Sn-Ag-Cu Research Articles

Effect of Pd Thickness on the Interfacial Reaction and Shear Strength in Solder Joints Between Sn-3.0Ag-0.5Cu Solder and Electroless Nickel/Electroless Palladium/Immersion Gold (ENEPIG) Surface Finish

Intermetallic compound formation at the interface between Sn-3.0Ag-0.5Cu (SAC) solders and electroless nickel/electroless palladium/immersion gold (ENEPIG) surface finish and the mechanical strength of the solder joints were investigated at various Pd thicknesses (0 μm to 0.5 μm). The solder joints were fabricated on the ENEPIG surface finish with SAC solder via reflow soldering under various conditions. The (Cu,Ni)6Sn5 phase formed at the SAC/ENEPIG interface after reflow in all samples. When samples were reflowed at 260°C for 5 s, only (Cu,Ni)6Sn5 was observed at the solder interfaces in samples with Pd thicknesses of 0.05 μm or less. However, the (Pd,Ni)Sn4 phase formed on (Cu,Ni)6Sn5 when the Pd thickness increased to 0.1 μm or greater. A thick and continuous (Pd,Ni)Sn4 layer formed over the (Cu,Ni)6Sn5 layer, especially when the Pd thickness was 0.3 μm or greater. High-speed ball shear test results showed that the interfacial strengths of the SAC/ENEPIG solder joints decreased under high strain rate due to weak interfacial fracture between (Pd,Ni)Sn4 and (Cu,Ni)6Sn5 interfaces when the Pd thickness was greater than 0.3 μm. In the samples reflowed at 260°C for 20 s, only (Cu,Ni)6Sn5 formed at the solder interfaces and the (Pd,Ni)Sn4 phase was not observed in the solder interfaces, regardless of Pd thickness. The shear strength of the SAC/ENIG solder joints was the lowest of the joints, and the mechanical strength of the SAC/ENEPIG solder joints was enhanced as the Pd thickness increased to 0.1 μm and maintained a nearly constant value when the Pd thickness was greater than 0.1 μm. No adverse effect on the shear strength values was observed due to the interfacial fracture between (Pd,Ni)Sn4 and (Cu,Ni)6Sn5 since the (Pd,Ni)Sn4 phase was already separated from the (Cu,Ni)6Sn5 interface. These results indicate that the interfacial microstructures and mechanical strength of solder joints strongly depend on the Pd thickness and reflow conditions.

Electrochemical migration behaviors of lead-free 64Sn-35Bi-1Ag solder on FR-4 PCB board plated with Cu

Electrochemical migration (ECM) characteristic is a new reliability failure. A parallel surfacial electrode system coupling with scanning electron microscope (SEM), energy dispersive X-ray analysis (EDAX), and X-ray Diffraction (XRD) technologies is designed to research ECM behavior of 64Sn-35Bi-1Ag (SBA) solder in 3.5 g/L NaCl solution and compare to that of Sn37Pb and Sn-3.0Ag-0.5Cu (SAC) solders. Results show that SBA solder is more susceptible to ECM failure than Sn37Pb solder, which is more difficult than SAC solder. The affected factors of ECM are given as follows: the solder compositions, the loaded electric field, ECM time, etc. The electrochemical reaction mechanism of ECM process is achieved. EDAX and XRD analyses show that the main contents on dendrites of SBA solder after ECM test are Sn, hardly any Bi, a little Ag, which illustrates that the order of ion migration capacity is shown as follows: Sn>Ag>Bi.

