The effects of solder deformation on the wetting characteristics and interfacial reaction of Sn–3.5Ag solders on Cu substrates in fluxless soldering

The effects of solder deformation on the wetting characteristics during fluxless soldering were studied when deformed Sn–3.5Ag solder balls were reacted with Cu or oxidized Cu substrates. The Cu surfaces were oxidized at 100 °C for 2 or 4 h in air. After the 760 μm diameter solder balls were deformed on the substrates under 0–30 N, they were then reflowed at 300 °C for 30 s without flux. An optical microscope and a scanning electron microscope equipped with energy dispersive spectroscopy were used to measure the wetting angles and to characterize interfacial microstructures. As solder deformation increased, the wetting angle of solder bumps on the Cu or oxidized Cu substrates decreased and the spreading area increased. The oxide layer on the Cu surface decreased the wettability of the solders. Intermetallic compound (IMC) growth was suppressed in the solder interface when the solder reacted with oxidized Cu, while the IMC thickness increased with solder deformation. Solder deformation exposed a fresh Sn surface and improved contact between the solder and Cu substrate, thereby increasing the wettability of the solders.     

Formation of Sn–Bi solder alloys by sequential electrodeposition and reflow

Eutectic Sn–Bi alloy is gaining considerable attention in the electronic packaging industry because of its favorable properties such as low melting temperature, good wettability, and good mechanical properties. Miniaturization of electronic devices requires small solder bumps, a few tens of micrometers in diameter. Electrodeposition is a reliable technique for the deposition of small volume of solder. This work focuses on the formation of eutectic Sn–Bi solder by reflowing a metal stack containing sequentially electrodeposited Sn and Bi layers. The effects of layering sequence on the composition and microstructure of the resulting alloy is investigated. Irrespective of the layering sequence, a homogeneous microstructure is achieved after reflow. The microstructure of the reflowed samples is the same as that of a metallurgically processed Sn–Bi alloy. Near-eutectic alloy with the composition Sn–54.6 wt% Bi is obtained by the sequential electrodeposition method.     

Microstructures formed from mixed solders on copper substrate

Abstract

With the development and use of a variety of Pb free solders, it is probable that some solder joints in electronic assemblies may be made with solders of two different compositions. To investigate possible microstructures resulting from such procedure, samples were prepared using small balls of four different Sn–Ag–Cu (SAC) Pb free solders, as well as Sn–Zn–Al solder, melted together with eutectic Pb–Sn solder paste and also various SAC solder pastes, on a copper substrate. It was observed that using eutectic Pb–Sn solder paste with an SAC solder ball introduced some Pb–Sn eutectic microstructure and changed the ternary eutectic present from Ag₃Sn–Cu₆Sn₅–Sn to Ag₃Sn–Pb–Sn. Use of an SAC solder paste with Sn–Zn–Al solder introduced an apparent Ag–Cu–Zn ternary compound, replacing Zn lamellae of the Sn–Zn eutectic. With eutectic Pb–Sn solder paste, the Pb–Sn–Zn ternary eutectic was formed. It was noted that use of a high Sn solder results in rapid dissolution of the copper substrate.     

Modifying the mechanical properties of lead-free solder by adding iron and indium and using a lap joint test

Eliminating lead in electronics is an environmental consciousness that is taken prior to manufacturing. Lead-free solder has recently been developed to advance that goal. One of the most common types of lead-free solder is Sn–Ag–Cu(SAC). Adding alloying elements can modify the properties of SAC. The present study is devoted to the research and development of SAC for microelectronic packaging applications. The effects of iron and indium addition to SAC were investigated. Four different samples were fabricated by casting: Sn–3.6Ag–0.9Cu, Sn–3.6Ag–0.9Cu–0.2Fe, Sn–3.6Ag–0.9Cu–0.6Fe, and SAC-InCe. Reliability tests were done on Cu and Ni–P substrate. The shear strength of the joint was improved by decreasing the intermetallic compound (IMC) thickness; so the IMCs thickness must be controlled, because the formation of IMC leads to joint embrittlement at the interface. In conclusion, the addition of In and Fe can improve mechanical properties, such as shear strength, but the addition of In appears to be more effective for increasing the fracture toughness. The addition of Fe lowers the wetting angle and it can effectively improve the solder reliability, this improvement in shear behavior for the samples, which were reflowed on Cu substrate, is enhanced compared with the Ni substrate, but 0.6% Fe addition for the Cu substrate illustrates an decrease in fracture strain, because of abnormal growth of Cu₆Sn₅Whiskers in this case.     

