Reliability of Pb-free solder alloys in demanding BGA and CSP applications

Based on the electronics industry\´s move to lead-free soldering, and increasing popularity of portable electronics, new concerns about solder joint reliability have emerged. Consequently, there is a resulting industry-wide effort to develop and understand new lead-free alloys for improved solder joint reliability. Until now, most of the work has focused on improving drop shock reliability which is a critical attribute for portable electronics. Significant work has been done to reduce the silver level in common SnAgCu Pb-free alloys, which lowers the bulk alloy modulus. Also, this work has focused on further modification of the characteristics of these "low-Ag" alloys using various micro-alloy additives. The net result of micro-additive addition is to either 1) alter the bulk alloy characteristics by changing the bulk microstructure and altering the formation and growth of intermetallics in the solder itself, or 2) control the interfacial intermetallic layer(s). Alloy microstructure can also be altered by thermal history of the solder. Therefore process conditions play an equally important role in determining the overall solder joint reliability. The alloy composition, both the Ag-level and presence of certain micro-additives, can have a profound effect on both drop-shock and temperature cycling reliability. At Cookson Electronics, we have an aggressive program to develop and study new alloys for BGA and CSP applications. We have investigated a wide range of low-Ag SnAgCu alloys with a broad selection of alloy additives. This paper examines the effect of micro-alloy additives on solder joint reliability and microstructure. Solder joint reliability as measured by drop-shock, high-speed ball pull, and temperature cycling tests is discussed in terms of the micro- structural characteristics.