Contact angle analysis and intermetallic compounds formation between solders and substrates under formic acid atmosphere

In 3D packaging, an increase in the solder bump density and reductions in solder-bump height are the roadblocks to using gel or liquid fluxes for the soldering process. Fluxless soldering has become a popular technology, and the formic acid (FA) atmosphere soldering is one of the most promising fluxless soldering techniques. In this study, the effects of FA atmosphere on contact angle and the intermetallic compounds formation between three different solders (Sn-58Bi, Sn-3.0Ag-0.5Cu, and Sn-0.5Sb) and two different substrates (Cu and Ni/Cu) were investigated. The results indicate that the wettability of those soldering systems of FA soldering is very similar to that of soldering with the solder flux. Moreover, the effect of pre-heat treatment on the wettability of the Sn-3.0Ag-0.5Cu solder (SAC)/Cu by FA soldering was investigated. With the increase in the pre-heat time, the soldering wettability was improved. And there was no significant difference in the interfacial reactions of SAC/Cu between FA and flux soldering according to the microstructural study results before and thermal aging. This paper verifies that the FA atmosphere is suitable for various lead-free solders and substrates to achieve fluxless soldering and then provides a theoretical reference for the industrialization of formic acid atmosphere in electronic packaging.