Manufacturability and reliability screening of lower melting point, Pb-free alloys containing bismuth

Abstract

This paper explores the manufacturability and reliability of three Pb-free Bi-containing alloys in comparison with conventional SAC305 and SnPb assemblies. The first alloy included in the study is a Sn-based alloy with 3.4%Ag and 4.8%Bi, which showed promising results in the National Center for Manufacturing Sciences and German Joint projects. The other two alloy variations have reduced Ag content, with and without Cu. BGA and leaded components were assembled on medium-complexity test vehicles using these alloys, as well as SAC305 and SnPb as baseline alloys, for comparison. Test vehicles were manufactured using two board materials, 170°C glass transition temperature (Tg) and 155°C Tg, with three surface finishes: ENIG, ENEPIG, and OSP. The accelerated temperature cycling (ATC) testing was done at -55°C to 125°C with 30-min dwells and 10°C/min ramps, for 3,000 cycles. Detailed microstructure examination before and after ATC testing is described, as is failure analysis. All three experimental alloys showed excellent performance in harsh-environment thermal cycling. Vibration testing at two G-force test conditions with resistance failure monitoring was performed on the daisy-chained components. A detailed description of the technique for the vibration testing using 2 G and 5 G harmonic dwells is provided. The lowest failure rate found at both the 2 G and 5 G levels was for the Cu-containing alloy known as Violet. These results provide data for further statistical analysis leading to the choice of proper combinations of the solder alloys, board materials, and surface finishes for high-reliability applications.