A review on effect of minor alloying elements on thermal cycling and drop impact reliability of low‐Ag Sn‐Ag‐Cu solder joints

PurposeThe purpose of this paper is to discuss the reliability of board level Sn‐Ag‐Cu (SAC) solder joints in terms of both thermal cycling and drop impact loading conditions, and further modification of the characteristics of low Ag‐content SAC solder joints using minor alloying elements to withstand both thermal cycle and drop impact loads.Design/methodology/approachThe thermal cycling and drop impact reliability of different Ag‐content SAC bulk solder will be discussed from the viewpoints of mechanical and micro‐structural properties.FindingsThe best SAC composition for drop performance is not necessarily the best composition for optimum thermal cycling reliability. The content level of silver in SAC solder alloys can be an advantage or a disadvantage depending on the application, package and reliability requirements. The low Ag‐content SAC alloys with different minor alloying elements such as Mn, Ce, Bi, Ni and Ti display good performance in terms of both thermal cycling and drop impact loading conditions.Originality/valueThe paper details the mechanical and micro‐structural properties requirements to design a robust bulk SAC solder joint. These properties provide design and manufacturing engineers with the necessary information when deciding on a solder alloy for their specific application.

Isothermal Aging Effects on the Thermal reliability Performance of Lead-Free Solder Joints

Electronics assemblies containing solder joints are often exposed to elevated temperatures for prolonged periods of time. The time-at-temperature stress impacts the overall package reliability of the assembled circuitry due to evolving materials, microstructural, and mechanical properties. It is especially important to understand the impact of isothermal aging on the long term behavior of lead (Pb)-free solder joints which operate in harsh environments. In this study, we have explored the effects of elevated temperature isothermal aging on the reliability of Sn-Ag-Cu (SAC) assemblies on board level packages. As the isothermal aging temperature increases, the Weybull characteristic lifetime for SAC 105 and 305 solder joints is drastically reduced compared to Sn-37Pb.In parallel mechanical studies on bulk solder specimens, the creep rate for SAC105, 305 rapidly increases with aging. A full test matrix with varying aging temperatures and solder alloys was considered. Package sizes ranged from 19mm, 0.8mm pitch ball grid arrays (BGAs) to 5mm, 0.4mm pitch μBGAs. The test structures were built on three different board finishes (ImSn, ImAg and SnPb). Storage condition temperatures were 25°C, 55°C, 85°C and 125°C with aging over time periods of 0, 6, and 12 months. Subsequently, the specimens were thermally cycled from −40°C to 125°C with 15 min dwell times at the high temperature. It was found that the thermal performance of lead-free fine-pitch packages significantly degrades up to 55–60% after aging at elevated temperature. The dominant failure mode can be associated with the growth of Cu6Sn5 intermetallic compounds (IMC) during the aging, particularly on the pad side.

Effect of Ni-Coated Carbon Nanotubes on the Corrosion Behavior of Sn-Ag-Cu Solders

In this paper, varying weight percentages of Ni-coated carbon nanotubes (Ni-CNTs) was incorporated into Sn-Ag-Cu (SAC) solder matrix, to form composite solder. Up to 0.05% of Ni-CNTs were successfully incorporated. After that, the electrochemical behavior of Sn-Ag-Cu with varying weight percentages of Ni-coated carbon nanotubes (Ni-CNTs) composite solder were investigated in 0.3% Na2SO4 solution by using potentiodynamic polarization method. Scanning microscopy was used to characterize the samples after the electrochemical tests. It was observed that the corrosion resistance decreased when the content of Ni-CNTs reached a certain level.

Influence of trace alloying elements on the ball impact test reliability of SnAgCu solder joints

In this work, we present ball impact test (BIT) responses and fracture modes obtained at an impact velocity of 0.8 m/s on SAC (Sn–Ag–Cu) package-level solder joints with a trace amount of Mn or RE (rare earth) additions, which were bonded with substrates of OSP Cu and electroplated Ni/Au surface finishes respectively. With respect to the as-mounted conditions, the Ni/Au joints possessed better impact fracture resistance than those with Cu substrate. Subsequent to aging at 150 °C for 800 h, multi-layered intermetallic compounds emerged at the interface of the Ni/Au joints and gave rise to degradation of the BIT properties. This can be prevented by RE doping, which is able to inhibit the growth of interfacial IMCs during aging. As for aged Cu joints, the Mn-doped samples showed the best performance in impact force and toughness. This was related to the hardened Sn matrix, and most importantly, a greater Cu 3Sn/Cu 6Sn 5 thickness ratio at the interface. Compared to Cu 6Sn 5, Cu 3Sn with a similar hardness but greater elastic modulus possessed better plastic ability, which was beneficial to the reliability of solder joints suffering high strain rate deformation if no excess Kirkendall voids formed.

