Voiding reduction and leadframe interactions in high-lead solder die attach for QFN applications

The development of solutions for high power packaging systems requires the use of a die attach material that has high thermal conductivity and low electrical resistance. However, solder paste, one of which meets this design requirement, is often affected by solder voids that form during the reflow process. By using a 3-factor full factorial design of experiment, this study looks into the effects of reflow profile, nitrogen flow rate, and leadframe type (A and B) on single and cumulative voids produced during the reflow process. Also, the effect of the location of specific nitrogen vents in the machine with respect to the package during reflow processing will be discussed. Analysis of data collected shows that use of higher N2 flow rate, leadframe type A, and reflow profile with higher time above liquidus and soak time reduces both single and cumulative void dppm by more than 95%. This is attributed to lower oxidation and increased flux outgassing during solder reflow.