Interfacial reactions of SAC305 and SAC405 solders on electroless Ni(P)/immersion Au and electroless Ni(B)/immersion Au finishes

The formation of intermetallic compounds (IMCs) on solder pads is strongly affected by the type of PCB surface finish since different finishes may lead to several different interfacial reactions and IMCs formation. Since the joint reliability is also determined by the type of the interfacial IMC layer between the solder and substrate, understanding how such intermetallic compounds form and grow during soldering and subsequent thermal ageing is essential. In this paper, experimental results of the effect of three different surface finishes namely: electroless nickel (phosphorus)/immersion gold, electroless nickel (boron)/ immersion gold and bare copper (for comparison) on the formation and growth of interfacial reactions during soldering and thermal ageing with Sn-3Ag-0.5Cu and Sn-4Ag-0.5Cu solders are presented. Several techniques of materials characterization including optical, image analysis, scanning electron microscopy and energy dispersive X-ray analysis were used to examine and quantify the intermetallics in terms of composition, thickness and morphology. The results showed that after soldering on Ni-Au finishes the reaction layer was found to consist of only one layer of (Cu, Ni)₆Sn₅ with a needle-shape morphology. In addition, the results from SEM with energy dispersive x-ray (EDX) have revealed that isothermal aging at 150°C has caused the thickening and coarsening of IMCs as well as changing them into more spherical shape. During soldering on Ni(B)/Au finish it was observed that closer control of the reflow temperature was very important compared to soldering on Ni(P)/Au finish. Selective etching of solder joints also revealed that the morphology of the intermetallic formed is different across a single solder joint from the centre to the outside edge.