Phenomenological Study of the Effect of Microstructural Evolution on the Thermal Fatigue Resistance of Pb-Free Solder Joints

Unlike SnPb solders, the thermal fatigue reliability of the Sn-Ag-Cu (SAC) solders is believed to be influenced significantly by both the initial and evolving microstructures. This paper presents a phenomenological study of the relationship between the initial SAC solder joint microstructure, the evolving microstructure, and the thermal fatigue performance measured by accelerated temperature cycling (ATC). To reflect the board assemblies that are in field use, commercial surface mount components with multiple geometries and materials and from different package assemblers were joined to the board with different lead free SAC alloys. The initial microstructures of the board level solder joints were altered in a variety of ways including: 1) varying the solder joint cooling rate; 2) varying the number of solder reflow exposures; and 3) exposure to different isothermal temperature exposures. In all cases the solder joint microstructure was exposed to one or more of these treatments prior to exposure to temperature cycling. In addition, some of the test boards were exposed to different cycling dwell times to determine if the microstructural evolution that occurred during ATC testing effected the respective characteristic lifetimes of the joints. The microstructural evolution was tracked and characterized with optical metallography and scanning electron microscopy. These results could have practical implications in terms of limiting the ability to develop acceleration factors and effective strain-based models for predicting Pb-free solder joint life.

Comparison of Extensive Thermal Cycling Effects on Microstructure Development in Micro-alloyed Sn-Ag-Cu Solder Joints

Pb-free solder alloys based on the Sn-Ag-Cu (SAC) ternary eutectic have promise for widespread adoption across assembly conditions and operating environments, but enhanced microstructural control is needed. Micro-alloying with elements such as Zn was demonstrated for promoting a preferred solidification path and joint microstructure earlier in simple (Cu/Cu) solder joints studies for different cooling rates. This beneficial behavior now has been verified in reworked ball grid array (BGA) joints, using dissimilar SAC305 (Sn-3.0Ag-0.5Cu, wt.%) solder paste. After industrial assembly, BGA components joined with Sn-3.5Ag-0.74Cu-0.21Zn solder were tested in thermal cycling (−55°C/+125°C) along with baseline SAC305 BGA joints beyond 3000 cycles with continuous failure monitoring. Weibull analysis of the results demonstrated that BGA components joined with SAC + Zn/SAC305 have less joint integrity than SAC305 joints, but their lifetime is sufficient for severe applications in consumer, defense, and avionics electronic product field environments. Failure analysis of the BGA joints revealed that cracking did not deviate from the typical top area (BGA component side) of each joint, in spite of different Ag3Sn blade content. Thus, SAC + Zn solder has not shown any advantage over SAC305 solder in these thermal cycling trials, but other characteristics of SAC + Zn solder may make it more attractive for use across the full range of harsh conditions of avionics or defense applications.

An investigation of microstructure and mechanical properties of novel Sn3.5Ag0.5Cu–XTiO2 composite solders as functions of alloy composition and cooling rate

