Thermal properties of Sn-based solder alloys

During last few decades, emerging environmental regulations worldwide, more notably in Europe and Japan, have targeted the elimination of Pb usage in electronic assemblies due to the inherent toxicity of this element. This situation drives to the replacement of the Sn–Pb solder alloy of eutectic composition commonly used as joining material to suitable lead-free solders for microelectronic assembly. Sn-based alloys containing Ag, Cu, Bi, and Zn are potential lead-free solders, usually close to the binary or ternary eutectic composition. For this reason a great effort was directed to establish reliable thermophysical data fundamental to interpret the solidification process and fluidity of alloys belonging to these systems. In this work, an analysis of the solidification process of pure Sn, binary Sn–Ag, Sn–Cu, Sn–Bi, Sn–Zn, Sn–Pb and ternary Sn–Ag–Cu eutectic alloys was carried out using computer aided-cooling curve analysis and differential scanning calorimetry.     

Reliability of Pb-free solders for harsh environment electronic assemblies

Abstract

Since 2006 and the implementation of environmental regulations, the electronic industry has moved to Pb-free solders. Harsh environment industries that were exempted from the regulations will soon have to follow suit. However, a suitable replacement solder for use in harsh environments still has to be validated and reliability models are yet to be established. In this review, research that led to the selection of currently used Pb-free alloys and the continuing search for high reliability alloys are described. Sn pest and Sn whiskers, potential major threats for electronics operating in harsh environments, are highlighted. This review also focuses on the microstructure, mechanical properties and deformation mechanisms of Pb-free alloys. Emphasis is placed on Sn–Ag–Cu alloys, now considered to be the alloys of choice for replacement of Sn–Pb solders. The reliability of Pb-free electronic assemblies is studied, focusing on thermal fatigue, believed to be the main source of failure through creep–fatigue mechanisms. The validity of models for Pb-free solder joints life time prediction is assessed and the lack of cohesiveness among the available reliability data is examined.     

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.     

Electrochemical composite deposition of Sn–Ag–Cu alloys

The dominant materials used for solders in electronic assemblies over the past 60 years have been Pb–Sn alloys. Increasing pressure from environmental and health authorities has stimulated the development of various Pb-free solders. One of the most promising replacements is eutectic or near-eutectic Sn–Ag–Cu alloys produced by electrodeposition. In this study, simple and “green” Sn–Cu-citrate solutions with suspended Ag particles have been developed and optimized for electrochemical composite deposition of eutectic and near-eutectic Sn–Ag–Cu solder films. Different plating conditions, including solution concentration, current density, agitation and additives, are investigated by evaluating their effects on plating rate, deposit composition and microstructure.     

Investigation of microstructures and tensile properties of a Sn-Cu lead-free solder alloy

In recent years, the pollution of environment from lead (Pb) and Pb-containing compounds in microelectronic devices attracts more and more attentions in academia and industry, the lead-free solder alloys begin to replace the lead-based solders in packaging process of some devices and components. In this work, microstructures and mechanical properties of the lead-free solder alloy Sn99.3Cu0.7(Ni) are investigated. This paper will compare the mechanical properties of the lead-based with lead-free solder alloys (Sn99.3Cu0.7(Ni) and 63Sn37Pb). The tensile tests of lead-based and lead-free solder alloys (Sn99.3Cu0.7(Ni) and Sn63Pb37) were conducted at room and elevated temperature at constant strain rate; the relevant tensile properties of Sn99.3Cu0.7(Ni) and Sn63Pb37 were obtained. Specifically, the tensile strength of this lead-free solder- Sn99.3Cu0.7(Ni) in 25^∘C, 50^∘C, 75^∘C, 100^∘C, 125^∘C was investigated; and it was found that tensile strength of the lead-free solder decreased with the increasing test temperature at constant strain rate, showing strong temperature dependence. The lead-free solder alloy Sn99.3Cu0.7(Ni) was found to have favorable mechanical properties and it may be able to replace the lead-based solder alloy such as Sn63Pb37 in the packaging processes in microelectronic industry.     

Mechanical behavior of NiTi shape memory alloy fiber reinforced Sn matrix “smart” composites

The detrimental effects of Pb on the environment and human health have provided the driving force for replacement of Pb–Sn solders with Pb-free alternatives. Sn-rich Pb-free solder alloys with silver and copper alloying additions have higher strength but lower elongation-to-failure than Pb–Sn solders. Thus, these alloys are more susceptible to failure under mechanical shock, drop, and thermal fatigue conditions. In this article, mechanical tensile testing of NiTi–Sn3.5Ag single fiber composites demonstrates superelastic behavior of the composite with 85% strain recovery. Fatigue experiments show an evolution in damage over cycles, and an S–N curve shows sharp transition between a nearly vertical low-cycle fatigue behavior and the high-cycle fatigue regime. The solder composite exhibits constant fatigue strength over the superelastic range of the NiTi fiber.     

