IMC Grains Research Articles

Study on Thermal Cycling Reliability of Epoxy-Enhanced SAC305 Solder Joint.

In this work, epoxy was added into commercial Sn-3.0Ag-0.5Cu (SAC305) solder paste to enhance the thermal cycling reliability of the joint. The microstructure and fracture surface were observed using a scanning electron microscope/energy dispersive spectrometer (SEM/EDS), and a shear test was performed on the thermally cycled joint samples. The results indicated that during the thermal cycling test, the epoxy protective layer on the surface of the epoxy-enhanced SAC305 solder joint could significantly alleviate the thermal stress caused by coefficients of thermal expansion (CTE) mismatch, resulting in fewer structural defects. The interfacial compound of the original SAC305 solder joints gradually coarsened due to the accelerated atomic diffusion, but epoxy-enhanced SAC305 solder joints demonstrated a thinner interfacial layer and a smaller IMC grain size. Due to the reduced stress concentration and the additional mechanical support provided by the cured epoxy layer, epoxy-enhanced SAC305 solder joints displayed superior shear performance compared to the original joint during the thermal cycling test. After 1000 thermal cycles, Cu-Sn IMC regions were observed on the fracture surfaces of the original SAC305 solder joint, exhibiting brittle fracture characteristics. However, the fracture of the SAC305 solder joint with 8 wt.% epoxy remained within the solder bulk and exhibited a ductile fracture mode. This work indicates that epoxy-enhanced SAC305 solder pastes display high thermal cycling reliability and could meet the design needs of advanced packaging technology for high-performance electronic packaging materials.

Investigation of isothermal aged Sn-3Ag-0.5Cu/Sn58Bi-Co hybrid solder joints on ENIG and ENEPIG substrate with various mechanical performances

The stability of solder joints is of utmost critical in electronic equipment. The integration of packaged devices in these systems often leads to the generation of heat, which might have a significant impact on how long the equipment lasts when exposed to high temperatures. It is imperative to perform an isothermal aging test on the solder joints in order to replicate the extended use of these joints. This research looked into how the mechanical characteristics and IMC growth of hybrid solder joints were affected by isothermal aging over 240 h, 480 h and 720 h. The reliability of solder joint under isothermal aging condition was improved by adding Pd coating and Co element. The results revealed that Pd coating over ENIG (Electroless Nickel Immersion Gold) pad reduced the growth rate of IMC in SAC305/SnBi hybrid solder joints during isothermal aging. ENIG solder joint strength dropped by 57.66% after 720 h of isothermal aging, and practically all fractures were IMC fractures. ENEPIG (Electroless Nickel Electroless Palladium Immersion Gold) shear strength lowered 31.01%, IMC fracture mode 20%, strong reliability at high temperature. IMC development was suppressed in SAC305/SnBi-Co hybrid solder joints by higher Co content. After 720 h isothermal aging, SAC305/SnBi-Co solder joints had better shear strength than non-Co joints. According to fracture morphology, SAC305/SnBi-Co hybrid solder joints reliability improves owing to the change of IMC morphology and grain refining.

Rapid formation, grain refinement and shear property of high-temperature-stable full IMC joints

Solid liquid interdiffusion (SLID) bonding that can fabricate high-melting point full IMC joints is playing an increasing key role in 3rd-generation semiconductor power electronic device packaging for high temperature applications. In this study, rapid formation of Cu/intermetallic compounds (IMCs)/Cu–8Ni joints with fine (Cu,Ni)6Sn5 and (Cu,Ni)3Sn grains was achieved by SLID bonding Cu/Sn/Cu–8Ni structure. The formation and evolution of the IMCs and the elemental distribution and shear property of the full IMC joints during bonding and subsequent aging were systematically investigated. Due to the Ni doping, the growth of the (Cu,Ni)6Sn5 IMC was significantly accelerated during bonding while that of the Cu3Sn phase was strongly suppressed during both bonding and aging. The total IMC growth rate in the Cu/Sn/Cu–8Ni solder joints was approximately 44 times faster than that in the Cu/Sn/Cu solder joints under the same bonding condition. After aging at 260 °C for up to 400 h, the full IMC joints showed good thermostability composing of fine IMC grains and no Kirkendall voids were observed. The shear strength of the full IMC joints slightly dropped from 63.17 MPa after bonding to 54.35 MPa after aging. The proposed SLID bonding using Cu/Sn/Cu–8Ni structure was proved to be a promising method for forming high-temperature-stable full IMC joints for 3rd-generation semiconductor power device packaging or 3D packaging applications.