Abstract In the present study, the influence of both TiO 2 nanoparticle addition and cooling rate on the melting temperature, microstructure, and mechanical behaviour of Sn3.5Ag0.5Cu (SAC) solder alloys was studied. The composite solders were prepared by mechanical mixing of TiO 2 nanoparticles with SAC solder. With the addition of TiO 2 nanoparticles into the eutectic SAC alloy, a novel SAC composite solder was successfully prepared. The melting temperature for the SAC composite solders was found to be only 1.56 °C higher than that of the SAC solder, indicating that the novel SAC composite solder is fit for existing soldering process. The cooling rate and TiO 2 nanoparticle addition affected the solidification of the microstructure dramatically. Notably, SEM observation of the microstructure of the SAC composite solders under the rapid-cooled condition revealed fine dot-like nano-Ag 3 Sn IMC in the solder matrix. The ultimate tensile strength (UTS), 0.2% yield strength (0.2YS), and microhardness of the SAC composite solder increased with the increase of TiO 2 nanoparticle content by 0.25–1.0 wt.% and the cooling rate, which could be attributed to the dispersion strengthening mechanisms. However, the ductility of the composite solders was found to decrease because of microporosity at the Ag 3 Sn network grain boundary.

Influence of Surface Segregation on Wetting of Sn-Ag-Cu (SAC) Series and Pb-Containing Solder Alloys

Wetting of Sn-Ag-Cu (SAC) series solder alloys to solid substrates is strongly influenced by surface segregation of low-level bulk impurities in the alloys. We report in situ and real-time Auger electron spectroscopy measurements of SAC alloy surface compositions as a function of temperature as the alloys are taken through the melting point. A dramatic increase in the amount of surface C (and frequently O) is observed with temperature, and in some cases the alloy surface is nearly 80 at.% C at the melting point. The C originates from low-level impurities incorporated during alloy synthesis and inhibits wetting because C acts as a blocking layer to reaction between the alloy and substrate. A similar phenomenon has been observed over a wide range of (SAC and non-SAC) alloys synthesized by a variety of techniques. That solder alloy surfaces at melting have a radically different composition from the bulk uncovers a key variable that helps to explain the wide variability in contact angles reported in previous studies of wetting and adhesion.

Silver nanoparticles effect on the wettability of Sn‐Ag‐Cu solder pastes and solder joints microstructure on copper

PurposeThe purpose of this paper is to investigate the influence of silver nanoparticle additions on the wetting properties of Sn‐Ag‐Cu (SAC) solder paste. In this investigation, the basic solder paste contained 85 wt.% of commercial Sn 96.5 Ag 3 Cu 0.5 powder (with the particle sizes in the range of 20‐38 μm) and 15 wt.% of self‐prepared middle activated rosin flux. To this paste was added 0.5, 1, 2 and 4 wt.% of self‐prepared silver nano‐powders of different grain sizes (from 9 to 138 nm). After the pastes had stabilized, their wetting properties were tested. The main goal of these investigations was to improve the wetting properties of SAC solder paste and to find correlations between the results of the wetting of solder paste with nanoparticles on the copper substrate with the microstructure of the solder joints.Design/methodology/approachThe following methods were applied for the wetting solder paste investigation: spreading on the copper substrate, contact angle measurement on the copper and wetting on a FR‐4 laminate double sided with an 18‐μm thick copper foil. The investigations were performed at temperatures of 220, 230, 240 and 250°C. Cross‐sectioning was performed on the solder paste after reflow on the copper substrate. For the microstructural analysis of the “nano” modified solder joints obtained at 250°C, standard metallographic procedures were applied. Changes in the microstructure, the thickness of the inter‐metallic compounds (IMCs) and their chemical compositions were observed by means of scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS).FindingsAs expected, a higher silver nanoparticle addition to the SAC solder paste resulted in better wetting properties on copper. The results indicated the possibility of an improvement of the reflow soldering process by using SAC solder paste with silver nanoparticles and by lowering its soldering temperature. An improvement was also observed in the wettability with a decrease in the silver nanoparticle grain size. Also, the wettability proceeded at a lower temperature (20°C lower) than that for the SAC paste, without the nano‐additives. For the 4 per cent silver nanoparticle addition, Ag3Sn star‐like IMCs were also found, which grew with the lowering of the silver nanoparticle grain size.Research limitations/implicationsFurther studies are necessary for confirmation of the practical application, especially of the mechanical properties, as well as the reliability properties of the solder joints, for the chosen solder paste with silver nanoparticles.Practical implicationsTaking into account the wetting data, the best results of the “nano” SAC solder pastes were obtained for the highest addition of the silver nanoparticles. It was found that the spreading on copper was higher and the contact angles were lower for the SAC solder paste with 4 per cent (by wt.) of 138‐nm grain size silver nanoparticles. A comparison of SAC solder pastes with a 4 per cent silver nanoparticle addition but of a different grain size (138‐9 nm), suggested a further improvement in wetting properties with lowering of the silver nanoparticle grain size. The results suggested the possibility of an improvement in the reflow soldering process by using SAC solder paste with silver nanoparticles and by lowering its soldering temperature.Originality/valueSpreading, wetting and contact angle measurement methods were used for the wetting determination of the SAC solder paste with the silver nanoparticles on copper under the same temperature conditions. Also, the microstructure of the solder joints obtained at 250°C was determined with the use of SEM and EDS methods. The results obtained made it possible to draw conclusions regarding the correlation between the output of the wetting results and the amount and the grain size of the added silver nanoparticles, and also the microstructure and thickness of the IMCs of the “nano” solder joints.

