Dissolution and interface reactions between the 95.5Sn-3.9Ag-0.6Cu, 99.3Sn-0.7Cu, and 63Sn-37Pb solders on silver base metal

Dissolution and intermetallic compound (IMC) layer development were examined for couples formed between 99.9 silver (Ag) and molten 95.5Sn-3.9Ag-0.6Cu (wt pct), 99.3Sn-0.7Cu, and 63Sn-37Pb solders, using a range of solder temperatures and exposure times. The interface reactions that controlled Ag dissolution were sensitive to the solder composition. The Ag₃Sn IMC layer thickness and interface microstructure as a whole exhibited nonmonotonic trends and were controlled primarily by the near-interface solder composition. The kinetics of IMC layer growth were weakly dependent upon the solder composition. The processes of Ag dissolution and IMC layer growth were independent of one another.     

On the Asymmetric Growth Behavior of Intermetallic Compound Layers During Extended Reflow of Sn-Rich Alloy on Cu

When solder interconnects are fabricated, a Sn-based alloy is melted between two substrates with metallization layers, such as Cu or Ni. From the reaction between Sn and Cu, a Cu₆Sn₅ intermetallic compound (IMC) layer is formed at the solder/Cu interfaces. The morphology of the IMC layer greatly influences the mechanical behavior of the solder joint. Here, we report on the characterization of a novel, asymmetric growth behavior of IMC layers in Sn-3.9Ag-0.7Cu solder joints, based on gravity-induced spalling of the IMC.     

Interfacial Reactions,Microstructure, and Strength of Sn-8Zn-3Bi and Sn-9Zn-Al Solder on Cu and Au/Ni (P) Pads

The Sn-8Zn-3Bi and Sn-9Zn-Al Pb-free solders were used to mount passive components onto printed circuit boards (PCBs) with electroless Ni immersed Au (ENIG) finishing layers using a reflow soldering process. The component mounted boards were aged at 150 °C for 200 to 1100 hours. The interfacial reactions and microstructure of the interfaces between the solders and the pads were observed using scanning electron microscopy and energy-dispersive spectrometry (EDS). Both solder joints on the two pads had similar interfacial microstructures; i.e., a very thin γ ₂-AuZn₃ layer was formed at the interface of the solder and Ni-P layer. The γ ₂-AuZn₃ layer transformed to an ε-AuZn₈ intermetallic compounds (IMC) with a consistent thickness during aging. Zinc atoms redeposited onto the IMC layer increased with increasing aging time. After aging at 150 °C for various times, the shear strengths of the ENIG and organic solderability preservative (OSP) joints were evaluated. The shear strength of the Sn-8Zn-3Bi solder joint was better than that of the Sn-9Zn-Al solder joint. All of the solder joints deteriorated after aging; however, the degradations of the OSP solder joints were more evident than those of the ENIG solder joints.     

合金元素Cr,Al对Sn-Ag-Cu基无铅钎料高温抗氧化和润湿性的影响

研究了少量合金元素Cr，Al对Sn-3．0Ag-0．5Cu（305）无铅钎料高温抗氧化性的影响。钎料在液态下的表面颜色变化以及热重分析表明，Cr，Al能明显改善305合金钎料的抗氧化性能。通过合金元素Cr，AI的抗氧化机制和X射线衍射分析得出：Al和Cr在钎料表面形成致密氧化膜，形成“阻挡层”，抑制了钎料的氧化。同时也比较了合金元素Cr，Al对305钎料润湿性能的影响，结果表明：单独加Al不利于钎料的铺展，少量的Cr和Al同时加入对钎料的铺展没有太大的影响。实验证实：Cr和Al的共同作用明显提高了Sn-3．0Ag-0．5Cu钎料的高温抗氧化性，同时对钎料的润湿性也没有恶化作用。     

Influence of reflow and thermal aging on the shear strength and fracture behavior of Sn-3.5Ag solder/Cu joints

The mechanical behavior of Sn-rich solder/Cu joints is highly sensitive to processing variables such as solder reflow time, cooling rate, and subsequent thermal aging. In this article, we focus on the lap shear behavior of Sn-3.5Ag/Cu joints as a function of solder yield strength and intermetallic thickness. Experimental results showed that the shear strength of the solder joints is primarily controlled by the mechanical properties of the solder, and not the intermetallic thickness. The thickness of intermetallic, however, controlled the fracture mode of the solder joints. At intermetallic thicknesses greater than 20 μm, brittle fracture between Cu₆Sn₅ and Cu₃Sn was the most common failure mechanism. Finite-element simulations were carried out to evaluate the effect of solder properties and of intermetallic thickness and morphology on lap shear behavior. The finite-element simulations corroborated the experimental findings, i.e., that increased solder strength results in increased joint strength. The simulations also showed that thicker intermetallics, especially of nodular morphology, yielded higher local plastic shear strain and work hardening rate.     

