To define the reflow process, you need to know what is going on with all variables that impact the process, many of which surprisingly are not considered on many modern SMT lines. Lucky for you, that’s why I wrote this guide! Before diving in, let’s first take a look at what is actually going on with the reflow process.
Each PCB will have an applied thickness of solder paste at each pad, which is intended to react and bond with the alloy on the component. Each component lead will be prepped or tinned with a complementary material to facilitate bonding of the solder alloy in the paste to the pad under certain conditions.
Solder paste is, generally, spheres of metal, flux and solvents that aid in the phase change of the paste from semi-solid, to liquid to vapor, and the metal from solid to liquid. Understanding that solvents react at a lower temperature than that of the metal, you may begin to see the intended chain of reactions during reflow. You may also be amazed by the complex chemistry that takes place when PCBs and components come into contact with solder paste in an environment of low oxygen and high temperatures. The bonding chain of reactions and interactions is nothing short of miracle that it works at all. This is really cool stuff and the subject for entire books, but this profiling guide is NOT an academic discussion, so let’s keep it on point: how to profile.
Your product, of course, is processed through a reflow oven, which has come a long way to becoming much more stable, controllable, repeatable and uniform in heat distribution. Despite all the advances and improvements, one fact has not changed: a reflow oven’s singular purpose is to solder components, or if you will, to create a profile that is in spec. Your job is to define, measure and improve upon this profile for your product. The reflow oven is just a means to an end.
