In-situ observation of fluxless soldering of Sn-3.0Ag-0.5Cu/Cu under a formic acid atmosphere

Abstract

Formic acid, which can provide efficient reduction for most solders, is widely used in fluxless soldering. In this study, to establish a reliable fluxless soldering process, a formic acid atmosphere was used to reduce the oxides of Sn-3.0Ag-0.5Cu solders and Cu substrates. The wetting behavior of Sn-Ag-Cu solder balls on Cu pads under a formic acid atmosphere was observed in situ, and the wettability was investigated by evaluating the contact angle and spreading rate. We confirmed the effect of formic acid on the interfacial reaction between the solder and Cu substrate. After the fluxless soldering, impact tests were conducted on the formed solder bumps. The results indicate that the contact angle of solder under a formic acid atmosphere was similar to that of the solder using rosin mildly activated (RMA) liquid flux; however, its spreading rate was much lower than that using the RMA flux. Some residues were found around the solders formed under a formic acid atmosphere after the reflow process (the substrate turned gray), whereas the same area on the substrate remained clean when using the RMA flux. Some pores were observed near the intermetallic compound layers in the samples soldered under a formic acid atmosphere, but the pores barely affected the impact strength.