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Because of the environmental health implications of Pb, and legislation backing Restriction on Hazardous Substances (RoHS) in electronic devices, Sn-based lead-free solders are developed. Near eutectic Sn-Ag-Cu (SAC) compositions is the most widely used Pb-free solder in the electronic industries. However, its high melting temperatures (>217 ℃) rem...
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SnAg -Cu (SAC) solders generally have better thermal cycling performance than eutectic Sn-Pb. However, their performance deteriorates significantly as the harshness of the thermal cycle increases, and the high-Ag SAC solders that perform best in thermal cycling have relatively poor drop impact properties. Therefore, there is a drive to develop a new generation of Pb-free solders that have improved thermal cycling performance under the temperature ranges relevant to emerging applications (e.g. in automotive, avionics, and defense), while also having acceptable performance under drop, shock and vibration loading. The new Pb-free solder alloys entering the market have taken a variety of alloy design approaches. However, a common theme in most is the addition of one or more of Bi, Sb and In to existing Pb-free compositions. Often the Bi, Sb and In additions are the most concentrated addition in the alloy and lead to the formation of new phases as well as dissolving in the β-Sn. In this work, the development of phases, microstructure and the distribution of Bi, Sb and In are investigated in a range of third generation Pb-free solder joints. The focus is on the influence of Bi, Sb and/or In combinations on the intermetallic layers on Cu and Ni-based substrates, the primary and eutectic solidification phases, phase formation due to solid state precipitation, and the overall β-Sn grain structure of BGA solder joints. INTRODUCTION The expanding electronic content in the automotive and aerospace sectors, including the electrification of vehicles and our increasing reliance on sensors, is driving the need for new solder alloys with improved reliability that can operate under increasingly harsh environments. Often this involves joints operating at higher temperature, cycling through larger temperature ranges while enduring higher current densities and steeper temperature gradients in combination with vibration and shock loading. In response to these demands, a third generation of Pb-free solder alloys is under development and entering service.