Annealing effect to constitutive behavior of Sn–3.0Ag–0.5Cu solder

Sn–Ag–Cu based solder alloys are replacing Sn–Pb solders in electronic packaging structures of commercial electric devices. In order to evaluate the structural reliability, the mechanical property of solder material is critical to the numerical simulations. Annealing process has been found to stabilize material properties of Sn–37Pb solder material. In the current study, the annealing effect on tensile behaviour of Sn–3.0Ag–0.5Cu (SAC305) solder material is investigated and compared with Sn–37Pb solder. It is found that the tensile strength for both materials are more stabilized and consistent after the annealing process, nevertheless, the annealing process will improve the plasticity of SAC305 solder dominated by dislocation motion, and impede the occurrence of hardening deformation in Sn–37Pb solder dominated by grain-boundary sliding mechanism. Furthermore, the annealing effect is quantified in the proposed constitutive model based on unified creep–plasticity theory. The parameters are calibrated against the measured stress–strain relationships at the tensile strain rates ranging from 1?×?10^?4 to 1?×?10^?3 s^?1. The numerical regressions for dominant parameters in the proposed model reveal the intrinsic differences between SAC305 and Sn–37Pb solders under annealing treatment.     

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．     

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.     

Review on microstructure evolution in Sn–Ag–Cu solders and its effect on mechanical integrity of solder joints

The use of Pb-bearing solders in electronic assemblies is avoided in many countries due to the inherent toxicity and environmental risks associated with lead. Although a number of “Pb-free” alloys have been invented, none of them meet all the standards generally satisfied by a conventional Pb–Sn alloy. A large number of reliability problems still exist with lead free solder joints. Solder joint reliability depends on mechanical strength, fatigue resistance, hardness, coefficient of thermal expansion which are influenced by the microstructure, type and morphology of inter metallic compounds (IMC). In recent years, Sn rich solders have been considered as suitable replacement for Pb bearing solders. The objective of this review is to study the evolution of microstructural phases in commonly used lead free xSn–yAg–zCu solders and the various factors such as substrate, minor alloying, mechanical and thermo-mechanical strains which affect the microstructure. A complete understanding of the mechanisms that determine the formation and growth of interfacial IMCs is essential for developing solder joints with high reliability. The data available in the open literature have been reviewed and discussed.     

Effects of metallic nanoparticle doped flux on the interfacial intermetallic compounds between lead-free solder ball and copper substrate

Lead free solders currently in use are prone to develop thick interfacial intermetallic compound layers with rough morphology which are detrimental to the long term solder joint reliability. A novel method has been developed to control the morphology and growth of intermetallic compound layers between lead-free Sn–3.0Ag–0.5Cu solder ball and copper substrate by doping a water soluble flux with metallic nanoparticles. Four types of metallic nanoparticles (nickel, cobalt, molybdenum and titanium) were used to investigate their effects on the wetting behavior and interfacial microstructural evaluations after reflow. Nanoparticles were dispersed manually with a water soluble flux and the resulting nanoparticle doped flux was placed on copper substrate. Lead-free Sn–3.0Ag–0.5Cu solder balls of diameter 0.45 mm were placed on top of the flux and were reflowed at a peak temperature of 240 °C for 45 s. Angle of contact, wetting area and interfacial microstructure were studied by optical microscopy, field emission scanning electron microscopy and energy-dispersive X-ray spectroscopy. It was observed that the angle of contact increased and wetting area decreased with the addition of cobalt, molybdenum and titanium nanoparticles to flux. On the other hand, wettability improved with the addition of nickel nanoparticles. Cross-sectional micrographs revealed that both nickel and cobalt nanoparticle doping transformed the morphology of Cu₆Sn₅ from a typical scallop type to a planer one and reduced the intermetallic compound thickness under optimum condition. These effects were suggested to be related to in-situ interfacial alloying at the interface during reflow. The minimum amount of nanoparticles required to produce the planer morphology was found to be 0.1 wt.% for both nickel and cobalt. Molybdenum and titanium nanoparticles neither appear to undergo alloying during reflow nor have any influence at the solder/substrate interfacial reaction. Thus, doping of flux with appropriate metallic nanoparticles can be successfully used to control the morphology and growth of intermetallic compound layers at the solder/substrate interface which is expected to lead to better reliability of electronic devices.     

