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July 2001

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Tue, 31 Jul 2001 07:26:59 -0500
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Steve,
The problem that you describe sounds vaguely familiar.  Whilst at CSL, we
examined a problem with open circuits in high aspect ration vias, under
BGAs, on about the thickness of board you describe.  The client had solder
mask that plugged the vias on the wave solder side of the board.  The mask
went on before gold plating.  Consequently, the sulfuric acid from the gold
plating remained in the vias and cause circumferential cracking in the
barrels of the holes.  The client had not had a problem with the vias until
they dropped under 13 mil diameter.  With vias larger than 13 mil, there
was enough opening for water rinsing to get down in the vias.  With vias
smaller than 13 mils, the surface tension of the water prevented it from
getting down into the via.  Imagine trying to clean out a capillary tube
and you get the general idea.  I wrote about this in Circuits Assembly
magazine.  The articles were in the Process Rx columns titled "It Cracks Me
Up" and "Sulfates in Vias:  The Rest Of The Story".  I can furnish them if
you desire, or you can probably get them from the CSL web site
www.residues.com.

I would say that you have to microsection to find out what is really going
on with the vias.

Doug Pauls
Rockwell Collins

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