Hi Steve! There are a couple of possible causes for having a blue tint on a
solder joint and most of them relate to the oxide condition. I have seen
various cleaning systems (terpene, soaps, alcohols, etc.) stain the
tin/lead oxide surface leaving a non-shiny, cosmetic appearance change -
very similar in appearance to a thin oil film oil on water. A second
possible cause is surface etching of the solder joint by the cleaning
chemistry. I have some wonderful pictures of solder joints on an assembly
which underwent a dozen passes (yes a dozen and don't ask why!)  through an
aqueous cleaning system using soap saponifier - the joints are purple! (we
called them "Barney boards"). The multiple cleaning passes resulted in an
etching of the tin on the solder fillets leaving a heavy lead oxide - I
used SERA testing and microsectioning to verify the oxide changes. The
solder joints were very purple and very reliable.  There have also been
some industry reports/investigations which discuss a visual solder joint
appearance change due to overall solder joint surface texture (e.g. when
solder joints get rough they look different). Hope this helps.

Dave Hillman
Rockwell Collins
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