Reactions and mechanical properties between AuSn20 solders and metalized Al–Si alloys for electronic packaging application

AuSn20 (mass fraction) lead-free solder reacting with the Au/Ni-metalized AlSi50 substrate during reflowing and aging processes were investigated in this study. The single lap shear strength, fracture behavior and microstructure evolution characteristics of the joints are detected. It is found that only a thin (Ni,Au)₃Sn₂ layer forms at the interface between the AuSn20 solder and Ni metalized AlSi50 alloy. But a composite Intermetallic compound (IMC) layer of (Ni,Au)₃Sn₂ and (Au,Ni)Sn is formed in the aged joints, due to the continuous interfacial reactions during aging process. The growth of the composite IMC layer is governed by the volume diffusion of the constituent elements at 120, 160 and 200 °C. The shear strength decreases with the increasing aging time and temperature, which is caused primarily by the growth of the IMC layer. The presence of faceted structures on the fracture surfaces of these specimens is indicative of a brittle failure mode for the joints.     

Intermetallic reactions in a Sn-3.5Ag-1.5In solder ball-grid-array package with Au/Ni/Cu pads

In this paper, the interfacial reactions between Sn-3.5Ag solder and Sn-3.5Ag-1.5In solder and Au/Ni/Cu pads in ball-grid-array (BGA) packages during solid aging were investigated by microstructural observations and phase analysis. During the solid aging, the intermetallic compound (IMC) layer in Sn-3.5Ag/Au/Ni/Cu solder joints evolved from the (Ni, Au)Sn₄ phase to the Ni₃Sn₄ phase, but the rate of growth of the IMC layer did not change significantly. While, in Sn-3.5Ag-1.5In/Au/Ni/Cu solder joints, the phases evolved from the (Ni, Au)Sn₄ and Ni₃Sn₄ phases into Ni₃(Sn, In)₄ phase. The distribution of In atoms in the solder alloy weakened interatomic force in the Sn-3.5Ag-1.5In solder alloy and the involvement of In atoms in the interfacial reaction generated more energy of distortion of the Ni₃(Sn, In)₄ and (Ni, Au)(Sn, In)₄ lattices. These both accelerated the diffusion of Sn atoms and the rate of growth of the whole IMC layer, but this effect reduced gradually after prolonged aging.     

感应加热重熔无铅钎料凸台的界面反应及剪切性能

感应自发热重熔（ISHR）技术在电子互连的应用中具有明显的三维选择性加热和快速加热等优点．该方法能够很好地解决由于无铅钎料的应用引起的日益严重的诸多问题,如球栅阵列中各钎料球受热不均匀和芯片基板与钎料球同时受热等．为此,采用ISHR进行了无铅钎料Sn3．5Ag在Au／Ni／Cu焊盘上的重熔实验、高温老化实验以及凸台剪切实验．由实验结果可知钎料凸台可以提供足够的剪切强度．文中讨论了界面反应和金属间化合物的演化．在老化期间界面处生长了连续的Ni3Sn4金属间化合物层,同时在钎料体内部生成了分散的（Aux,Ni1-x）Sn4化合物．金属间化合物的生长速度与老化时间的平方根成正比。由此可以判断金属间化合物的生长是一种扩散控制过程．     

Effect of 1 wt% ZnO nanoparticles addition on the microstructure,IMC development,and mechanical properties of high Bi content Sn–57.6Bi–0.4Ag solder on Ni metalized Cu pads

In the present study, we investigate the performance of 1 wt% ZnO nanoparticles addition to Sn–57.6Bi–0.4Ag lead-free solder and thereby study the effect of nanoparticle addition on the hardness and the inter-metallic compound (IMC) growth, under different reflow conditions. The interfacial morphology of both the plain Sn–57.6Bi–0.4Ag solder and the composite Sn–57.6Bi–0.4Ag/1 % ZnO solder containing nanoparticles on Ni metalized Cu pads ball grid array substrates and the distribution of nanoparticles on the composite solder were characterized metallographically by using scanning electron microscope (SEM). Prior research efforts by others has found strong evidence that the thickness of the IMC layer increases substantially with the increase in the number of reflow cycles. At the interfaces between solder ball and substrate, scallop-shaped Sn–Ni–Cu IMC layer was found in both the plain solder and the composite solder. Meanwhile the SEM results indicated that the growth of IMC of the composite solder joint was different compared to the plain solder with same number of reflow cycles and reflow time. During the shear test, the shear strength of the composite solder samples containing nanoparticle reinforcement were consistently higher than the plain solder joints due to the fact that the IMC growth has been retarded substantially by the nanoparticle addition. The fracture surface of plain solder exhibited a brittle fracture mode with a relatively smooth surface while doped solder joints showed typical ductile failures with very rough dimpled surfaces.     

