There’s no point in beautiful solder connections if the system is unreliable.
Cosmetically acceptable solder joints are important but only if they are achieved in a reliable fashion. Wetting issues can be eliminated by employing stronger fluxes, but the resulting ionic contamination can cause failures. And overheating components leads to premature failures. Neither ionic contamination nor overheating can be seen but they can be deadlier than unattractive solder. Anyone can melt solder onto components; very few people do it reliably.
Reliability can’t be inspected in.
A cosmetically defective solder connection is almost certainly unreliable. However, cosmetically acceptable alone proves nothing. True reliability requires doing the job right the first time, every time.
There’s no such thing as “cold” solder.
Again, words matter. The logical corrective action for “cold solder” is more heat, and that is the normal industry approach to the problem. But, unless the solder actually froze before completing its flow (as in a plated through-hole with heavy mass), what people call “cold solder” is just poor wetting. IPC-A-610G, 1.8.2, defines “cold solder” as “A solder connection that exhibits poor wetting, and that is characterized by a grayish porous appearance. (This is due to excessive impurities in the solder, inadequate cleaning prior to soldering, and/or the insufficient application of heat during the soldering process.)” The British have a better term, “dry solder,” that accurately represents the underlying solderability issue.
Perfect soldering is easy – 98% perfect is hard.
I began this series by telling you that “perfect soldering is simple.” So, it seems appropriate to end with the same theme. Soldering is actually a fairly straightforward science. If we meet a small number of requirements imposed by nature, the solder will do the work and produce perfect connections. But we must comply with all the requirements. A process that violates just one of nature’s requirements produces visually unacceptable connections; it is necessary to set other parameters to sub-optimal levels that compensate for the initial flawed setting. Perfect soldering really is easy, but it doesn’t happen by luck or guesswork. Read our next article in the soldering series making sense of wave soldering.
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