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36 SMT007 MAGAZINE I JULY 2021 Flux and Cleaning— How Clean Is Clean? Part 1 SMT Solver Feature Column by Ray Prasad, RAY PRASAD CONSULTANCY GROUP e subjects of flux and cleaning are inter- related; one cannot be discussed without the other. e selection of fluxes and cleaning pro- cesses plays a critical role in the manufacturing yield and product reliability of electronic assem- blies. Once soldering is accomplished, any cor- rosive material le on the surface must be com- pletely removed aer soldering. In this column, I discuss various types of fluxes and why we use them, followed by var- ious types of cleaning materials and processes. In my next column, I will discuss cleanliness requirements to know whether the boards have been cleaned enough to meet their functional requirements for their intended applications. Why Do We Use Flux? We use flux to be able to join two metallic sur- faces. Joining is done by welding, brazing, or soldering. ey are essentially the same except they are done at different temperatures. In all these three joining processes, we use flux to remove oxides so that a strong bond between the mating surfaces can be accomplished. e key difference among these processes is that there is no need for cleaning aer welding and brazing. But when it comes to electronic assemblies, cleaning is a critical process. You are not done aer the joining processes until you remove the residues. Otherwise, field fail- ures are almost certain depending on what components and flux types are being used. e common metallic surfaces for joining in the soldering process are copper and tin. Like most metals, tin and copper have the natural tendency to oxidize. However, to accomplish intermetallic bonds between them, you need to get rid of oxides. Flux helps provide a fresh clean surface to accomplish the intermetallic bond between copper and tin (it is the same for both tin-lead and lead-free) to achieve a reliable solder joint.