Interfacial reaction of Sn–2.0Ag–2.5Zn solder on Cu and Ni–W substrates

The interfacial reactions of Sn–2.0Ag–2.5Zn solder on Cu and Ni–W substrates after soldering and subsequent aging have been investigated in this study. Ni–W alloy layers with tungsten content of 3.0 and 10.0 at.% were electrodeposited on copper substrate. The interfacial micrographs of solder joints prepared at 250 °C for 15 s and aged at 150 °C for 24, 96 and 216 h are shown. Double-layer IMC composed of Cu₅Zn₈ and Ag₃Sn was observed at the interface of Sn–2Ag–2.5Zn and Cu couple, which was compact and acted as a barrier layer to confine the further growth of Cu–Sn IMC. On Ni–W barrier layer, a thin Ni₃Sn₄ film appeared between the solder and Ni–W layer, whose thickness decreases with the increase of W content. During the aging process, a thin layer of the Ni–W substrate transforms into an amorphous bright layer, and the thickness of amorphous layer increased as aging time extended. Referring to the elemental line-distribution and the thickness of different layers at the interface, the formation of the bright layer is caused by the fast diffusion of Sn into Ni–W layer.     

Effect of Cu diffusion through Ni on the interfacial reactions of Sn3.5Ag0.75Cu and SnPb solders with Au/Ni/Cu substrate during aging

The effect of Cu diffusion through Ni on the interfacial reactions of Sn3.5Ag0.75Cu (SAC) and SnPb solders with Au/Ni/Cu substrate during aging was investigated in this study. AuSn₄ needle-like intermetallic compound (IMC) formed at the interfaces of both SAC and SnPb solder joints after laser reflow. In SnPb solder joints, a layer-type ternary IMC, (Au,Ni)Sn₄, formed over a thin layer quaternary IMC (Au,Ni,Cu)₃Sn₄ after aging at 150 °C for 4 days, in which Cu came from the Cu pad below the Ni layer through diffusion. This (Au,Ni,Cu)₃Sn₄ IMC evolved into a new kind of quaternary IMC (Au,Ni,Cu)₆Sn₅ with further aging at 150 °C for 8 days because more Cu diffused through Ni and was involved into the IMC formation. For SAC solder joints, needle-like binary AuSn₄ IMC in as-soldered state changed into a layer-type quaternary IMC (Au,Ni,Cu)Sn₄. The quaternary IMC (Au,Ni,Cu)₆Sn₅, which has the same composition as those found in SnPb solder joints, was developed below the (Au,Ni,Cu)Sn₄ layer after aging at 150 °C for 8 days. It was found that Cu diffusion played a key role in the interfacial reaction and compound formation.     

Analysis of tin whisker growth on lead-free alloys with Ni presence under thermal shock stress

This paper presents the results of whiskers formation after thermal shocks on various tin-rich materials used in electronics: Sn100, Sn99Cu1, Sn97Cu3, Sn99.3Cu0.7Ni, Sn99.3Cu0.7AgNiGe and Sn99Ag0.3Cu0.7NiGe. Alloys plated over an Ni/Au sublayer were compared with those plated directly over a Cu layer. Scanning electron microscope (SEM) imaging was applied to determine whisker densities, lengths and types. Different degrees of susceptibility to whisker formation were found for all the investigated alloys subjected to 1500 shocks in a cyclic temperature range of −45 °C to +85 °C. Although the whisker growth rates for the alloys plated over an Ni/Au sublayer were lower than for the alloys plated directly over a Cu layer, all the materials tested were more or less prone to whisker formation.     