The impact of intermetallic compound on microstructure, mechanical characteristics, and thermal behavior of the melt-spun Bi-Ag high-temperature lead-free solder

PurposeThis study aims to study the impact of intermetallic compound on microstructure, mechanical characteristics and thermal behavior of the melt-spun Bi-Ag high-temperature lead-free solder.Design/methodology/approachIn this paper, a new group of lead-free high-temperature Pb-free solder bearing alloys with five weight percentages of different silver additions, Bi-Agx (x = 3.0, 3.5, 4.0, 4.5 and 5.0 Wt.%) have been developed by rapidly solidification processing (RSP) using melt-spun technique as a promising candidate for the replacement of conventional Sn-37Pb common solder. The effect of the addition of a small amount of Ag on the structure, microstructure, thermal and properties of Bi-Ag solder was analyzed by means of X-ray diffractometer, scanning electron microscopy, differential scanning calorimetry and Vickers hardness technique. Applying the RSP commonly results in departures from conventional microstructures, giving an improvement of grain refinement. Furthermore, the grain size of rhombohedral hexagonal phase Bi solid solution and cubic IMC Bi0.97Ag0.03 phase is refined by Ag addition. Microstructure analysis of the as soldered revealed that relatively uniform distribution, equiaxed refined grains of secondary IMC Bi0.97Ag0.03 particles about 10 µm for Bi-Ag4.5 dispersed in a Bi matrix. The addition of trace Ag led to a decrease in the solidus and liquidus temperatures of solder, meanwhile, the mushy zone is about 11.4°C and the melting of Sn-Ag4.5 solder was found to be 261.42°C which is lower compared with the Sn-Ag3 solder 263.60°C. This means that the silver additions into Bi enhance the melting point. The results indicate that an obvious change in electrical resistivity (?) at room temperature was noticed by the Ag addition. It was also observed that the Vickers microhardness (Hv) was increased with Ag increasing from 118 to 152 MPa. This study recommended the use of the Bi-Ag lead-free solder alloys for higher temperature applications.FindingsSilver content is very important for the soldering process and solder joint reliability. Based on the present investigations described in this study, several conclusions were found regarding an evaluation of microstructural and mechanical deformation behavior of various Bi-Ag solders. The effect of Ag and rapid solidification on the melting characteristics, and microstructure of Bi-Ag alloys were studied. In addition, the mechanical properties of Bi with different low silver were investigated. From the present experimental study, the following conclusions can be drawn. The addition of Ag had a marked effect on the melting temperature of the lead-free solder alloys, it decreases the melting temperature of the alloy from 263.6 to 261.42°C. Bi-Ag solders are comprised of rhombohedral Hex. Bi solid solution and cubic Ag0.97Bi0.03 IMC is formed in the Bi matrix. The alloying of Ag could refine the primary Bi phase and the Bi0.97Ag0.03 IMC. With increasing Ag content, the microstructure of the Bi-Ag gradually changes from large dimples into tiny dimple-like structures. The refinement of IMC grains was restrained after silver particles were added into the matrix. The inhibition effect on the growth of IMC grains was most conspicuous when solder was doped with Ag particles. As a result, the Vickers microhardness of the Bi-Ag lead-free solder alloys was enhanced by more than 100% ranging from 118.34 to 252.95 MPa. Bi-Ag high-temperature lead-free solders are a potential candidate for replacing the tin-lead solder (Sn-37Pb) materials which are toxic to human and the environment and has already been banned.Originality/valueThis study recommended the use of the Bi-Ag lead-free solder alloys for high-temperature applications.

