UNDERCOOLING OF 63SN-37PB SOLDER PASTES

Large-scale undercoolings and long incubation periods for solidification of 63Sn-37Pb solder pastes were observed through a differential scanning calorimeter, as functions of cooling rate and contact surface (aluminium versus copper). The undercooling reduced significantly with decreased cooling rate, but even at a rate of 2 degrees C/ minute, the undercooling was as large as 35 degrees C for a no-clean solder paste in an aluminium sample pan. Poor wetting conditions (weak flux nonwetting surfaces) led to large undercoolings and long incubation periods. The undercooling during a typical reflow process in electronic assembly was determined by a simulated DSC test, to be about 50 degrees C for a no-clean solder paste on bare copper surface. The large undercooling is probably caused by the reduced degree of the heterogeneous nucleation for solidification due to the small size of a solder joint and poor wetting with its contact surface.