Thermal properties and interfacial reaction between the Sn-9Zn-<Emphasis Type="Italic">x</Emphasis>Ag lead-free solders and Cu substrate

The thermal properties and interfacial reaction between the Sn-9Zn-xAg lead-free solders and Cu substrate, such as solidus and liquidus temperatures, heat of fusion, intermetallic compounds, and adhesion strength, have been investigated. Two endothermic peaks appear in the DSC curve when the Ag content in the Sn-9Zn-xAg solder alloy is above 1.5 wt pct. The solidus temperatures of the Sn-9Zn-xAg solder alloys are around 197 °C, but the liquidus temperatures decrease from 225.3 °C to 221.7 °C and 223.6 °C with increasing the Ag content in the solder alloy from 1.5 to 2.5 and 3.5 wt pct, respectively. Three intermetallic compounds, namely, Cu₆Sn₅, Cu₅Zn₈, and Ag₃Sn are observed at the Sn-9Zn-xAg/Cu interface. The Cu₅Zn₈ is formed close to the Cu substrate, Ag₃Sn is adjacent to it, and Cu₆Sn₅ is nearest the Sn-9Zn-1.5Ag solder alloys. A bi-structural Cu₆Sn₅ layer with hexagonal η-Cu₆Sn₅ and monoclinic η′-Cu₆Sn₅ is found at the Sn-9Zn-1.5Ag/Cu interface due to Ag dissolution. A maximum adhesion strength of 10.7±0.8 MPa is obtained at the Sn-9Zn-2.5Ag/Cu interface as soldered at 250 °C for 30 seconds.     

Solidification Condition Effects on Microstructures and Creep Resistance of Sn-3.8Ag-0.7Cu Lead-Free Solder

Metallurgical, mechanical, and environmental factors all affect service reliability of lead-free solder joints and are under extensive study for preparation of the transition from Sn-Pb eutectic soldering to lead-free soldering in the electronic industry. However, there is a general lack of understanding about the effects of solidification conditions on the microstructures and mechanical behavior of lead-free solder alloys, particularly on the long-term reliability. This study attempts to examine the creep resistance of the Sn-Ag-Cu eutectic alloy (Sn-3.8Ag-0.7Cu, SAC387) with a variety of solidification conditions with cooling rates ranging from 0.3 °C/s to 17 °C/s. Results indicate that solidification conditions have a major influence on the creep resistance of SAC387 alloy; up to two orders of magnitude change in the steady-state creep rates were observed at low stress levels. An understanding of the mechanical property change with microstructures, which are determined by the solidification conditions, should shed some light on the fundamental deformation and fracture mechanisms of lead-free solder alloys and can provide valuable information for long-term reliability assessment of lead-free solder interconnections. This article is based on a presentation made in the symposium entitled “Solidification Modeling and Microstructure Formation: in Honor of Prof. John Hunt,” which occurred March 13–15, 2006 during the TMS Spring Meeting in San Antonio, Texas, under the auspices of the TMS Materials Processing and Manufacturing Division, Solidification Committee.     

Intermetallic growth and mechanical behavior of low and high melting temperature solder alloys

The presence of an intermetallic is often an indication of good wetting in a solder joint. However, excessive intermetallic growth and the brittleness of the intermetallic layer may be detrimental to joint reliability. This study examined the growth and mechanical behavior of interfacial intermetallics between copper and six solder alloys commonly used in electronics assembly. The solder alloys tested were 60Sn-40Pb, 63Sn-37Pb, 95Sn-5Sb, 96.5Sn-3.5Ag, 50Pb-50In, 50Sn-50In, and 40In-40Sn-20Pb. The 50Sn-50In and 40In-40Sn-20Pb exhibited faster solid state growth of the intermetallic layer at 100 °C as compared to the near-eutectic Sn-Pb control solder. The 50In-50Pb had a slower growth rate, relative to 63Sn-37Pb, at the aging temperature of 170 °C due to slower reaction rate kinetics of indium with copper. The 96.5Sn-3.5Ag and 95Sn-5Sb had similar intermetallic growth rates at 170 °C and 205 °C, and the aging was comparable to that of the 63Sn-37Pb alloy. The 95Sn-5Sb solder/copper intermetallic had a faster growth rate of the Cu₃Sn layer than was observed in the Sn-Ag or Sn-Pb alloys. Modified fracture toughness and low load indentation tests were used to characterize the mechanical behavior of the intermetallics. The intermetallics were harder than both the base metal and the solder alloy. The fracture behavior of the joints in tension was dependent upon the strength of the solder alloy. Solders with low strengths failed in the solder by plastic deformation. The failure of solders with higher strengths was dependent upon intermetallic thickness. When the intermetallic was thin, fracture occurred in the solder or at the solder/ intermetallic interface. As the interfacial intermetallic thickened, the fracture path moved into the intermetallic layer.     