Suppressing effect of 0.5 wt.% nano-TiO 2 addition into Sn–3.5Ag–0.5Cu solder alloy on the intermetallic growth with Cu substrate during isothermal aging

This study investigated the effects of adding 0.5 wt.% nano-TiO 2 particles into Sn3.5Ag0.5Cu (SAC) lead-free solder alloys on the growth of intermetallic compounds (IMC) with Cu substrates during solid-state isothermal aging at temperatures of 100, 125, 150, and 175 °C for up to 7 days. The results indicate that the morphology of the Cu 6Sn 5 phase transformed from scallop-type to layer-type in both SAC solder/Cu joints and Sn3.5Ag0.5Cu–0.5 wt.% TiO 2 (SAC) composite solder/Cu joints. In the SAC solder/Cu joints, a few coarse Ag 3Sn particles were embedded in the Cu 6Sn 5 surface and grew with prolonged aging time. However, in the SAC composite solder/Cu aging, a great number of nano-Ag 3Sn particles were absorbed in the Cu 6Sn 5 surface. The morphology of adsorption of nano-Ag 3Sn particles changed dramatically from adsorption-type to moss-type, and the size of the particles increased. The apparent activation energies for the growth of overall IMC layers were calculated as 42.48 kJ/mol for SAC solder and 60.31 kJ/mol for SAC composite solder. The reduced diffusion coefficient was confirmed for the SAC composite solder/Cu joints.

Solder Joint Reliability of SnAgCu Solder Refinished Components Under Temperature Cycling Test

SnAgCu (SAC) solder is being offered as a lead-free termination finish. SAC finish is obtained by dipping the terminals of components into molten SAC solder. However, the reliability of solder joints formed with SAC solder refinished components needs to be determined in order to evaluate the effects of the SAC solder refinishing process. In this paper, the strength of solder joints with SAC solder refinished thin small outline packages (TSOPs) was evaluated by the shear test. The reliability of solder joints formed with SAC solder refinished components, including TSOPs and resistors, was evaluated by a temperature cycling test. Original Sn finished components were used as a baseline for comparison. It was found that SAC solder refinishing increased solder joint strength. SAC solder refinishing decreased the fatigue life of solder joints of TSOPs and increased the fatigue life of resistors under temperature cycling conditions.