Development of Sn–Ag–Cu and Sn–Ag–Cu–X alloys for Pb-free electronic solder applications

The global electronic assembly community is striving to accommodate the replacement of Pb-containing solders, primarily Sn–Pb alloys, with Pb-free solders due to environmental regulations and market pressures. Of the Pb-free choices, a family of solder alloys based on the Sn–Ag–Cu (SAC) ternary eutectic (T _eut. = 217°C) composition have emerged with the most potential for broad use across the industry, but the preferred (typically near-eutectic) composition is still in debate. This review will attempt to clarify the characteristic microstructures and mechanical properties of the current candidates and recommend alloy choices, a maximum operating temperature limit, and directions for future work. Also included in this review will be an exploration of several SAC + X candidates, i.e., 4th element modifications of SAC solder alloys, that are intended to control solder alloy undercooling and solidification product phases and to improve the resistance of SAC solder joints to high temperature thermal aging effects. Again, preliminary alloy recommendations will be offered, along with suggestions for future work.     

Issues related to the implementation of Pb-free electronic solders in consumer electronics

Consumer electronic applications are the primary target of the Pb-free initiative and package assembly and performance is affected by the move from eutectic Sn–Pb to Pb-free solder alloys. This paper outlines the key issues and mitigation possibilities for package assembly using Pb-free solders: High temperature reflow, Interfacial reactions, and Reliability. At the high temperatures required to reflow Pb-free alloys, moisture absorbed into the package can result in delamination and failure. The reaction of the Pb-free solder with Ni and Cu metallizations results in interfacial intermetallics that are not significantly thicker than with Sn–Pb but provide a path for fracture under mechanical loading due to the increased strength of the Pb-free alloys. The reliability issues discussed include thermomechanical fatigue, mechanical shock, electromigration and whiskering. The Pb-free alloys tend to improve thermomechanical fatigue and electromigration performance but are detrimental to mechanical shock and whiskering. Design trade-offs must be made to successfully implement Pb-free alloys into consumer applications.     

Rare-earth additions to lead-free electronic solders

The research in lead(Pb)-free solder alloy has been a popular topic in recent years, and has led to commercially available Pb-free alloys. Further research in certain properties to improve aspects such as manufacturability and long term reliability in many Pb-free alloys are currently undertaken. It was found by researchers that popular Pb-free solders such as Sn–Ag, Sn–Cu, Sn–Zn and Sn–Ag–Cu had improved their properties by doping with trace amounts of rare earth (RE) elements. The improvements include better wettability, creep strength and tensile strength. In particular, the increase in creep rupture time in Sn–Ag–Cu–RE was 7 times, when the RE elements were primarily Ce and La. Apart from these studies, other studies have also shown that the addition of RE elements to existing Pb-free could make it solderable to substrates such as semiconductors and optical materials. This paper summarizes the effect of RE elements on the microstructure, mechanical properties and wetting behavior of certain Pb-free solder alloys. It also demonstrates that the addition of RE elements would improve the reliability of the interconnections in electronic packaging. For example, when Pb-free-RE alloys were used as solder balls in a ball grid array (BGA) package, the intermetallic compound layer thickness and the amount of interfacial reaction were reduced.     

Life expectancies of Pb-free SAC solder interconnects in electronic hardware

The transition from lead (Pb) bearing solder to Pb-free solder has arisen in response to government restrictions on the use of lead (Pb) by the European Union. As a result, electronic manufacturers have sought a material comparable to the conventional 63Sn37Pb solder that has been traditionally used to assemble electronic hardware. Based on extensive review of various solder combination, the majority of electronic manufacturers appear to be adopting a tin–silver–copper (SAC) solder as a popular Pb-free solder replacement. Significant investments have been made by many researchers to characterize the material behavior and durability of this solder system. While the exact composition of the SAC solder is still in question, it now appears that the 96.5Sn3.0Ag0.5Cu (SAC305) solder is gaining wider acceptance as the favored Pb-free replacement, for surface mount assemblies that are going to be subjected predominantly to cyclic thermal environments. This paper presents a review of our current understanding of the life expectancy of Pb-free SAC solder interconnects for electronic hardware. To this end, the paper focuses on material characterization of SAC solder, as well as its temperature cycling and vibration fatigue reliability. From this review, SAC solder interconnects are shown to be suitable for providing adequate life expectancies for temperature cycling in electronic hardware. However, it is clear that there are differences between SAC and the conventional Sn37Pb solder, that need to be understood in order to design reliable electronic hardware.     