Microstructure and shear property of Ni-coated carbon nanotubes reinforced InSn-50Ag composite solder joints prepared by transient liquid phase bonding

InSn-50Ag composite solder joints doping Ni-coated carbon nanotubes (Ni-CNTs) were fabricated by transient liquid phase (TLP) bonding, and then the effect of Ni-CNTs contents (x = 0, 0.01, 0.03, 0.05, 0.07, 0.1 wt%) on the microstructure and shear property of the composite solder joints was examined. The results showed that the microstructure of Cu/InSn-50Ag-x(Ni-CNTs)/Cu composite solder joints consisted of the interfacial region bamboo-type Cu3(In, Sn), solder center region Ag3In, Ni3Sn4, Ag particles and Ni-CNTs. The appropriate amounts of Ni-CNTs (about 0.07 wt%) are conducive to form a compact composite solder joint and restrain the growth of interfacial Cu3(In, Sn) IMC layer, and the morphologies of IMC layer are determined by Ni-CNTs suppressing atom diffusion and IMC grain orientation. The shear strength of composite solder joints relates to Ni-CNTs content, and the maximum shear strength reaches 42 MPa of Cu/InSn-50Ag-0.07(Ni-CNTs)/Cu. It is regulated by uniform distribution of Ni-CNTs, refined Ag3In grains and interface IMC layer with near-continuous voids. The fracture of the composite solder joints doped Ni-CNTs occurred near to the boundary between the interfacial IMC layer and solder center region, the fracture mechanism is ductile fracture.

Effects of SiC nanowires on reliability of Sn58Bi-0.05GNSs/Cu solder joints

In this work, SiC nanowires (SiC NWs) reinforced SBG (Sn-58Bi-0.05GNSs) composite solder was prepared using powder metallurgy route. The effect of SiC NWs on melting temperature, wetting behavior, shear properties, microstructure of the prepared solder joints and interfacial reaction were studied in detail. Results reveal that incorporating SiC NWs can develop the wetting behavior and shear properties of solder joint but has a little effect on melting temperature. The microstructure of solder is refined markedly with the addition of SiC NWs, which is one of the reasons for the increase in the shear strength of the solder joints. Additionally, the dimension of Cu6Sn5IMC grains diminishes with the doping of SiC NWs, which resulted in the thinning of Cu6Sn5IMC layer. Thence, the addition of SiC NWs may be an effective way to improve the reliability of solder joints.

Interfacial microstructure evolution of solder joints by doping Cu nanoparticles into Ni(P) electroless plating

To enhance the effects of electroless Ni–P plating on inhibiting atom diffusion in Sn-58Bi joint systems, adding nano-sized metals into coating was regarded as an efficient method. Therefore, Cu nanoparticles were chosen as the additive in current study, and the interfacial microstructure evolution of solder joints by doping Cu nanoparticles into Ni(P) electroless plating were investigated. Experimental results revealed that growth rates of IMCs at the joint interface remarkably decreased with increasing content of Cu nanoparticles. In the Ni(P)–0.8 g/L Cu based joint, transformation from (Ni,Cu)3Sn4 to (Cu,Ni)6Sn5 occurred since more Cu atoms supplied by the coating participated in the interfacial reaction between solder and coating. Meantime, sizes of IMC grains at each isothermal aging stage decreased with increasing content of Cu nanoparticles, which could be attributed to introduction of potent nuclei. The IMC growth was mainly volume diffusion-controlled and followed parabolic laws. Diffusion coefficients were analyzed to be 1.18 × 10–2 μm2/h, 2.89 × 10–4 μm2/h and 2.56 × 10–4 μm2/h in Ni(P), Ni(P)–0.4 g/L Cu and Ni(P)–0.8 g/L Cu-based joint systems, respectively, suggesting that diffusion coefficient gradually decreased with increasing content of Cu nanoparticles.

Effect of Thermal Cycling on Interfacial Microstructure and Mechanical Properties of Sn-0.3Ag-0.7Cu-(α-Al2O3) Nanoparticles/Cu Low-Ag Solder Joints

