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February 1999

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Subject:
From:
Phil Hinton <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Mon, 8 Feb 1999 18:37:10 EST
Content-Type:
text/plain
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Guenter,

For the equilibrium to be reach the intermetallic has to be in contact with
molten solder for more than a few seconds.  If you take 3 pieces piecec of
2oz.foil and dip them it in a solder pot for 1, 2 and 3 minutes and section
them you will find that the intermetallic is always the same thickness, but
that the copper foil has decreased in thickness depending on the dwell time.
If you leave the foil in the pot for a couple of more minutes you will be left
with only the end of the piece you are holding.   If you do a reflow or fusing
operation of 1,2 and 3 minutes on foil coated with tin/lead plate you will
fiind that the intermetallic is thicker on the one with most time, often over
3 microns.  HASL produces intermetallics in the range of a couple of microns,
but may reach equilibrium and decrease the copper thickness .  Diffusion and
mobility of the elements in the solder is an important factor.  This increases
with temperature and hoiw high above the melting temperature it is.  If you
bake the boards or preheat boards near the solder melting temperature for much
time you will find that the intermetallic thickness is greater on those with
the longest preheat.  If you wave solder a board several times or slow the
conveyor down, you will notice copper thickness at the bottom knee got
considerably thinner while the center of the hole did not decrease that much
and the intermetallic is thinner at the bottom knee than at the top knee.

So what Lea is saying  may or may not happen in a solder joint depending on
how long the joint is heated, agitation, preheat time and temperature and rate
of cool down.  I think that most joints have enough solder available for the
intermetallic thickness to reach equilibrium, if they are allowed to be in the
molten state long enough, and enrichment of the solder with copper is a minor
factor.

Phil Hinton

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