Electrochemical corrosion behaviour of Sn–Ag–Cu (SAC) eutectic alloy in a chloride containing environment

AbstractThe corrosion behaviour of the Sn94.5Ag3.8Cu1.5 (SAC) eutectic alloy was investigated in 0.1 M NaCl solution by potentiodynamic polarization and impedance spectroscopy measurements and compared with that of the conventional Sn73.9Pb23.1 eutectic solder employed for a long time in the packaging of microelectronic components and devices. Scanning electron microscopy (SEM) and electron probe microanalysis (EPMA) were used to characterize the SAC eutectic alloy prior to and after the electrochemical tests. The electrochemical results indicated that the Sn–Ag–Cu eutectic alloy exhibits better corrosion behaviour than the Sn–Pb eutectic solder in NaCl solution. The presence of a corrosion products layer constituted by tin oxy‐chloride was detected at the surface of both alloys investigated after the electrochemical tests. The better corrosion behaviour of SAC eutectic alloy compared to Sn–Pb eutectic solder is ascribed to the formation of a more compact surface film of corrosion products with improved protective properties owing to the presence of copper and silver, as revealed by EPMA.

Improving Low-Ag Pb-Free Solder Performance by Adding Bi, Ni Elements

In this study, some Ni, Bi elements were added into low-Ag (less than 1%) Sn-Ag-Cu (SAC) solder. The effect of these additive elements on the solderability, intermetallic compounds (IMCs), and electromigration performance of low-Ag SAC (LASAC) solder were investigated. With the increase of Bi content in LASAC-0.05Ni-xBi (x=0, 2.0, 2.5, 3.0, 3.5, 4.0) solders, the peak melting point decreases while the wetting area of solder alloy increases. With the addition of Ni, the IMC between Cu pad and LASAC solder transforms from Cu6Sn5 to (Cu1-xNix)6Sn5 and the morphology of the IMC turns from bulk-like to needle-like. Either the addition of Bi or Ni will slow down the IMCs growth rate during high temperature storage aging (HTS) at 180°C and has a positive effect on electromigration performance of LASAC soldering.

Interfacial reaction and dissolution behavior of Cu substrate in molten Sn‐3.8Ag‐0.7Cu in the presence of Mo nanoparticles

PurposeIn electronic packaging, when solid copper comes in contact with liquid solder alloy, the former dissolves and intermetallic compounds (IMCs) form at the solid‐liquid interface. The purpose of this paper is to study the effect of the presence of molybdenum nanoparticles on the dissolution of copper and the formation of interfacial IMC.Design/methodology/approachCu wire having a diameter of 250 μm is immersed in liquid composite solders at 250°C up to 15 min. Composite solder was prepared by adding various amount of Mo nanoparticles into the Sn‐3.8Ag‐0.7Cu (SAC) solder paste. The dissolution behavior of Cu substrate is studied for SAC and Mo nanoparticles added SAC solders. The IMCs and its microstructure between the solder and substrate are analyzed by using conventional scanning electron microscope (SEM) and field emission SEM. The elemental analysis was done by using energy‐dispersive X‐ray spectroscopy.FindingsGenerally, the dissolution of the substrate increases with increasing immersion time but decreases with the increase of the content of Mo nanoparticles in the solder. The IMC thickness increases with increasing the reaction time but Mo nanoparticles can hinder the growth of IMC layer. The presence of Mo nanoparticle is found to be effective in reducing the dissolution of copper into SAC solder.Originality/valueThe paper shows that molybdenum nanoparticles in liquid SAC solders have a prominent effect on the substrate dissolution rate and the interfacial IMC between the SAC solder and copper substrate.

Constitutive Modeling for SAC Lead-Free Solder based on Cyclic Loading Tests using Stepped Ramp Waves

Finite element analyses for evaluating fatigue strength of solder joints in electronic packages should employ a constitutive model which can describe cyclic deformations precisely. Therefore, the model should be constructed based on detailed information about the inelastic strain generated under the cyclic loading. We previously investigated both the plastic and creep strains of Sn-Ag-Cu (SAC) lead-free solder under cyclic loading experimentally by conducting cyclic loading tests using stepped ramp waves. In this study, we construct an elastic-plastic-creep constitutive model based on the experimental results, such as the stress-plastic strain relations, and the stress-creep strain relations obtained by the above-mentioned tests. The model considers the characteristics of the plastic and creep deformations in cyclic loading where loading direction changes. Simulations of cyclic loading of the SAC solder under several loading conditions are conducted using the proposed model and the results are compared with experiments.