Interfacial reaction between Au–Sn solder and Au/Ni-metallized Kovar

Gold-tin (Au–Sn) solder and Kovar alloy have been widely used in many fields such as mechanical engineering, atomic energy industry, aerospace facility, and electronic devices. Solder bonds strongly to the metallized substrate by forming intermetallic compounds (IMCs) at the interface. The IMC layer may adversely affect the reliability of the joints due to excessive growth and thermal fatigue during storage and service. Therefore, knowledge of the interfacial reactions between the Au–Sn solder and Au/Ni-metallized Kovar in microelectronic and optoelectronic packaging is essential. In this study, the microstructural evolution and interfacial reactions between the Au–Sn solder and Au/Ni-plated Kovar substrate were studied during aging at 180 and 250 °C for up to 1,000 h. The microstructure of the Au–Sn/Ni/Kovar joint was stable during aging at 180 °C. The solid-state interfacial reaction was much faster at 250 °C than at 180 °C. The joints aged at 250 °C fractured along the interface, thereby demonstrating brittle failure possibly because of the brittle IMC layer at the interface. The complete consumption of the thin Ni layer significantly weakened the joint interface during aging at 250 °C and clearly demonstrated the need for a thicker Ni layer in order to ensure the high temperature reliability of the Au–Sn/Ni/Kovar joint above 250 °C.     

Interfacial microstructure and hardness of nickel (Ni) nanoparticle-doped tin–silver–copper (Sn–Ag–Cu) solders on immersion silver (Ag)-plated copper (Cu) substrates

Sn–Ag–Cu composite solder has been prepared by adding Ni nanoparticles. Interfacial reactions, the morphology of the intermetallic compounds (IMC) that were formed, the hardness between the solder joints and the plain Cu/immersion Ag-plated Cu pads depending on the number of the reflow cycles and the aging time have all been investigated. A scallop-shaped Cu₆Sn₅ IMC layer that adhered to the substrate surface was formed at the interfaces of the plain Sn–Ag–Cu solder joints during the early reflow cycles. A very thin Cu₃Sn IMC layer was found between the Cu₆Sn₅ IMC layer and the substrates after a lengthy reflow cycle and solid-state aging process. However, after adding Ni nanoparticles, a scallop-shaped (Cu, Ni)–Sn IMC layer was clearly observed at both of the substrate surfaces, without any Cu₃Sn IMC layer formation. Needle-shaped Ag₃Sn and sphere-shaped Cu₆Sn₅ IMC particles were clearly observed in the β-Sn matrix in the solder-ball region of the plain Sn–Ag–Cu solder joints. Additional fine (Cu, Ni)-Sn IMC particles were found to be homogeneously distributed in the β-Sn matrix of the solder joints containing the Ni nanoparticles. The Sn–Ag–Cu–0.5Ni composite solder joints consistently displayed higher hardness values than the plain Sn–Ag–Cu solder joints for any specific number of reflow cycles–on both substrates–due to their well-controlled, fine network-type microstructures and the homogeneous distribution of fine (Cu, Ni)–Sn IMC particles, which acted as second-phase strengthening mechanisms. The hardness values of Sn–Ag–Cu and Sn–Ag–Cu–0.5Ni on the Cu substrates after one reflow cycle were about 15.1 and 16.6 Hv, respectively–and about 12.2 and 14.4 Hv after sixteen reflow cycles, respectively. However, the hardness values of the plain Sn–Ag–Cu solder joint and solder joint containing 0.5 wt% Ni nanoparticles after one reflow cycle on the immersion Ag plated Cu substrates were about 17.7 and 18.7 Hv, respectively, and about 13.2 and 15.3 Hv after sixteen reflow cycles, respectively.     

Influence of Thermal Cycling on the Microstructure and Shear Strength of Sn3.5Ag0.75Cu and Sn63Pb37 Solder Joints on Au/Ni Metallization

The influence of thermal cycling on the microstructure and joint strength of Sn3.5Ag0.75Cu （SAC） and Sn63Pb37 （SnPb） solder joints was investigated. SAC and SnPb solder balls were soldered on 0.1 and 0.9 μm Au finished metallization, respectively. After 1000 thermal cycles between -40℃ and 125℃, a very thin intermetallic compound （IMC） layer containing Au, Sn, Ni, and Cu formed at the interface between SAC solder joints and underneath metallization with 0.1 μm Au finish, and （Au, Ni, Cu）Sn4 and a very thin AuSn-Ni-Cu IMC layer formed between SAC solder joints and underneath metallization with 0.9 μm Au finish. For SnPb solder joints with 0.1 μm Au finish, a thin （Ni, Cu, Au）3Sn4 IMC layer and a Pb-rich layer formed below and above the （Au, Ni）Sn4 IMC, respectively. Cu diffused through Ni layer and was involved into the IMC formation process. Similar interfacial microstructure was also found for SnPb solder joints with 0.9μm Au finish. The results of shear test show that the shear strength of SAC solder joints is consistently higher than that of SnPb eutectic solder joints during thermal cycling.     

