The Morphology Evolution and Voiding of Solder Joints on QFN Central Pads with a Ni/Au Finish

In this paper, we report on a comprehensive study regarding the morphology evolution and voiding of SnAgCu solder joints on the central pad of two different packages – QFN and an Agilent package called TOPS – on PCBs with a Ni/Au surface finish. Samples were isothermally aged at the equivalent of 0, 2, 7 and 14 years service life. Representative solder joints were cross-sectioned and analyzed using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) in order to investigate the evolution of the solder joint morphology as a function of Au content and isothermal aging. IMC thickness was measured. The effect of Au content on the void percentage was studied as well. The results show that if copper is available to dissolve into the solder joint, the AuSn4 IMC from the bulk does not migrate to the interface as a result of thermal aging. The IMC thickness grew with aging as expected, however with Cu base metallization the IMC was dominated by Cu6Sn5, and with Ni base metallization on both sides of the joint the IMC was dominated by AuSn4. Voiding analysis showed that thick Au metallization on thermal pads leads to more voiding and larger standoff height.