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REFLOW: The secret to a high tensile strength! (Part 1)

At a recent customer visit, I had the opportunity to discuss "the process".  What we typically call "the process", is that magic that happens from when the separate parts go in at the start of the line, and the finished product comes out of the reflow oven.  This discussion was focused on reflow, and why it is important.  Reflow is the high-wire balancing act of the SMT circus.  Reflow is a balancing act because a good profile is a split between too little and too much.




Typically, we configure the reflow profile to work with the available solder and components, to give the highest tensile strength possible.  So, we know what the end goal is, and we adjust what we have to achieve that goal.  Besides tensile strength, some secondary goals are good wetting, solid intermetallic formation, homogeneous solder joint and a small, tight crystal structure.  All of these are achieved through process management of the reflow process.




There are four parts of the reflow process that are adjusted to achieve the goals we have in mind, namely highest possible tensile strength.  They are ramp rate, time above liquidus (TAL) peak temperature and cool down rate.  Each one of these has its own effect on the final solder joint, and each one is important.