Microstructural analysis of lead-free solder alloys

Among the many issues related to the performance of lead-free solder alloys, the dependence of their mechanical properties on the microstructure and the stability of the microstructure stability are some of the most important issues. A comprehensive understanding of the process-microstructure-property relationships is essential. Toward that goal, a microtextural analysis is performed using orientation imaging microscopy (OIM) for alloy Sn-3.8Ag-0.7Cu (wt pct) processed at four different temperatures. Sn-3.8Ag-0.7Cu is one of the most promising lead-free solder alloys that has shown superior mechanical properties to other candidate lead-free solder alloys. However, a comprehensive understanding of their microstructure and the dependence of microstructure on processing conditions are still lacking. In the present work, a detailed microstructure characterization with respect to phase compositions, grain size and size distributions, texture, and orientation relationships between various phases are performed. The measured microstructural features are correlated with the soldering temperatures.     

Effects of indium on the mechanical properties of ternary Sn-In-Ag solders

The effects of indium on the mechanical properties of ternary Sn-In-Ag solders have been reported by using tensile tests at various homologous temperatures in this study. Indium depresses the melting temperature of Sn-3.5Ag solder. However, a hard In-rich phase was formed when the indium content was raised to 15 pct, which affected the ultimate tensile strengths (UTSs) and elongation of the ternary Sn-In-Ag alloys. Their UTSs increased, but the elongation decreased by increasing the indium content. While raising the homologous temperature to η=0.796, the apparent activation energy for creep of a Sn-10In-2.8Ag alloy was about 26.9 kJ/mol. Two kinds of plastic-flow mechanisms controlled the Sn-20In-2.8Ag alloy deformation under various homologous temperature ranges. A typical intergranular creep-fracture mode was observed in the Sn-10In-2.8Ag solder. However, a dimple structure with flat plate-like facets of In-rich phase was found on the fractured surfaces of Sn-15(20)In-2.8Ag solders, which indicated that an initial crack had nucleated locally at the In-rich/γ-matrix interface.     

Dissolution and Interface Reactions between Palladium and Tin(Sn)-Based Solders: Part II. 63Sn-37Pb Alloy

The interface microstructures and dissolution behavior were studied, which occur between 99.9 pct Pd substrates and molten 95.5Sn-3.9Ag-0.6Cu (wt pct, Sn-Ag-Cu) solder. The solder bath temperatures were 513 K to 623 K (240 °C to 350 °C). The immersion times were 5 to 240 seconds. The IMC layer composition exhibited the (Pd, Cu)Sn₄ (Cu, 0 to 2 at. pct) and (Pd, Sn) solid-solution phases for all test conditions. The phases PdSn and PdSn₂ were observed only for the 623 K (350 °C), 60 seconds test conditions. The metastable phase, Pd₁₁Sn₉, occurred consistently for the 623 K (350 °C), 240 seconds conditions. Palladium-tin needles appeared in the Sn-Ag-Cu solder, but only at temperatures of 563 K (290 °C ) or higher, and had a (Pd, Cu)Sn₄ stoichiometry. Palladium dissolution increased monotonically with both solder bath temperature and exposure time. The rate kinetics of dissolution were represented by the expression At ⁿ exp(∆H/RT), where the time exponent (n) was 0.52 ± 0.10 and the apparent activation energy (∆H) was 44 ± 9 kJ/mol. The IMC layer thickness increased between 513 K and 563 K (240 °C and 290 °C) to approximately 3 to 5 μm, but then was less than 3 μm at 593 K and 623 K (320 °C and 350 °C). The thickness values exhibited no significant time dependence. As a protective finish in electronics assembly applications, Pd would be relatively slow to dissolve into molten Sn-Ag-Cu solder. The Pd-Sn IMC layer would remain sufficiently thin and adherent to a residual Pd layer so as to pose a minimal reliability concern for Sn-Ag-Cu solder interconnections.     