Precipitation of large Ag3Sn intermetallic compounds in SnAg2.5 microbumps after multiple reflows in 3D-IC packaging

Microbumps have been adopted as interconnects between Si chips in 3D integrated-circuit packaging. The solder volume of a microbump decreases dramatically due to fine-pitch requirement and it is approximately two orders smaller in magnitude than that of a traditional flip-chip solder joint. The metallurgical reactions in the microbumps may behave quite differently to those in flip-ship bumps. Liquid-state metallurgical reactions were examined in SnAg2.5 microbumps with Ni metallization. The results indicate that large particles of Ag₃Sn intermetallic compounds (IMCs) precipitate after a 10-min reflow on microbumps with 4.0-μm-thick solder, which does not occur with flip-chip solder bumps. It is proposed that the Ag concentration in the remaining solder may increase as Sn reacts with Ni. The increase in the Ag concentration is mainly responsible for the occurrence of the large Ag₃Sn precipitates. The formation of these Ag₃Sn IMCs would be detrimental to the mechanical properties of the microbumps.     

Wetting behavior and elastic properties of low alpha SAC105 and pure Sn solder

The issue of soft error in microelectronics packaging have necessitated the development of low alpha (LA) activity solders as, as solders are found to be major sources of radiation in electronic devices that causes soft error. Low alpha ray emitter Sn and Sn–1.0 %Ag–0.5 %Cu (SAC105) solders were prepared and their alpha activity was measured using Ultra Low Background Alpha Particle Counting Systems. The solders are confirmed to be low alpha solders having activity less than 0.005 α/h/cm² (LC3 Grade). Microstructure of the LA SAC105 and Sn are characterized using scanning electron microscopy. Wetting balance tests are conducted to assess their solderability at different temperatures. Elastic properties i.e. Young Modulus and hardness were determined using Vickers hardness test and Nanoindentation technique. All the solders show good wettability and solderability having zero cross time less than 1.5 s. The wettability of low alpha solders is better as compared to the normal ones. Hardness results show the reduction in hardness of the LA solders as compared to normal ones. This is attributed to removal of radioactive trace impurities as the latter results in high hardness due to solid-solution hardening mechanism. Nanoindentation results shows the scatter in the values of hardness and Young’s Modulus for different IMCs phases found in solder joint with Cu and in bulk.     

Effect of ZnO nanoparticles addition on thermal, microstructure and tensile properties of Sn–3.5 Ag–0.5 Cu (SAC355) solder alloy

Regarding to the development of Sn–Ag–Cu (SAC) lead-free solders for advance electronic components, the effect of 0.5 wt% nano-sized ZnO particles on the thermal, microstructure and tensile properties of Sn–3.5 wt% Ag–0.5 wt% Cu (SAC355) lead-free solder alloy is investigated. The results showed that addition of 0.5 wt% nano-sized ZnO particles into the conventional lead-free SAC355 solder caused a slight increase of its liquidus temperature by about 1.1 K. Metallographic observations of SAC355–0.5 wt% ZnO (composite solder) revealed an obvious refinement in the microstructure compared with the SAC355 (non-composite) solder. Consequently, addition of nano sized-ZnO particles could improve the stress–strain characteristics proof stress (σ_y0.2) and ultimate strength (σ_UTS). This was rendered to suppressing effect of ZnO on the coarsening of the intemetallic compounds (IMCs) Ag₃Sn and Cu₆Sn₅ during the solidification process in the composite solder and subsequently dispersion strengthening is considered to be the dominating mechanism. This will allow the use of SAC355 composite lead-free solder alloy, to be consistent with the conditions of usage for conventional SAC solder alloys and to overcome the serious problem of the excessive growth of IMCs and the formation of microvoids in the SAC lead-free solder alloys.     

