Development of high-temperature solders: Review

The objective of this review is to study the interfacial intermatallic compounds (IMCs) between Sn–Ag–Cu based solders and common substrates, which play a crucial role in solder joints typically present in Pb-free electronics manufacturing. The microstructural evolution of IMCs at the solder/substrate interfaces is analyzed, while the models and theories describing the formation/growth mechanism of interfacial IMCs are also introduced. We focus on the influence of a variety of factors that have been reported recently, including substrates, minor alloying, mechanical stress, electromigration and thermomigration etc., as full understanding of the mechanisms that determine the formation and growth of interfacial IMCs is important to reach for developing high reliability solder joints. In the end of this review, the characteristics of the IMCs are compared and illustrated, which have marked effect on the mechanical properties and fracture behavior as well as reliability of solder joints.

A review on the interfacial intermetallic compounds between Sn–Ag–Cu based solders and substrates

Development of Au–Ge based candidate alloys as an alternative to high-lead content solders

Design of lead-free candidate alloys for high-temperature soldering based on the Au–Sn system

Passive solar evaporation represents a promising and environmentally benign method of water purification/desalination. Plasmonic nanoparticles have been demonstrated as an effective approach for enhancing solar steam generation through a plasmonic heating effect, nonetheless the efficiency is constrained by unnecessary bulk heating of the entire liquid volume, while the noble metals commonly used are not cost-effective in terms of availability and their sophisticated preparation. Herein, a paper-like plasmonic device consisting of a microporous membrane and indium nanoparticles (In NPs/MPM) is fabricated through a simple thermal evaporation method. Due to the light-weight and porous nature of the device, the broadband light absorption properties, and theoretically the excellent plasmonic heating effect from In NP which could be even higher than gold, silver and aluminium nanoparticles, our device can effectively enhance solar water evaporation by floating on the water surface and its utility has been demonstrated in the solar desalination of a real seawater sample. The durability of the device in solar seawater desalination has also been investigated over multiple cycles with stable performances. This portable device could provide a solution for individuals to do water/seawater purification in under-developed areas with limited/no access to electricity or a centralized drinking water supply.

Plasmonic heating from indium nanoparticles on a floating microporous membrane for enhanced solar seawater desalination

The fluxless solderability of pure indium on gold-coated surfaces is investigated in this study using measurements of wetting angle and joint strength. This study focuses on the effects of indium’s native oxides and those which form during heat treatment. The initial oxide thicknesses are obtained by heating indium samples at various temperatures, and then, during the reflow above the melting temperature, oxidation at the interface is either allowed in open air or prevented by reflowing in an inert environment. The testing results presented include characterization of indium oxide (In2O3), the effects of indium oxide on joint strength, and the effect of hot pressing during reflow process.

/pdf/effect-of-oxidation-on-indium-solderability-1krlmnn67s.pdf

Effect of Oxidation on Indium Solderability

Fundamental knowledge on the oxidation behavior of pure indium, commonly used as a low-temperature, fluxless soldering material in micro-electro-mechanical system (MEMS) devices, is of importance as it influences the solder joint reliability. A thermodynamic model of the oxidation and reduction behavior of indium is developed by constructing an Ellingham diagram, and by using H2(g) reactions. Partial pressure (p) of H2O was shown to be the critical parameter in creating a reducing environment in the applicable solder reflow temperature range. Verification of the thermodynamic models was then carried out through heating and melting of indium in controlled glove box environments by adjusting p(H2)/p(H2O). The nanometer scale thickness of the oxide layer grown on indium was measured by a spectroscopic ellipsometer. The growth mechanism for oxidation in air below 220 °C follows Uhlig’s logarithmic law where electron transport is the rate-controlling mechanism, implying that there is an incubation period for the onset of initial oxidation. Its activation energy was found to be 0.65 eV.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0884291400031502

Oxidation and reduction behavior of pure indium

This study characterized the temperature-dependent constitutive parameters (yield strength, ultimate tensile strength, elastic modulus, strain hardening exponent) from the mechanical behavior of five high-temperature solders, 95Sn-5Sb, 95Pb-5In, 90Pb-10Sn, 92.5Pb-5Sn-2.5Ag, and 93Pb-3Sn-2In-2Ag, chosen such that T
 m > 518 K. To model appropriately their mechanical responses under high-temperature thermal cycling, where the temperatures exceed 473 K, the material’s parameters must be determined as a function of temperature. Uni-axial tensile tests were, therefore, carried out between 298 K and 473 K to determine the constitutive behavior of each solder. 95Sn-5Sb exhibited the highest strength over the temperature range tested except near 473 K. Pb-based alloys with a higher degree of solid solution (>5%) showed greater strengthening than those primarily strengthened by coarse precipitates. Additionally, microstructure changes in 90Pb-10Sn and 95Sn-5Sb were shown to be responsible for unexpected mechanical behavior at elevated temperatures.

Microstructure Evolution and the Constitutive Relations of High-Temperature Solders

This study examines the high temperature creep behavior of several Pb-based alloys. All compositions tested were found to follow power-law dislocation creep in the strain rate range of 10 −9 –10 −3  s −1 . Both the stress exponent and activation energy were measured from 298 to 473 K to identify the rate controlling mechanism for creep deformation. Creep of 95Pb–5In, 92.5Pb–5Sn–2.5Ag, 93Pb–3Sn–2Ag–2In was rate limited by dislocation climb from the observed stress exponent. A transition in the controlling climb mechanism from pipe diffusion to lattice diffusion was observed around 0.7 T m . Creep of 90Pb–10Sn was, however, rate limited by viscous solute drag rather than dislocation climb due to the greater concentration of Sn in Pb. The enhancement in self-diffusion of Pb was dependent on the degree of solid solution with solute atoms. The outcome of this work identifies variables related to the alloy elements that control creep behavior of Pb-based alloys used in high temperature applications where traditional solders cannot be used.

Effect of alloying elements on the creep behavior of high Pb-based solders

Predominant high melting point solders for high temperature electronics (operating temperatures over 200°C) are high Pb containing alloys. The new regulation will have banned the use of Pb in all electronics by 2019. We designed a "new" class of solder alloys with high bismuth (Bi) content (&#x2265; 70 wt.%) to evaluate their potential as a high temperature Pb-free solder. Although having high melting point (271°C), pure Bi exhibits poor thermal conductivity, limited plasticity and wetting issues on common metallized surface such as Ni and Cu. In this work, alloying elements such as Sb and Cu were added to Bi in an attempt to negate such shortcomings by creating innovative alloy microstructures and optimum alloy compositions. As an extension to this work, a transient liquid phase (TLP) bonding between Bi and Ni was also investigated as Bi in a molten state reacts strongly with Ni to be converted into a Bi3Ni or BiNi intermetallic layer. Once formed, these phases can be stable until reaching ~ 470°C (with Bi3Ni), or ~ 617°C (with BiNi). We tested the reliability of the die attached coupons using Bi-Sb-Cu solders and Bi-Ni TLP bonds under high temperatures (up to 200°C for Bi solders and up to 350°C for TLP bonds), thermal shock (from -55 to 200°C), and high temperature storage (at 200°C). The resultant data indicate the potential of these two Bi systems as Pb-free alternatives for high temperature electronics.

Harry Schoeller

Papers

Effect of Oxidation on Indium Solderability

Oxidation and reduction behavior of pure indium

Microstructure Evolution and the Constitutive Relations of High-Temperature Solders

Effect of alloying elements on the creep behavior of high Pb-based solders

Exploring Bismuth as a New Pb-Free Alternative for High Temperature Electronics