The evolution of interfacial microstructures and mechanical properties of joints soldered with Sn-0.3Ag-0.7Cu (SAC0307) and SAC0307-0.12Al2O3 nanoparticles (NPs) subjected to thermal cycling were investigated. The joint soldered with SAC0307-0.12Al2O3 displayed an enhanced thermal cycling shear force with a ductile fracture mode when compared with the original alloy whose fracture mode showed a mixed feature of ductile and brittle. The enhanced thermal cycling shear force was attributed to a pinning effect by Al2O3 NPs on interfacial IMC grain growth. Even after 1200 thermal cycles, SAC0307-0.12Al2O3 solder was still structurally characterized by a much more refined microstructure than the non-reinforced solder alloy. Theoretical analysis on the growth of interfacial IMC layer showed that with the addition of Al2O3 NPs, the average growth coefficients of total interfacial IMCs (DT) and Cu3Sn IMCs (\( D_{{{\rm{Cu}}_{3} }} \)) were decreased from 9.2 × 10−11 cm2/h to 5.6 × 10−11 cm2/h, and from 6.9 × 10−11 cm2/h to 4.1 × 10−11 cm2/h, respectively. Hence, a much thinner IMC layer was produced at the SAC0307-0.12Al2O3/Cu interface, thus contributing to an enhanced shear resistance.

Size effect on interface reaction of Sn–xCu/Cu solder joints during multiple reflows

At present, electronic products are developing in the direction of miniaturization and integration, which leads to the downsizing of solder bump in the packaging process. Moreover, micro solder bumps often require undergoing multiple reflow processes due to the improvement of packaging technology, which has a great influence on interface reaction. Hence, it is necessary to study the effects of solder composition, bump size and reflow cycle on interfacial reaction between solder alloys and Cu substrates. In this experiment, Sn–xCu (x = 0, 0.7, 2.0 wt%) alloys with diameter of 200 µm, 500 µm, and 800 µm were soldered to Cu substrates at 250 °C for 1 min, and then reflowed 20 cycles totally. The size effect of micro solder joints on the growth of IMC after multiple reflows was analyzed. At the same time, the impact of Cu concentration inside the bulk solder on the interfacial reaction during multiple reflows was explored. This experiment finds that the diameter of IMC grains increases with the decrease of solder ball diameter after one reflow cycle, and a significant size effect occurs in Sn/Cu solder bump. As the number of reflow cycle increases, the size effect on interface reaction is more pronounced. The most direct kinetic factor of this phenomenon is that the average Cu concentration in the small-sized solder ball rises faster than the others. When the number of reflow cycle reach to nine times, the lateral growth rate of IMC grains begins to surpass the longitudinal growth rate, and the morphology of IMC grains becomes flat. This phenomenon is especially evident in the small-sized solder ball. Moreover, the addition of Cu element in solder promotes ripening reaction resulting in the lateral growth of IMC. Cu6Sn5 micro particles appearing at the Sn/Cu, Sn–0.7Cu/Cu interface hinder the grain boundary motion and inhibit the lateral annexation of IMC grains, thereby suppressing the lateral growth of IMC.

Shear strength and fracture surface analysis of Sn58Bi/Cu solder joints under a wide range of strain rates

The influences of the strain rate on the shear strength and failure mode of Sn58Bi/Cu solder joints were investigated. After reflowing, some Kirkendall voids were observed at the neighborhood of the Cu3Sn/Cu interface or in the inner Cu3Sn layer. In addition, another type of void could also be observed inside the Sn58Bi eutectic solders, and its size was much larger than that of Kirkendall voids. Some Bi particles were obviously found to segregate at the interface between the Cu-Sn IMC and the Sn58Bi solder. The single lap shear test results indicated that the strain rate had an important influence on the shear strength and failure mode of Sn58Bi/Cu solder joints. The shear strength of joints demonstrated increment at first and then decrement as the strain rate increased from 3.33 × 10−4 s−1 to 3.33 s−1. It was observed that all Sn58Bi/Cu solder joints broke in a mixed-type fracture mode under a wide range of strain rates. Additionally, more broken IMC grains were exposed on the fracture face and more fracture occurred within the IMC layer with increasing strain rate. Furthermore, the fracture path gradually moved from the solder side to the inner IMC side as the strain rate increased.

