Effect of deviating from the reflow process window for lead-free assembly

Abstract

The primary objective of this research is to determine the influence of deviating from the standard reflow process window for lead-free soldering. As the industry is moving towards lead-free technology, SAC solder alloy (melting point 217°C) is being viewed as a potential alternative to conventional Sn/Pb solder alloy (melting point 183°C). The higher melting point of lead-free solders demand higher reflow process temperature profiles.

Since current electronic components can withstand a maximum temperature only in the range 245-250°C, the process window for reflow soldering has narrowed. It is expected that even minor deviations outside of the window could have a dramatic effect on the process outcome and the component's ability to function properly.

In this research project, peak reflow temperature (Tpeak) and time above liquidus (TAL) were varied across the solder paste manufacturer recommended specifications (standard process window), in order to study the effect of process window deviation. Solder joint quality, shear strength, microstructure and intermetallic of the joints were used as metrics to determine the influence of deviating from the process window.

Interestingly, across the process window, as peak temperature and TAL varied, solder joint appearance varied from uncoalesced to shiny to dull solder joints. Significant solder wicking on the component leads was also observed. ANOVA analysis and t-test were performed in order to understand the influence of process window deviation on solder joint strength.