Ben,
I wrote a paper while at Johanson Dielectrics titled OOOH Colors it 
Must Be Lead Free a few years ago and it is still on their web site. The 
color shift comes from tin oxide thickness growth due to peak 
temperature and cumulative temperature exposure. Metal colors are 
commonly used in tempering steel and are an accurate indication for 
temperature.

John Maxwell

On Fri, 12 Aug 2011 14:56:17 -0400, Gumpert, Ben wrote:
> Technetters,
>
> I seem to remember this coming up previously, but I couldn't find it
> in the archives... What causes solder joints to have a blue tint?
>
> I have a card where most of the solder joints across the assembly
> have a blue tint to them. It's Sn63Pb37 solder on a SnPb HASL board
> and a variety of part finishes - but the pads that weren't pasted
> don't exhibit the blue color. This is the only card in the lot that
> has this condition, but I don't know what process it may have seen
> differently from the others.
>
> Some kind of oxide?
>
> Thanks in advance,
> Ben
>
>
> 
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