Examining Formic Acid Reflow in Power Device Packaging for Flux-Free Soldering

Die attach soldering for power device packaging traditionally involves the use of a flux, either in a solder paste or a tacky flux used with preform solder. One of the roles of the flux is to remove oxides from the surfaces being soldered allowing for good coalescence of the solder after melting and also good wetting of the solder to the substrate and die surfaces. When soldering using a flux, post reflow flux residues are inevitable. A cleaning step is required to remove the post reflow flux residues as uncleaned flux residues can impact the reliability and electrical performance of power devices. This paper details using reflow systems with formic acid (HCOOH) to perform the soldering. Formic acid when heated to temperatures above 180oC “activates” and reduces metal surface oxides. The reaction product is metal formate, CO2 and H2O. By utilizing formic acid, a traditional flux component is no longer required for soldering, this means no post reflow flux residue and subsequently no need for cleaning. The formic acid soldering process is discussed in detail, explaining the intricacies of the reflow profile and how the profile varies based on different equipment types. Also examined are the different solder materials suitable for formic acid soldering, these include preform solders and also a novel “flux-free” solder paste that contains no rosins or activators and leaves virtually no residue post reflow. This process is also positive with regards to sustainability as electricity and chemical waste is reduced by not cleaning.