Harsh service environment effects on the microstructure and mechanical properties of Sn–Ag–Cu-1 wt% nano-Al solder alloy

This paper investigates the electrical and mechanical performances of eutectic Sn-3Ag-0.5Cu (wt%) solder with the addition of Al nanoparticles. The study revealed that the elastic moduli, electrical resistivity and damping properties of such solder alloy were improved. Further, interfacial reaction phenomena on Au/Ni-plated Cu pad ball grid array substrate during isothermal aging and thermal cycle was evaluated in terms of the formation and growth kinetics of intermetallic compound (IMC) layer. A structural analysis confirmed that at their interfaces a ternary (Cu, Ni)-Sn IMC layer was adhered at the substrate surface. The thickness of this IMC layer was increased with increasing the duration of the isothermal aging and thermal cycle without any defects. In addition, the formation of Ag₃Sn, Cu₆Sn₅, Sn–Al–Ag and AuSn₄ IMC phases were evenly distributed in the solder matrix which acts as the second phase reinforcement. The measured shear strength and microhardness indicated that the exposure of the solder joints to the thermal cycles make the joints degraded faster than the situation in isothermal aging.     

Wettability and interfacial whiskers of Sn–9Zn–0.5Ga–0.08Nd solder with Sn,SnBi and Au/Ni coatings

The wettability and interfacial whiskers of Sn–9Zn–0.5Ga–0.08Nd solder were investigated. The results indicated that Sn–9Zn–0.5Nd–0.08Nd solder has shown good wettability using water soluble flux and self made flux based on wetting balance testing, and solder with SnBi coating has shown superiority wettability among all three coatings. Moreover, no whisker was observed in Sn–9Zn–0.5Ga–0.08Nd solder with different coatings. But in Sn–9Zn–0.5Ga–1Nd solder, fewer whiskers were observed in solder with SnBi coating compared with Sn and Au/Ni coatings, which can be attributed to the refinement effect of Bi.     

Intermetallic compounds evolution between lead-free solder and cu-based lead frame alloys during isothermal aging

This paper investigated the intermetallic compounds (IMCs) formation between SnAgCu solder and four Cu-based lead frame alloys during reflow soldering and isothermal aging. Scanning Electron Microscope (SEM) and Energy Dispersive X-ray (EDX) were used to study the cross-sectional microstructure and stoichiometric information. Optical Microscope (OM) was used to measure the mean thickness of IMCs. It was found that Ni and Sn trace element have important influences on the interfacial reactions. After soldering, for the case of Sn-Ag-Cu solder on Cu-Sn-Cr-Zn, Cu-Sn-P and Cu-Fe-P-Zn-Pb, an island type Cu6Sn5 and a thin Cu3Sn layer were formed at the interface. However, for the case on Cu-Ni-Si-Mg alloy, no Cu3Sn was detected and only a layer of ternary (Cu,Ni)6Sn5 was confirmed and some Cu6Sn5 particles was observed to disperse in the bulk solder. The top morphology of IMCs was also characterized after the solder was selectively etched away. The IMCs on the Cu-Ni-Si-Mg showed long rod-like shape, whilst the IMCs on the other three alloys appeared round. After different duration of aging at 150°C, all the IMCs grew thicker and the grain size became larger. The rod-like IMCs on Cu-Ni-Si-Mg gradually transformed into round shape and it was relatively smaller compared to that on the other three alloys. Moreover, the growth rate of IMCs on Cu-Ni-Si-Mg is the fastest among the four alloys.     

