A Systematic Procedure for Selection of a Lead-free Solder Paste at an Electronics Manufacturing Environment

摘要

Historically, tin-lead has been the joining material of choice in the manufacturing of electronic products. Environmental and heath concerns, due to the leaching of lead from landfills into ground water, have given rise to legislations that restrict the use of lead in electronics. These legislations, in addition to the globalization of markets, manufacturing and competition have become the primary drivers for lead elimination from electronics. Therefore, the transition of solder for electronics assembly from a tin-lead composition to a lead-free composition is imminent. Several alternative solder alloys (and their fluxes) have been researched for electronics assembly in the last few years. The Electronics Manufacturing Service (EMS) providers are required to decide upon a lead-free alternative without the help of specific industry standards and guidelines. The objective of this research was to develop a systematic selection process for choosing a 'preferred' lead-free solder paste based on its print and reflow performances. After a detailed study of industry preferences, published experimental data, and recommendations of various industrial consortia, a near eutectic tin-silver-copper (Sn/Ag/Cu also known as SAC) composition was selected as the preferred alloy for evaluation. Commercially available tin-silver-copper solder pastes with a no-clean chemistry were extensively investigated in a simulated manufacturing environment. A total of nine SAC pastes from seven manufacturers were evaluated in this investigation. A eutectic (Sn/Pb) solder paste was used as a baseline for comparison. While selecting the best lead-free paste, it was noted that the selected paste has to perform as good as, if not better than, the current tin-lead paste configuration used in electronics manufacturing for a particular application. The quality of the solder pastes was characterized by a series of analytical and assembly process tests consisting of, but not limited to, printability test, solder ball test, slump test, and post reflow characteristics such as tendency to form voids, self-centering and wetting ability. Each paste was evaluated for desirable and undesirable properties. The pastes were then scored relative to each other in each individual test. An aggregate of individual test scores determined the best paste.