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Date: | Wed, 22 Nov 95 11:03:50 PST |
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In 7 years of experience as Process Engineer for HASL I have seen both
sides of the coin. In the beginning, one of our major customers wanted the
solder as flat as we could make it. As pitches shrank, minimum solder
thickness requirements increased, especially for 20 mil QFP's. I, like
you, have never seen a soldering defect directly attributed to thin
solder. But, I have had boards returned for the "thin solder defect".
Typically solder thickness on discrete pads will be ~50 microinches. Those
pads tend to look dull compared to a 20 mil pitch QFP site with 400
microinches of solder. That visual difference can lead to rejection.
There is always a very thin coating of eutectic solder over the
intermetallic layer (IMC) on HASL processed product. Granted, a thin
solder coating may have an increased lead content due to the consumption
of tin into the IMC. Assembly problems can arise if the product is stored
in poor conditions and a heavy lead oxide layer forms (difficult to remove
with today's water and no clean fluxes).
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______________________________ Reply Separator _________________________________
Subject: Minimum Solder Thickness @ Assembly
Author: [log in to unmask] at corp
Date: 11/22/95 9:23 AM
As fabricators, we have had it drilled into us that "flat solder is best"
for assembly. We have never received a return from any customer for "thin
solder", but I can't say the opposite is true. My process engineers desire
to alter our hot air solder leveling parameters to eliminate any future
chances of returns for thick solder. Our SPC data suggests that lowering the
peak solder thicknesses results in exceptionally thin solder (IMO) on
certain pad geometries. My question:
How thin is too thin? As assemblers, when (at what thickness or process
conditions) would you return boards to the fabricator?
[log in to unmask] (Glynn Shaw)
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