Suppressing Ni3Sn4 formation in the Sn–Ag–Cu solder joints with Ni–P/Pd/Au surface finish

The interfacial reactions of Sn–3.0Ag–0.5Cu solder jointed with electroless Ni–P/immersion Au (ENIG) and electroless Ni–P/electroless Pd/immersion Au (ENEPIG) were investigated. Cu₆Sn₅ grew rather slower in ENEPIG samples among all aging conditions as compared with ENIG. Furthermore, the second phase, Ni₃Sn₄, started to form in the ENIG aged joints, but not in the ENEPIG aged joints. It was demonstrated that ENEPIG could inhibit the formation of Ni₃Sn₄, which further decreased the growth of columnar Kirkendall voids inside the Ni₃P layer. With less voids formed in the Ni₃P layer, it is expected that the reliability of ENEPIG joints would be superior to that of ENIG joints. Detailed mechanisms of Ni₃Sn₄ suppression and void formation were discussed and proposed.     

Characters of multicomponent lead-free solders

In the process of electronic packaging, such as flip chip technology, under bump metallization (UBM) can be consumed gradually by solder during soldering. Then dissolution of Ni, Au and Cu from UBM into the solder may change the original solder to a multicomponent one especially under the trend of miniaturization. It is quite necessary to evaluate the properties of the multicomponent solders that have new composition after soldering. In this study, the microstructure, thermal and mechanical properties of five types of multicomponent lead-free solders, i.e. Sn–2Cu–0.5Ni, Sn–2Cu–0.5Ni–0.5Au, Sn–3.5Ag–0.5Ni, Sn–3.5Ag–1Cu–0.5Ni and Sn–3.5Ag–2Cu–0.5Ni (all in wt% unless specified otherwise) were investigated. Comparison with eutectic Sn–0.7Cu, Sn–3.5Ag and Sn–3.5Ag–0.7Cu solders was made. There was no obvious difference of the melting point between the multicomponent lead-free solders and the eutectic ones. For Sn–2Cu–0.5Ni solder, Cu₆Sn₅ and (Cu,Ni)₆Sn₅ intermetallic compounds (IMCs) formed. In the case of Sn–2Cu–0.5Ni–0.5Au, besides (Cu,Ni)₆Sn₅, (Cu,Au)₆Sn₅ and (Cu,Ni,Au)₆Sn₅ were also observed. The IMCs formed in Sn–3.5Ag–0.5Ni solder were Ag₃Sn and Ni₃Sn₄. In both Sn–3.5Ag–1Cu–0.5Ni and Sn–3.5Ag–2Cu–0.5Ni solders, Ag₃Sn and (Cu,Ni)₆Sn₅ were detected. The mechanism for the formation of the IMCs was discussed. Tensile test was also conducted. The fractography indicated that all of the multicomponent lead-free solders exhibited a ductile rupture.     

三价铈离子掺杂氯化钾的光致发光性能

采用水热蒸发法制备了KCl∶Ce3+荧光粉。测量并分析了材料在室温下的真空紫外激发光谱及相应的发射光谱。结果表明激发谱显示6个峰,峰位分别为149、194、206、219、233和251nm。其中149nm的激发峰是基质吸收引起的;194、206、219、233和251nm是Ce3+离子的4f→5d跃迁引起的。发射峰显示双峰结构,峰位分别是311和326nm。此峰对应于Ce3+离子的5d→4f(2F5/2,2F7/2)跃迁。     

Characteristics of Laser Reflow Bumping of Sn3.5Ag and Sn3.5Ag0.5Cu Lead-Free Solder Balls

ead-free Sn3.5Ag and Sn3.5Ag0.5Cu solder balls were reflowed by laser to form solder bumps. Shear test was performed on the solder bumps, and SEM/EDX （scanning electron microscopy/energy dispersive X-ray spectrometer） was used to analyze the formation of intermetallic compounds （IMCs） at interface region. A finite element modeling on the temperature gradient and distribution at the interface of solder bump during laser reflow process was conducted to elucidate the mechanism of the IMCs growth direction. The results show that the parameters window for laser reflow bumping of Sn3.5Ag0.5Cu was wider than that of Sn3.5Ag. The shear strength of Sn3.5Ag0.5Cu solder bump was comparable to that of Sn3.5Ag solder bump, and was not affected obviously by laser power and irradiation time when appropriate parameters were used. Both laser power and heating time had a significant effect on the formation of IMCs. A continuous AuSn4 intermetallic compound layer and some needle-like AuSn4 were observed at the interface of solder and Au/Ni/Cu metallization layer when the laser power is small. The formation of needle-like AuSn4 was due to temperature gradient at the interface, and the direction of temperature gradient was the preferred growth direction of AuSn4. With increasing the laser power and heating time, the needle-like AuSn4 IMCs dissolved into the bulk solder, and precipitated out once again during solidification along the grain boundary of the solder bump.     