Tensile properties and wettability of SAC0307 and SAC105 low Ag lead-free solder alloys

In this article, the tensile properties of low Ag lead-free solder alloys, SAC0307 and SAC105, are examined under various strain rates and temperatures. The wettability of these solders on Cu pad is also characterized by using different fluxes. The SAC305 and Sn37Pb solder alloys are also studied for comparison. Our results show that the properties of all solder alloys are dependent on the strain rate and temperature. The ultimate tensile strength increases monotonously with the increment of strain rate. Both SAC0307 and SAC105 alloys possess lower strength and higher elongation ratio than SAC305 and Sn37Pb alloys. For all the fluxes used in this study, the SAC0307 and SAC105 alloys show the similar wettability to SAC305, whereas worse than that of Sn37Pb alloy. Increasing the activity of the flux does not improve the wettability of the SAC solder alloys on Cu pad effectively.     

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.     

Synthesis and DSC study on Sn3.5Ag alloy nanoparticles used for lower melting temperature solder

The traditional Sn–Pb eutectic solder alloys are being phased out from the electronics industry due to the toxicity of lead (Pb), leading to the development and implementation of lead-free solders. Sn3.5Ag lead-free solder alloy, considered to be one of the promising alternatives to replace the traditionally used Sn–Pb solder, however, still has some weaknesses, such as its higher melting temperature than that of the Sn–Pb solder alloy. A possible way to decrease the melting temperature of a solder alloy is to decrease the alloy particle size to the nanometer range. Sn3.5Ag nanoparticles with different size distribution were synthesized using chemical reduction method by applying NaBH₄ as reduction agent. The melting properties of these synthesized nanoparticles were studied by differential scanning calorimetry (DSC), and size-dependent melting temperature depression of these nanoparticles has been observed. Gibbs–Thomson equation was used to analyze the size-dependent melting temperature property, giving a good prediction of the melting temperature depression for the Sn-based lead-free solder alloy nanoparticles.     

Microstructural Evolution and Cracking of Pb-free Ball Grid Array Assemblies under Thermal Cycling

Two kinds of CBGA （ceramic ball grid array） assemblies were made by reflow soldering process using two different Pb-free solders. Microstructural evolution and cracks induced by thermal cycling in CBGA assemblies were examined by scanning electron microscopy （SEM） and finite element method （FEM）. Before thermal cycling, intermetallic compounds （IMCs） Cu6Sn5 and Ag3Sn were observed at the solder interface between Cu and Ag metallizations, respectively. After thermal cycling, another IMC Cu3Sn was observed near the Cu pad in both two assemblies and the layers of Cu6Sn5 and Ag3Sn became thicker. As a result of thermal cycling, cyclic stress and strain were accumulated in the solder joint leading to fatigue cracking. Both experiments and FEM revealed that cracks preferred to initiate at the corner of each solder joint. Multi-modes of the crack propagation were found in the two assemblies. Based on Coffin-Manson equation, the thermal fatigue life was calculated and the predicted life showed good agreement with the experimental results.     

Thermal property,wettability and interfacial characterization of novel Sn–Zn–Bi–In alloys as low-temperature lead-free solders

The effect of indium (In) addition on thermal property, microstructure, wettability and interfacial reactions of Sn–8Zn–3Bi lead-free solder alloys has been investigated. Results showed that addition of In could lower both solidus and liquidus temperatures of the solder alloys with wettabilty significantly improved. The spreading area of Sn–8Zn–3Bi–1.0In was increased by 34% compared to that of Sn–8Zn–3Bi. With the increase of In content, Zn-rich precipitates were smaller in size and distributed more uniformly, which might be beneficial for mechanical properties and corrosion resistance of the solders. The intermetallic compounds (IMCs) formed between Sn–8Zn–3Bi–xIn solder/Cu substrate was identified as Cu–Zn with a scallop layer adjacent to the solder and a flat layer to the substrate. The addition of In slightly influenced the thickness of the IMCs. The newly developed Sn–Zn–Bi–In solder system has great potential to replace the Sn–Pb solders as low-temperature lead-free solders.     

Tensile properties and thermal shock reliability of Sn-Ag-Cu solder joint with indium addition

The thermal shock reliability and tensile properties of a newly developed quaternary Sn-1.2Ag-0.5Cu-0.4In (wt%) solder alloy were investigated and compared to those of ternary Sn-Ag-Cu based Pb-free solder alloys. It was revealed that the Sn-1.2Ag-0.5Cu-0.4In solder alloy shows better thermal shock reliability compared to the Sn-1.0Ag-0.5Cu and Sn-3.0Ag-0.5Cu solder alloys. The quaternary alloy has higher strength than Sn-1.0Ag-0.5Cu alloy, and higher elongation than Sn-3.0Ag-0.5Cu alloy. It was also revealed that the addition of indium promotes the formation of Ag3(Sn, In) phase in the solder joint during reflow process.     

