Pb-free solders have been widely used in SMT applications for many years, with the SnAgCu or "SAC" alloy becoming the most common composition. However, the rapid escalation in raw material costs have impacted solder prices, therefore creating renewed interest in cost-effective alternative solders. Due to the lower metals costs, lower silver (Ag) content solders have emerged as the leading candidates to supplant the most common three percent silver containing, or "SAC305" alloy. The Ag contained inside the current Pb-free solders, however, plays an important role in preserving joint reliability and the reduction of Ag content may have an adverse impact on product life. Also many of the low-Ag candidates have higher melting points which may increase the risk of damage to the substrate and component. These risk factors often prevent the use of low-Ag solder materials in the SMT process. This report investigates the feasibility of using a low-Ag solder as a lower cost alternative for the SMT process, with review of an experimental study identifying the effect of minute alloy composition modifications on joint strength and melting points. As a result of modifying the solid solution hardening of tin (Sn) inside low-Ag solder alloy with the addition of small amounts of Bismuth (Bi), Indium (In) and Antimony (Sb), an equivalent or superior reliability level was achieved when compared to the industry standard SAC305 alloy. The low-Ag solder paste developed has been successfully implemented into flat panel TV assemblies, contributing significantly to the customers' indirect materials cost reduction targets. This paper outlines development and characterization of this new low-Ag solder paste.
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