Microstructural evolution of low temperature solders in solder joints on a flexible substrate

Surface mount technology is widely practiced in present day electronic component assembly. Low temperature solders are finding increasing applications, primarily when polymeric materials are involved in the assembly process. Although there is a significant amount of data on conventional Pb-Sn solders reflowed at temperatures above 200°C, information on the behaviour of other solders is extremely limited. Hence, it is necessary to develop an understanding of the structure-property relationship, both after reflow and after long term use, in these alternate solders. The solder microstructure in an SMT joint and the reflow characteristics are influenced by a variety of factors, including solder composition, reflow parameters such as temperature and dwell time, other process variables, subsequent temperature exposure, etc. In this study, two different solders, Pb-Sn-Bi solder and Pb-Sn-Ag solder, have been used to surface mount various components on a polyester substrate using various process conditions. The solder joint microstructures have been examined using cross-sectional scanning electron microscopy. The effects of some of the process variables on the solder microstructure are outlined. Differences between the behaviour of the solders are also highlighted. The solder microstructure is discussed on the basis of the reflow profiles and the flux used