions, namely, the thickness and morphology of the intermetallic layers, which are correlated with the shear strength of the solder joint as well as its electrical resistance. A model joint was made by joining two “L-shaped” copper coupons with three Pb-free solders, Sn-3.5Ag (SA), Sn-3.8Ag-0.7Cu (SAC), and Sn-3.5Ag-3Bi (SAB) (all in wt.%), and combined with two surface finishes, Cu and Ni(P)/Au. The thickness and morphology of the intermetallic compounds (IMCs) formed at the interface were affected by solder composition, solder volume, and surface finish. The mechanical and electrical properties of Pb-free solder joints were evaluated and correlated with their interfacial reactions. The microstructure of the solder joints was also investigated to understand the electrical and mechanical characteristics of the Pb-free solder joints.