Behavior of Sn-3.0Ag-0.5Cu solder/Cu fluxless soldering via Sn steaming under formic acid atmosphere

Formic acid (FA) atmosphere is promising to achieve chemical reduction at the oxidized surfaces of the solder and matrix material during soldering, replacing chemical flux being challenging as the continuous decrease in the pitch of solder bump in a three-dimensional integrated circuit. Although a previous study observed the effect of Sn steaming on the wettability of the solder during FA soldering, it has not been understood yet by adequate studies. This study demonstrates the fluxless soldering behavior of Sn-3.0Ag-0.5Cu (SAC) solder/Cu under FA atmosphere. Resembling vulcanian eruption Sn steaming occurred with a solid-state Sn and promoted Sn–Cu intermetallic compound (IMC) formations obviously at temperature (210 ° C) lower than the melting temperature of Sn (231 ° C). As indicated by in-situ observations, the IMC formations led to the final spreading area of solder under FA soldering was larger than that under the soldering with rosin mildly activated (RMA) flux. The wettability of SAC solder under FA atmosphere could be improved with continuous heating.