Influence of Zn additions on the interfacial reaction and microstructure of Sn37Pb/Cu solder joints

The effects of Zn (5 and 10 wt%) additions into Sn37Pb solder and isothermal solid state aging on the interfacial reactions between Sn37Pb–xZn solders and Cu substrates were investigated in this study. It was found that the addition of Zn changed the types and morphologies of interfacial IMC layers during reflowing and thereafter under aging condition. During reflowing, the planar-type Cu5Zn8 compound was the interfacial IMC for Sn37Pb–xZn (5 and 10 wt%) solder, while the scallop-type Cu6Sn5 was the interfacial IMC for Sn37Pb solder. After aging, the final interfacial structure for Sn37Pb–5Zn solder was solder/Cu5Zn8/Cu6(Sn,Zn)5/Cu, while solder/Cu6Sn5/Cu3Sn/Cu for Sn37Pb solder and solder/Cu5Zn8/Cu for Sn37Pb–10Zn solder, respectively. The Kirkendall voids disappeared with Zn addition into Sn37Pb solder. For the Sn37Pb–5Zn/Cu solder joint, the thickness of Cu6(Sn,Zn)5 layer increased, while the thickness of Cu5Zn8 layer decreased with aging time extended to 360 h due to the decomposition of the Cu5Zn8 IMC layer by diffusing Cu and Zn atoms into nether IMC layer, combining Sn atoms diffused from solder matrix to form Cu6(Sn,Zn)5 IMCs. Furthermore, the growth of Cu6Sn5 and Cu3Sn layers for Sn37Pb/Cu solder joint and the total IMC layer at the interface of Sn37Pb–xZn (x = 0, 5, and 10 wt%) solder with Cu substrate followed the diffusion control mechanism. Compared to the Sn37Pb–5Zn/Cu solder joint, higher Zn concentration depressed the growth of Cu5Zn8 layer for Sn37Pb–10Zn solder. In the end, refining effect on IMC grains was found by the addition of Zn into Sn37Pb solder and the 10 wt% Zn-doping significantly refined the interfacial IMC grains.

Geometrical outline evolution and size-inhibiting interaction of interfacial solder bubbles and IMCs during multiple reflows

By utilizing synchrotron radiation X-ray imaging technique for in situ observation, the geometrical outline evolution and size-inhibiting interaction of solder bubbles and IMCs during multiple reflows were investigated in this work. It was found that the interfacial bubbles can inhibit the growth of IMC grains caused by the hindering effect of bubbles on contact and reaction between solder and substrate; meanwhile, following the growth of IMC grains, the bubble size in return got smaller and its aspect ratio became larger with reflow cycle, which is the result of inhibiting effect of solid IMC grains on solder voids; Because of boundary segregation and thermomigration, Cu atoms tend to gather on the bubble boundary and form a Cu-rich phase, consequently resulting in the formation of IMCs at the gas/solder interface area.

OS0407 低銀鉛フリーはんだ/銅の接合界面性状に及ぼす時効の影響

The purpose of study is to investigate the influence of additional elements on the morphology of interfacial layers after aging. The interfacial reaction layers formed between the low Ag lead-free solders with the additional elements and Cu plate before and after aging were observed by OM and SEM. The addition of Ni was suppressed the growth of the reaction layer compared to SAC107 solder. The addition of Bi and Sb changed finer IMC grains at the reaction layer compared to SAC107 solder. After aging, the reaction layer formed on SAC107 and SAC107 with Bi grew thicker. On the other hands, the addition of Ni or Sb suppressed the growth of the reaction layer.

Modeling of Intermetallic Compounds Growth Between Dissimilar Metals

Abstract A model has been developed to predict growth kinetics of the intermetallic phases (IMCs) formed in a reactive diffusion couple between two metals for the case where multiple IMC phases are observed. The model explicitly accounts for the effect of grain boundary diffusion through the IMC layer, and can thus be used to explore the effect of IMC grain size on the thickening of the reaction layer. The model has been applied to the industrially important case of aluminum to magnesium alloy diffusion couples in which several different IMC phases are possible. It is demonstrated that there is a transition from grain boundary-dominated diffusion to lattice-dominated diffusion at a critical grain size, which is different for each IMC phase. The varying contribution of grain boundary diffusion to the overall thickening kinetics with changing grain size helps explain the large scatter in thickening kinetics reported for diffusion couples produced under different conditions.