Influence of POSS nano-particles on Sn–3.0Ag–0.5Cu–xPOSS/Cu composite solder joints during isothermal aging

Polyhedral oligomeric silsesquioxanes (POSS) nano-particles reinforced Sn–3.0Ag–0.5Cu–xPOSS (x = 1, 3, 5) composite solders were prepared and were reflowed on Cu substrates at 543 K. Then, the solder joints were isothermal aged at 393 K for 24 and 48 h, respectively. Microstructural evolution of the solder joints were observed by scanning electron microscopy and the influence of POSS nano-particles on the solder joint were investigated. The results showed that β-Sn primary phase was refined and the number of grain boundary increased with the addition of POSS nano-particles. The growth rates of intermetallic compounds (IMCs) layer were suppressed by the adsorption affection of POSS nano-particles to the IMCs layer during isothermal aging. Moreover, the dissolution process of the IMCs layer, which was accompanied by with the growth of the IMCs layer, changed the morphology of the IMCs layer. The growth rate and the dissolution rate of the IMCs layer in Sn–3.0Ag–0.5Cu–3POSS/Cu composite solder joint were the lowest.     

Cadmium oxide whisker crystals grown by the vapour-liquid-solid mechanism using various elements as growth initiators

CdO whisker crystals with micrometre thickness and length of up to 4.5 mm were grown by a simple technology described in previous publications of the authors. The whiskers grow on CdS crystal substrates, previously covered by a thin metal or silicon layer, on annealing in air at atmospheric pressure. The growth initiator used was Ag, Au, AI, Si, Mn, Ni, Pd or Pt. A study was carried out of the effect of impurities on the conditions and efficiency of growth, as well as on the morphology and composition of the whiskers obtained. The small crystals were examined using an optical microscope, a scanning electron microscope, X-ray structural analysis and X-ray microanalysis. The maximum speed of growth obtained with Au was about 1 mm h^–1. The whiskers grow with a face-centred cubic lattice. The largest crystals are in the form of a parallelipiped with facets of {100}. Data are obtained which support the vapour-liquid-solid mechanism of crystal growth. The possibility is also shown of growing whiskers on zinc or cadmium plates covered with a thin layer of Cu, Ag, Au or AI by applying the same technology.     

Microstructural variation and high-speed impact responses of Sn–3.0Ag–0.5Cu/ENEPIG solder joints with ultra-thin Ni–P deposit

Electroless Ni–P/electroless Pd/immersion Au (ENEPIG) with ultra-thin Ni–P deposit serve as a potential replacement of traditional ENEPIG surface finish because of its superior electrical performance in flip chip solder joints interconnection. However, the interfacial reaction and mechanical reliability of solder joints in ENEPIG with ultra-thin Ni–P layer is not yet well evaluated. In this study, we investigated the characteristic microstructure of interfacial intermetallic compounds and high-speed impact responses of Sn–3.0Ag–0.5Cu/ENEPIG attachments with 4.8, 0.3, and 0.05 μm Ni–P deposit. ENEPIG with Ni–P layer of 0.3 μm exhibited the eutectic structure dispreading in the solder alloys and layer-type P-rich IMCs at solder/metallization interface, while there was (Cu,Ni)₆Sn₅ precipitation in the solder but no P-rich IMCs layer formed in ENEPIG with 0.05 μm Ni–P layer. Slower interfacial reaction rate in ENEPIG with 0.3 μm Ni–P layer was attributed to the effect of electroless Ni–P diffusion barrier layer, which would further provide better impact resistivity than that of ENEPIG with 0.05 μm Ni–P deposit. Moreover, breach in P-rich IMCs and underneath (Cu,Ni)₆Sn₅ patch were observed in ENEPIG with 0.3 μm Ni–P layer. The growth mechanism was closely related to the Ni diffusion from surface finish and element redistribution.