The solder joint is not metal touching metal — it is a layered structure of intermetallic compounds, flux residue, and grain boundaries whose properties depend on alloy composition, peak reflow temperature, and cooling rate. Understanding the materials science explains why 63Sn/37Pb was nearly perfect, why RoHS forced a worse substitute on the industry, and why tin whiskers still haunt spacecraft and military hardware today.
Soldering
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Solder Metallurgy: Eutectics, Whiskers, and Why Lead-Free Is Harder