Interfacial reaction and failure mode analysis of the solder joints for flip-chip LED on ENIG and Cu-OSP surface finishes

The interfacial reactions and failure modes of the solder joints for flip-chip light emitting diode (LED) on electroless nickel/immersion gold (ENIG) and Cu with organic solderability preservatives (Cu-OSP) surface finishes were investigated in this study. The experimental results demonstrate that the interfacial reactions in the Au/Sn–Ag–Cu(SAC)/ENIG and Au/SAC/Cu systems are different but the failure mechanisms of the two types of solder joints are similar during the shear test. For the Au/SAC/ENIG system, the Au layer on the surface finish of the diodes dissolved into the molten solder and transformed into a continuous (Au, Ni)Sn4 IMC layer at the diode/solder interface during reflow and the interfacial IMC at the solder/ENIG interface is dendritic Ni3Sn4 IMC grains which are surrounded by (Au, Ni)Sn4. For the Au/SAC/Cu system, however, no IMC layers can be observed at the diode/solder interface. The interfacial IMC at the solder/Cu interface is (Cu, Au)6Sn5 and a Cu3Sn IMC layer at the (Cu, Au)6Sn5/Cu interface. Tiny (Au, Cu)Sn4 IMC grains distribute in the solder layer and surround the (Cu, Au)6Sn5 grains. For the two types of systems, the primary failure mode for the cathode is due to the broken of the Si-based insulation layer which led to a high residue stress and poor connection between the Si-based layer and the solder layer. Meanwhile, the failure of the solder joint for the anode is mainly because of the failure of the solder layer under the conductive via. The crack generally forms at this area and then propagated along the diode or the diode/solder interface.

Effect of alloying Cu substrate on microstructure and coarsening behavior of Cu6Sn5 grains of soldered joints

In order to investigate the effects of alloying Cu substrate on microstructure and coarsening behavior of Cu6Sn5 grains of soldered joints, in the paper, four types of wetting layers (Cu, Cu–8 wt% Zn, Cu–15 wt% Zn and Cu–30 wt% Zn) reaction with Sn–3.0Ag–0.5Cu (SAC305) solder were fabricated. The effects of reflowing and isothermal aging on the interface of soldered joints and the morphology of Cu6Sn5 grains were studied. Experimental results show that the scallop-shaped Cu6Sn5 formed at the interface of SAC/Cu after reflowing. After isothermal aging for 120 h, a bi-layer structure of Cu6Sn5 and Cu3Sn formed at the interface of SAC/Cu and some Ag3Sn grains decorated in Cu6Sn5, and Kirkendall voids were observed inside the Cu3Sn layer after aging for 240 h. However, Cu6Sn5 was the predominant reaction product and Cu3Sn and Kirkendall voids were not observed at the interface of SAC/Cu–Zn irrespective of aging time. The thickness of IMC layer increases with increasing aging time. Compared to SAC/Cu, the additions of Zn can effectively suppress the growth of IMC. During the aging process, the growth rate constants of IMC layers of SAC/Cu, SAC/Cu–8Zn, SAC/Cu–15Zn and SAC/Cu–30Zn were obtained by using fitting method from the experimental data, which were 7.78 × 10−17, 5.42 × 10−17, 2.51 × 10−17 and 1.46 × 10−17 m2 s−1, respectively. The mean diameter of IMC grains increases with the increasing aging time. The relationships of the mean diameter of Cu6Sn5 grains of SAC/Cu, SAC/Cu–8Zn, SAC/Cu–15Zn, SAC/Cu–30Zn interface joints and aging time were given, i.e., d = 1.49t0.237, d = 1.46t0.204, d = 1.43t0.203 and d = 1.71t0.197, respectively.

Solid state diffusion between Sn and Cu microcones on Cu microcones

Abstract Theoretical study of solid state diffusion mechanism in low-temperature bonding method based on Cu microcones is reported. A Sn layer was electrodeposited on Cu microcones as well as flat Cu substrate then annealed at bonding temperature 463 K. IMC grains of Sn/Cu-microcones diffusion samples grew with a relatively fine and uniform size. Cu 6 Sn 5 crystal orientation in Sn/Cu-microcones and Sn/flat-Cu diffusion samples were different according to X-ray diffusion patterns. Nucleation was more likely to occur near the pinpoints of the Cu microcones. The kinetic factors showed that the special morphology of Cu microcones was beneficial for the growth of intermetallic layer (Cu 6 Sn 5 ) in short annealing time and the grain boundary diffusion partially contributes to the rate-controlling process.

