I do love this question and hate it. I love it because it is so refreshing
seeing the "young" folks ask important questions in consideration of a
possible new technology. I hate it because it is very ancient "technology"
and dates me.
Hot oil leveling, really fusing, is the best damn process ever invented to
both to "level" a tin-lead plated surface (really quite flat to begin with),
also another old and best process invented to ensure solderable surfaces,
and to burn a few board shops down. The most solderalbe surface is one, in
my opinion, is one that has been electroplated with tin-lead - provided it
is hot oil leveled afterword. No oxides possible here except under extreme
conditions.
Actually the minimim .0003" mils of tin-lead plating, per the old
MIL-P-55110 requirements, is not leveled. It is "fused" making it a very
homogeneous surface suitable for all through hole applications. It is in no
way suitable for SMT applications as the fusing, again not leveling, causes
a "dome" to be formed on pads that can reach a majestic height of 1 mil or
so. Hot air leveling, or HAL/HASL, reaches no such lofty heights but is an
often ruiness, to SMT placement and soldering, hill and valley terrain with
other objectional, to me and many, characteristics.
I could go on forever about the gold old days firing up the "peanut" oil, as
the fusing medium was so fondly called, and smelling something like nothing
else on earth. Personally prefer polyimide cooking in an old PHI press. Two
of the reasons, besides Bud, I've so few brain cells left as evidenced by
this diatribe. But it is fun, or was?
And on it goes,
MoonMan
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