An alternative approach for the analysis of intermetallic compounds in SMT solder joints

In a properly wetted joining system, with tin/lead solder as the filler and copper as the base metals, a layer of intermetallic compounds is usually found at the interface of these two metals. This layer, mainly copper and tin, has profound effects on the joint's properties and reliability. Optical microscopy or scanning electron microscopy micrographs are traditionally used for analysing the type and thickness of these compounds. An alternative analytical approach is presented. By using scanning electron microscopy in conjunction with energy-dispersive X-ray analysis, the elemental constituents across the interface can be plotted. More detailed and precise information about the compound distribution can thus be deduced. The studies reported concentrate on infra-red reflowed joints. Unlike conventional wave-soldered joints, only Cu6Sn5 was found in these. The growth of intermetallic compounds at different aging temperatures, and with solder paste exposed to atmosphere for different times, was also studied.