A RELIABILITY INVESTIGATION OF Bi-42Sn-1Ag SOLDER PASTE: AN ALTERNATIVE TO Sn-3.5Ag-0.7Cu FOR LOW COST CONSUMER PRODUCTS
Author: V. Schroeder Company: Hewlett Packard Date Published: 6/12/2001
Advanced Packaging Technology
Abstract: Eutectic Bi-42Sn solder is a low melting point alternative to lead-based solders, particularly for low cost, consumer electronics. In earlier work, the mechanical properties of this solder have been enhanced by small additions of silver (0.5%-3%). In this study, Bi-42Sn-1Ag solder paste was used to assemble lead-free PBGAs (Sn-0.7Cu, Sn-3.5Ag, and Sn-3.5Ag-0.7Cu ball compositions) on lead-free PCB surfaces (organic solderability preservative (OSP), electroless nickel/immersion gold, and immersion tin). These lead-free assemblies exhibit accelerated thermal cycling (ATC) performance comparable to the performance of assemblies made with Sn-37Pb (40 min. cycles between –25 degrees C and 75 degrees C). In addition, some of these lead-free assemblies have bend strengths that are close to the strength of a comparable Sn-37Pb assembly. However, the mechanical properties of the lead-free assembly depend on the BGA ball and PCB surface finish metallurgy. To understand the thermal cycling and bend test behavior, cross-sections of solder joints and fracture surfaces from bend testing were evaluated.
Key words: Bi-Sn-Ag, Pb-free solder, thermal fatigue, bend strength.