Location of the Critical Solder Joint of a PBGA under Temperature Cycling Load for SAC and SnPb Solder. Results of Experiments vs. FE-Simulations on System and Board Level.

PBGAs with SnPb and SnAgCu (SAC) solder joints were stressed with temperature cycles on board- and system-level. A significant influence of the different solder materials on the location of the most damaged PBGA solder balls was observed in the experiment. The reason for this experimental finding was investigated and explained by FE–simulation. The simulations of the PBGAs were done on package-, board- and system-level (PCB within a metal housing). For the system level simulation a 2-step sub-model technique described in [1] was used. Through such an approach the transient PCB deformation and the transient temperature field within the ECU-housing can be incorporated into a creep simulation of the PBGA solder joints. The creep results for both SnPb and SnAgCu solder joints from the board- and system-level simulation were compared. The calculated damage factor due to the ECU-housing influence is different for PBGA with SnPb and SAC solder joints. The simulation results were validated step by step with measurements and experiments: warpage of the non-soldered PBGA, mechanical strain and temperature on the mounted PCB, crack length evaluation of all PBGA solder joints.

Diffusion Barrier Characteristics of Electroless Co(W,P) Thin Films to Lead-Free SnAgCu Solder

Diffusion barrier and bonding characteristics of amorphous and polycrystalline electroless Co(W,P) layers (α-Co(W,P) and poly-Co(W,P)) to SnAgCu (SAC) solder were investigated. In the SAC/α-Co(W,P) sample subjected to liquid-state aging at 250°C, the spallation of intermetallic compound (IMC) into solder, a nano-crystalline P-rich layer at SAC/Co(W,P) interface, and the recrystallized Co(W,P) containing Co2P precipitates were observed. As to the SAC/α-Co(W,P) sample subjected to solid-state aging at 150°C, a thick IMC layer neighboring to the P-rich layer formed at the solder/Co(W,P) interface. Liquid-state aging resulted in an IMC mixture without spallation whereas solid-state aging induced a layer-like IMC in SAC/poly-Co(W,P) samples. Amorphous W-rich layers were also observed in SAC/poly-Co(W,P) samples and it could not inhibit subsequent alloy reactions in the samples. Analytical results indicated the α-Co(W,P) is a sacrificial- plus stuffed-type barrier whereas the poly-Co(W,P) is mainly a sacrificial barrier. The activation energies of IMC growth were 110.7 and 81.8 kJ/mol for SAC/α-Co(W,P) and SAC/poly-Co(W,P) samples, respectively. High P content in α-Co(W,P) was found to degrade the bonding strength to the solder as revealed by the shear test and the control of P content would be a key issue for electroless plating layer applied to under bump metallurgy as the diffusion barrier.

Mechanism of the Delayed Growth of Intermetallic Compound at the Interface between Sn-4.0Ag-0.5Cu and Cu-Zn Substrate

A Cu−Zn wetting layer was very effective to depress the excessive growth of intermetallic compound (IMC). The effect of Zn addition to the Cu layer on the IMC growth and microvoid formation in the solder interface was similar to the effect of Zn addition into the Sn-Ag-Cu (SAC) solders. In this study, the mechanism of slow IMC growth at the SAC/Cu−Zn interfaces was investigated. As the aging time increased, Zn atoms accumulated at the Cu6Sn5/Cu interface and formed a Zn-rich layer. By adding Zn into the Cu wetting layer, the IMC growth was delayed due to the retardation of the formation of the Cu3Sn layer. Since the low driving force for the formation of Cu3Sn became smaller by adding Zn, the diffusion of Cu in Cu−Zn into SAC solder was delayed. Also, the CuZn phase formed at the Cu6Sn5/Cu−Zn interface plays a role as a diffusion barrier of interdiffusion of Cu and Sn.