Dissolution of Copper and Formation of IMC in Bulk Lead-Free Solders

Besides the popular reflow soldering practice, wave soldering is another established technique in the electronic packaging industry for obtaining the solder joints. In this study the dissolution behavior of copper and the growth of intermetallic compound (IMC) during wave soldering process have been investigated. The study involved two different solder compositions, viz. pure Sn and eutectic Sn–Ag solder. Dipping experiments were conducted for the temperature range of 221–300 °C and for dwell time starting form 5 s to 6 min. The average thickness of the intermetallic compound formed and the amount of copper dissolved was determined. Experimental results indicated that the amount and rate of copper dissolution were strongly affected by temperature of molten solder and dwell time. The initial rate of copper dissolution was very high but it decreased with time. Based on the experimental results, the kinetic parameters governing the dissolution of copper and growth of the IMC were determined for the two solders.     

Investigation on the intermetallic compound layer growth of SnZnGa/SnZnGaNd solder joints

Growth kinetics of intermetallic compound layers (IMCs) formed between SnZnGa/SnZnGaNd solders for re-flow soldering and Cu substrate during aging was investigated at temperatures between 100 and 150 °C. The Cu₅Zn₈ IMCs could be found by reacting SnZnGa/SnZnGaNd solders on Cu substrate, and it was found that the addition of rare earth Nd can decrease the thickness of the IMCs of SnZn/Cu solder joints. The apparent activation energies of Cu₅Zn₈ which were calculated as 48.76 kJ/mol (SnZnGa) and 56.99 kJ/mol (SnZnGaNd). The shear force of Sn9Zn0.5Ga0.08Nd solder joint after aging treatment was remarkably higher than those of Sn–9Zn and Sn9Zn0.5Ga solder joints. The results would provide support to the superiority of Sn9ZnGaNd solder which can be used in the electronic packaging instead of traditional SnPb solder.     

Microstructural damage analysis of SnAgCu solder joints and an assessment on indentation procedures

In solder joint reliability, solder/pad adjoining interface is crucial, the quality of which is determined by the metallization. In this paper, microstructural analyses of SnAgCu alloy and soldered joints are conducted in direct connection with the metallization. Solder balls, solder paste and cast SnAgCu are reflowed on Cu, Ni/Au and Cu/Ni(V)/Au. Substrate influence on bulk and interfacial metallurgy is examined and a correlation between interfacial microstructure and the corresponding damage paths is established. Damage localizes at the bonding interfaces with a strong influence of intermetallic layers and primary crystals. Crack propagation is studied with Cu and Ni/Au substrates and the cracking mechanism in principal directions is scrutinized. In BGA production, different reflow parameters are investigated, and an optimum bumping procedure is established. Nano-indentation is used for the mechanical characterization of the solder alloy. An assessment on indentation parameters for soft solders is conducted and the influence of Ag content on material properties of SnAgCu is presented.     

Multiple reflow study of ball grid array (BGA) solder joints on Au/Ni metallization

This paper evaluates the shearing behavior of ball grid array (BGA) solder joints on Au/Ni/Cu pads of FR4 substrates after multiple reflow soldering. A new Pb-free solder, Sn–3Ag–0.5Cu–8In (SACI), has been compared with Sn–3Ag–0.5Cu (SAC) and Sn–37Pb (SP) solders, in terms of fracture surfaces, shearing forces and microstructures. Three failure modes, ball cut, a combination of solder shear and solder/pad bond separation, and pad lift, are assessed for the different solders and reflow cycles. It is found that the shearing forces of the SP and SAC solder joints tend to increase slightly with an increase in the number of reflow cycles due to diffusion-induced solid solution strengthening of the bulk solder and augmentation of the shearing area. However, the shearing forces of the SACI solder joints decrease slightly after four cycles of reflow, which is ascribed to the thermal degradation of both the solder/intermetallic compound (IMC) and IMC/Ni interfaces. The SACI solder joints yield the highest strengths, whereas the SP solder joints give the smallest values, irrespective of the number of reflow cycles. Thickening of the interfacial IMC layer and coarsening of the dispersing IMC particles within the bulk solders were also observed. Nevertheless, the variation of shearing forces and IMC thickness with different numbers of reflow cycles was not so significant since the Ni under layer acted as an effective diffusion barrier. In addition, the initially-formed IMC layer retarded the further extensive dissolution of the pad material and its interaction with the solder.     

