IMC investigation on fracture surface between solder joints and different surface finish processes

In this work, IMC (Inter metallic compound) of lead free (Sn-Ag-Cu (SAC)) and lead (62Sn-36Pb-2Ag (SP)) solder joint on the four kinds of surface finished of ball grid array (BGA) pad, such as Immersion Tin (ImSn), Organic solderability preservatives (OSPs), Ni-P/Pd/Au (ENEPIG), and Ni-P/Au (ENIG) were investigated by Focus Ion Beam microscope (FIB). Sample after failure ageing conditions (Baking 200°C, Reflow process et al.) were followed with ball pull test, and one can find some residuum of SAC and SP solder on the fracture surface of surface finish process. These solder residuum had stronger adhesion with surface, and FIB was applied to observe the grain and crystal morphology of the solder residuum. It was found that the distribution of residual and blank solder have relation with IMC crystal lattice. That result indicated that the IMC structure became deformed after ball pull test. And the thermal aging will make IMC to grow. Some specific IMC have better solder joint reliability.