Study of Undercooling and Recalescence During Solidification of Sn62.5Pb36.5Ag1 and Sn96.5Ag3Cu0.5 Solders in Real Electronic Joints

Abstract

Undercooling and recalescence were studied using the differential scanning calorimetry (DSC) method on real electronic systems. Two solder pastes, Sn 62.5 Pb 36.5 Ag 1 and Sn 96.5 Ag 3 Cu 0.5 , were used for preparation of electronic joints. Various combinations of these solders and soldering pads with different surface finishes such as Cu, Cu-Ni-Au, Cu-Sn, and Cu-Sn 99 Cu 1 were used. During melting of both pastes, the Sn and Sn 99 Cu 1 surface finishes immediately dissolved in the solder and the Cu surface coating was exposed to the melt. Therefore, practically the same undercooling was found for the Cu, Cu-Sn, and Cu-Sn 99 Cu 1 coatings. The lowest undercooling was found for the Cu-Ni-Au surface finish for both solder pastes. If two separated electronic joints were made on the sample, two separate peaks were found in the DSC signal during solidification. In the sample with only one joint, only one exothermic peak was found. These findings were observed for all paste/surface finish combinations. These data were analyzed, showing that this effect is a consequence of undercooling and recalescence: Latent heat released during solidification of the joint increases the surrounding temperature and influences all the processes taking place.

Influence of latent heat released from solder joints on the reflow temperature profile

“…This mutual influence arises from undercooling combined with the high latent heat typical of Sn and its alloys. The results obtained by DSC measurements of PCB with a real electronic part soldered at two points are presented in our previous work [26]. From these results, both joints did not simultaneously solidify in most cases.…”