Whitepapers

Voiding of Lead-Free Soldering at Microvia

Dr. Hyoryoon Jo, Dr. Ning-Cheng Lee

Summary: Microvia technology is a critical element in high density interconnect development. It allows realization of low cost, high density, high speed and miniaturization for electronic devices. However, accompanied with all of the advantages described above is the observation of a high occurrence rate of voiding in the solder joints. Presence of voids in the solder joints often affects the mechanical properties of joints and deteriorates the strength, ductility, creep and fatigue life, due to the growth in voids, which could coalesce to form ductile cracks and consequently lead to failure. The deterioration could also be due to the enhanced magnitude of the stresses and strains of solder caused by voids. In addition, voids could also produce spot overheating, hence lessen the reliability of joints. Although voiding in typical solder joints has been studied extensively, very little work has been done on the emerging microvia applications which appear to be more prone to voiding problems. In this study, the effect of materials and processes on voiding in microvia, such as printing process, solder particle size, metal content, solderability of pads, reflow profile, and flux chemistry are studied. Results of investigation indicate that voiding was found to decrease with increasing number of print, increasing flux activity, decreasing solder powder size, decreasing metal content, decreasing peak temperature, and use of linear ramp profile instead of profile with a soaking zone. Voiding is affected by variation in flux chemistry. But the second pass for air reflow does not suffer deterioration in voiding. Among all, profile effects are relatively moderate, and double print, powder size, and flux activity effects are more pronounced. The voiding mechanisms for microvia applications are mostly similar to that using regular pads. Hole-filling capability is a new element contributing to voiding in microvia. Lead-free soldering does not introduce new voiding mechanism here.