As a PCB user we specify LPI, then HASL, followed by a secondary "plugging" 
operation from the second/bottom/solder side of the PCB using a screen printable
solder mask.  The plug aperture is large enough to adhere to the LPI around the 
via perimeter.  This does add cost since it is an additional process, but it 
avoids the issues of entrapped flux.  We specify this process whenever we have 
components on the second/bottom/solder side of the PCB and whenever via to via 
spacings need to be reduced.  It makes the wave soldering process much more 
forgiving and permits vacuum pull down for testing on double-reflow assemblies. 

Dick Krug, Racal-Datacom, Sunrise, Florida     [log in to unmask]
______________________________ Reply Separator _________________________________
Subject: FAB:  soldermask plugged vias
Author:  [log in to unmask] at ftl03
Date:    7/23/96 1:15 PM


Just curious:

We often have customers requesting that the (LPI) soldermask be 
screened/floodcoated to plug vias with soldermask.  I understand the 
reasons behind the design; however, I'm concerned about potential 
contaminates being trapped in the via, especially from the HAL or assembly 
process.  This soldermask process can produce vias that are partially to 
fully plugged from one (or both) sides of the PCB.  This can cause 
inadequate rinsing and cleaning of the vias during the HAL or assembly 
process.  

Any other ideas or concerns?  Has anyone looked at this issue in detail?


(This soldermask process can also produce exposed copper in the via, which 
has been discussed on this forum.  I believe the OSP surface finish would 
be a much safer choice with this via/soldermask design).

Thanks,

Dave Boggs
Merix Corporation
E-mail: [log in to unmask]

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