Microstructure,mechanical properties,and deformation behavior of Sn–1.0Ag–0.5Cu solder after Ni and Sb additions

Minor alloying addition to solders has been an important strategy to improve the integrity and reliability of Pb-free solders joint. In this study, the effects of 0.06Ni and 0.5Sb additives on the microstructure and solidification behavior as well as the creep properties of Sn–1.0Ag–0.5Cu (SAC105) alloys were investigated. Results show that alloying of Ni and Sb resulted in considerably reduced undercooling, increased eutectic area and extended volume fraction of proeutectic Sn of which the dendritic size was refined. Moreover, with the addition of Ni and Sb into SAC105, significant improvement in creep resistance of (210%) and (350%) is realized when compared with the SAC105 solder alloy. Likewise, the creep life time of SAC105 alloys was remarkably enhanced (2–3 times) with the minor alloying additions. An analysis of the creep behavior at elevated temperatures suggested that the presence of hard Ni₃Sn₄ IMC particles and the solid solution hardening effects which appeared, respectively, in the Ni-doped and Sb-doped alloys could increase the resistance to dislocation movement, which improves the creep properties.     

Electrochemical migration of Sn-Pb and lead free solder alloys under distilled water

Electrochemical migration (ECM) is a potential reliability problem in electronic soldering which might become more dangerous in lead free electronic devices. In this paper, electrochemical migration tests on Sn-Pb and lead free solder alloys were conducted under distilled water by applying constant voltages with a power supply. The susceptibility of the solder alloys to ECM and the effect of the composition on ECM behavior were studied. It is found that both Sn-Pb and lead free solders investigated in present research have susceptibility on ECM. Dendrites grow from cathode to anode and show different morphologies with the different migration elements involved. In Sn-37Pb and Sn-36Pb-2Ag solders, the main migration element is Pb. While for Sn-Ag and Sn-Ag-Cu solder alloys, Sn leads the migration. For Sn-8Zn-3Bi, both Sn and Zn can migrate. Furthermore, the effect of applied voltage on the time to short and short resistance was also investigated. As could be expected, the higher the voltage is, the shorter the failure time is. The electrochemical migration mechanism of the solder alloys was also discussed. The author is now at Fraunhofer IZM, Berlin, Germany     

The electrochemical corrosion behaviour of Pb-free Al-Zn-Sn solders in NaCl solution

The corrosion behaviour of Pb-free X(5Al-Zn)-YSn solders, where X is 9-50 and Fis 91-50, has been investigated with regard to open-circuit potential, galvanic corrosion and potentiodynamic polarization in a 3.5% NaCl solution. The galvanic current densities for 9 (5Al-Zn)-91Sn in NaCl solution are 25, 20 and 12.5 μA/cm² with respect to Cu, Ni-Cu-P and Ni-P, respectively. The corrosion behaviour has been compared with that of 63Sn-37Pb solder. A passivation behaviour is observed for all of the investigated Al-Zn-Sn solders. The magnitudes of the passivation current densities depend on the compositions of the solders and the potentials applied. The polarization behaviour of eutectic 9 (5Al-Zn)-91Sn solder is very similar to that of 63Sn-37Pb solder. SnO₂ is formed at polarization potentials below −330 mV, while SnCl₄ forms when the potential is at 120 mV. ZnCl₂ replaces SnCl₄ on polarizing at 120 mV for an extended period of 1 h.     

Electromigration issues in lead-free solder joints

As the microelectronic industry advances to Pb-free solders due to environmental concerns, electromigration (EM) has become a critical issue for fine-pitch packaging as the diameter of the solder bump continues decreasing and the current that each bump carries keeps rising owing to higher performance requirement of electronic devices. As stated in 2003 International Technology Roadmap for Semiconductors (ITRS), the EM is expected to be the limiting factor for high-density packages. This paper reviews general background of EM, current understanding of EM in solder joints, and technical hurdles to be addressed as well as possible solutions. It is found that the EM lifetimes of Pb-free solder bumps are between the high-Pb and the eutectic composition under the same testing condition. However, our simulation results show that the electrical and thermal characteristics remain essentially almost the same during accelerated EM tests when the Pb-containing solders are replaced by Pb-free solders, suggesting that the melting points of the solders are likely the dominant factor in determining EM lifetimes. The EM behavior in Pb-free solder is a complicated phenomenon as multiple driving forces coexist in the joints and each joint contains more than four elements with distinct susceptibility to each driving force. Therefore, atomic transport due to electrical and thermal driving forces during EM is also investigated. In addition, several approaches are presented to reduce undesirable current crowding and Joule heating effects to improve EM resistance.