微量Ga元素对低银系无铅钎料抗氧化性能的影响

电子封装波峰焊从有铅到无铅的转换过程中,由于无铅钎料中锡含量比传统Sn37Pb钎料高,导致波峰焊过程中氧化渣的产生量很大。其不仅造成生产中的浪费,还会影响焊接质量。控制钎料氧化渣的产生量是当前无铅波峰焊技术必须要解决的一个重要问题。研究了目前常用的Sn-0.3Ag-0.7Cu无铅钎料在模拟波峰炉中的抗氧化情况。主要研究了微量Ga元素的加入对该钎料抗氧化性的影响。通过钎料的润湿实验和氧化锡渣的产出量的比较可以发现微量Ga元素的加入可以提高钎料的抗氧化性能,Ga元素的最佳含量是0.02%(质量分数),Sn-0.3Ag-0.7Cu-0.020Ga的抗氧化有效温度范围是320℃以下,有效时间控制在120 min以下。利用俄歇能谱AES分析表明,微量元素Ga在焊料表面富集,O原子浓度的降低。热力学分析表明:Ga元素会在合金中优先氧化,阻碍钎料的进一步氧化;动力学分析表明:在保护膜内高价Ga离子使表面层离子排列空位增加并使电导率降低,是产生抗氧化性的原因。     

微量Ge对Sn-0.7Cu高温钎焊界面组织的影响

测试了Sn-0.7Cu和Sn-0.7Cu-0.012Ge钎料在不同钎焊温度下的润湿性,研究了Ge元素对老化过程中钎焊界面金属间化合物(IMC)层生长速率的影响。结果表明:2种钎料的润湿性相差不大,但添加了Ge元素的Sn-0.7Cu-0.012Ge钎料在不同钎焊温度下的漫流性得到了明显的改善,相应提高了约4.00%~5.00%;250℃和350℃钎焊温度下,Sn-0.7Cu/Cu钎焊界面IMC在150℃的老化条件下的生长速率分别为3.24×10-18m2/s和2.50×10-17m2/s,Sn-0.7Cu-0.012Ge/Cu钎焊界面IMC的生长速率分别为2.66×10-18m2/s和1.48×10-17m2/s。Ge元素在钎焊界面处富集,提高了界面IMC的致密性,阻碍了原子的扩散,在一定程度上抑制了界面IMC层的粗化与增厚。     

Infrared Brazing Fe<Subscript>3</Subscript>Al Using Ag-Based Filler Metals

The microstructural evolution and bonding shear strength of infrared brazed Fe₃Al using Ag and BAg-8 (72Ag-28Cu in wt pct) braze alloys have been studied. The Ag-rich phase alloyed with Al dominates the entire Ag brazed joints, and the shear strength is independent of the brazing time. The BAg-8 brazed joint contains Ag-Cu eutectic for all brazing conditions, and its shear strength increases slightly with increasing brazing time. The highest shear strength of 181 MPa is acquired from the joint infrared brazed at 1073 K (800 °C) for 600 seconds. A thin layer of Fe₃Al is identified at the interface between the brazed zone and the substrate for both braze alloys. An Al depletion zone in the Fe₃Al substrate next to the interfacial Fe₃Al is identified as the α-Fe phase. The dissolution of Al from the Fe₃Al substrate into the molten braze causes the formation of α-Fe in the Fe₃Al substrate.     

Influence of the Substrate on the Creep of SN Solder Joints

The creep rate of Sn solder joints is noticeably affected by joint metallization. Cu|Sn|Cu joints have significantly higher creep rates than Ni|Sn|Cu joints, which, in turn, have higher creep rates than Ni|Sn|Ni joints. Replacing Ni by Cu on both substrates increases the creep rate at 333 K (60 °C) by roughly an order of magnitude. The increased creep rate appears with no apparent change in the dominant creep mechanism; the change in the constitutive equation for creep (the Dorn equation) is in the pre-exponential factor. The decreased creep rate on substituting Ni is accompanied by an increase in the hardness of the polygranular solder but a decrease in the nanohardness of the grain interiors. The source of the strong influence of the Ni substrate appears to be the introduction of an array of Ni₃Sn₄ intermetallic precipitates along the grain boundaries. These precipitates inhibit grain boundary sliding, boundary reconfiguration, and grain growth during creep. The intermediate creep rate of the asymmetric Ni|Sn|Cu joint has two causes: a decrease in grain boundary mobility due to precipitate decoration and a restriction in the free volume of the joint due to rapid intermetallic growth from the substrate on the Ni side. The sources of this anomalous intermetallic growth are discussed.