Effects of Nano-TiO2 additions on thermal analysis, microstructure and tensile properties of Sn3.5Ag0.25Cu solder

For development of a lead-free composite solder for advance electrical components, a series of Sn3.5Ag0.25Cu (SAC) solders containing TiO₂ nanopowders have been studied. The results showed that the addition of 0.25–1 wt.% TiO₂ nanopowders into the lead-free SAC solder caused its liquidus temperature to increase by about 3.5–5.9 °C. Metallographic observations of the lead-free SAC composite solders revealed a reduction in the grain size of β-Sn, Ag₃Sn phase and Ag₃Sn phase located between the spacing lamellae. In terms of mechanical behavior, the addition of larger weight fractions of TiO₂ nanopowders in lead-free SAC solder matrix led to an improvement in microhardness, 0.2%YS and UTS. However, ductility of the lead-free SAC composite solders was observed to decrease.     

Dynamic observations of Sn-Pb solder reflow in a hot-stage environmental scanning electron microscope

Solidification behaviour of solders has a critical effect on the resulting microstructures and hence mechanical properties. Therefore, it is essential to understand the effects of soldering processing parameters on microstructure to engineer optimum microstructures. Microstructural changes in the solder that occur during the reflow process were studied in a hot-stage environmental scanning electron microscope (ESEM). An off-eutectic Sn60-Pb40 solder and a dispersion-strengthened solder with the same Sn-Pb ratio were reflowed in an ESEM, and changes in the microstructure were recorded on video tape. The dissolution and nucleation of grains during melting and solidification were observed. It was found that the microstructure of the conventional solder became coarser when it was allowed to solidify from a melt where the proeutectic lead was not completely dissolved. Grain refinement was observed in dispersion-strengthened solder where the dispersoids acted as heterogeneous nucleation sites.     

Solder joint reliability of Sn–0·7Cu and Sn–0·3Ag–0·7Cu lead-free solder alloys solidified on copper substrates with different surface roughnesses

Abstract

The bond shear test was used to assess the integrity of Sn–0·7Cu and Sn–0·3Ag–0·7Cu lead-free solder alloy drops solidified on copper substrates with smooth and rough surface finishes. Solder alloys solidified on smooth substrates required higher shear force compared to that on rough substrates. Sn–0·3Ag–0·7Cu alloy required higher shear energy than Sn–0·7Cu alloy. Solder alloys solidified on smooth substrate surfaces exhibited complete ductile failure. On rough copper surfaces, solder alloys showed a transition ridge characterized by sheared intermetallic compounds (IMCs) and the presence of dimples. The peak shear strength decreased with increase in contact area of the solder bond on the substrate. Smooth surface and the presence of minor amount of Ag in the solder alloy enhance the integrity of the solder joint.     

'Spalling' of SAC Pb free solders when used with nickel substrates

Abstract

The term "spalling" refers to the formation in a solder joint of a second layer of intermetallic compound, in addition to and separate from the usual single layer that normally forms on the substrate. It has been observed in Pb-free solders of the SAC type when used in conjunction with a nickel finish. Explanations offered to account for this phenomenon have been based on thermodynamic phase equilibria observed in isothermal sections of the Sn corner of the Cu–Ni–Sn system at temperatures 20°C or more above the solidification temperature of the solder, and without regard to reactions that occur at low temperatures, during freezing. Recent measurement of the liquidus projection of the Sn corner of the Cu–Ni–Sn system as shown the occurrence of a ternary quasiperitectic reaction in the composition range in which "spalling" is observed. Evaluation of the effect of this quasiperitectic reaction shows that its operation during freezing of the solder can account for the "spalling" phenomenon.