An experimental approach to pore-free reflow soldering

This paper addresses the mechanism of pore growth and reports the results of an investigation on pore-free reflow soldering. An experimental approach to obtain pore-free solder joints has been successfully developed. The method employed is to split the solder joints during the reflow soldering process. For the Sn63Pb37 solder paste with metal content of 90%, the solder joints produced by this method have no detectable pores compared with 7.5% area fraction of pores in normal IR reflow soldering. The joint strength also increases by about 20-40% as compared with that of normal solder joints. This method has promising applications, especially in the nitrogen reflow soldering technique, to yield pore-free and robust solder joints. A model for the pore growth is also proposed in the paper. It shows that flux residues usually occur together with the pores and are important to the pore development. Once entrapped by the molten solder, the pores and inclusions have difficulty escaping from the joints unless external forces or disturbances are applied.