Interfacial fracture toughness of Pb-free solders

Increasing environmental concerns and pending government regulations have pressured microelectronic manufacturers to find suitable alternatives to Pb-bearing solders traditionally used in electronics packaging. Over recent years, Sn-rich solders have received significant attention as suitable replacements for Pb-bearing solders. Understanding the behavior of intermetallics in Sn-rich solders is of particular concern as the microelectronics industry progresses towards Pb-free packaging. The formation of intermetallic compounds results from the reaction of the solder with the metallization on the substrate in the electronic package. While the presence of the intermetallic is an indication of good wetting, excessive growth of the intermetallic can have a dramatically adverse effect on the toughness and reliability of the solder joint. Understanding their fracture behavior will lend insight to their reliability under mechanical and thermomechanical strains. We investigated the intermetallic compound growth associated with Sn–0.7Cu and Sn–4.0Ag–0.5Cu solders on Ni–Au, Ni–Pd, and Cu substrates. (Ni,Cu) 3Sn 4 was present at the Ni interface for both solders but was coarser in the case of Ni–Pd. Cu 6Sn 5 and Cu 3Sn were observed for both solder types. The Cu 3Sn layer was similar in thickness and appearance for both solders, but the Cu 6Sn 5 was smoother and rounder in the case of Sn–0.7Cu. Additional time above liquidus resulted in growth of the Cu 6Sn 5 layer and eventual spalling of the IMC grains. The effect of the intermetallic on the toughness ( K Q ) of the solder joint was investigated using a modified compact tension specimen. Typical failure modes included bulk solder failure, intergranular separation, and intermetallic fracture, or cleavage. In some cases, additional time above solder liquidus was used to shift the dominant failure mode from that dominated by the bulk solder to interfacial delamination through the intermetallics. Solder joint fracture toughness was different between Ni–Sn and Cu–Sn interfacial intermetallics and was also affected by the relative intermetallic thickness. The relationship between solder and intermetallic microstructure and mechanical properties is discussed.

Thermal Cycling Aging Effect on the Reliability and Morphological Evolution of SnAgCu Solder Joints

In solder ball grid array (BGA) technology, solder joint reliability is one of the critical issues in microelectronics manufacturing industries. In this reliability aging study, Sn3.5AgO.7Cu solder joints were subjected to accelerated temperature cycling (ATC) test in TBGA assembly. Fatigue fracture occurred, very close to the solder/intermetallic compound (IMC) interface, at the TBGA component side due to the larger coefficient of thermal expansion (CTE) mismatch compared to the PCB side. During reflow, needle-type and scallop-type morphologies of (Cu,Ni)6Sn5 IMCs were formed at the TBGA component and PCB interfaces. In the process of thermal cycling, a layer of (Ni,Cu)3Sn4 IMC grew beneath the (Cu,Ni)6Sn5 IMC due to the out diffusion of Ni from the under bump metallization (UBM). After extended thermal cycling aging, Ni-Sn-P IMC was found between the (Ni,Cu)3Sn4 IMC and the In3P layer at the printed circuit board (PCB) interface. Grain ripening and spalling of (Cu,Ni)6Sn5 IMC grains into the solder joint was also observed in the process of thermal cycling. The spalling phenomena of (Cu,Ni)6Sn5 IMCs was caused by interface structure change and cyclic shear stresses and strains incurred during temperature cycling.

Reliability testing of WLCSP lead-free solder joints

The interfacial reactions of solder joints between Sn-4Ag-0.5Cu solder ball and a couple of presoldered pastes (Sn-7Zn-Al(30ppm) and Sn-3Ag-0.5Cu) were investigated in wafer-level chip-scale package (WLCSP). After appropriate surface mount technology reflow processes on printed circuit boards with a Cu/OSP (organic solderability preservative) surface finish, samples were subjected to 150°C high-temperature storage (HTS) for 1,000 h of aging or 1,000 cycles of a thermal cycling test (TCT). Sequentially, cross-section analysis is scrutinized by scanning electron microscopy/energy dispersive spectrometry and energy probe microanalysis to observe metallurgical evolution in the interface and solder buck itself. It was found that the degradation of the joint shear strength after TCT is more pronounced than that of the shear strength after HTS. Fracture surface analyses of the shear tests show that the degradation of the joint strength for HTS is solely due to the influence of the interfacial IMC grain growth, while the shear strength degradation for TCT is mainly due to the coefficient thermal expansion mismatch from the thermal cycling at the chip-solder interface and can lead to the occurrence of the crack.