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March 2000

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Subject:
From:
Phil Dutton <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Fri, 17 Mar 2000 10:30:57 +1030
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Hello TechNet gurus,

The discussion on whether to plug or not has been going on for a long time.
There appears to be many things to consider.
We've been recently having poor yields with 19x19 BGAs (1mm pitch) and are
trying to find the reason for this.
We have partially tented vias under the BGAs.(soldermask over the via pads,
but only up to the holes, leaving the holes open).
Via size is 26/12 and 24/12
Pads have been gold plated, but I'm not sure if the gold is heavy enough to
produce soldering problems.
Could we be having CTE mismatch problems, as we are using regular FR4
materials?

A summary of information that I have found, mainly on technet is as follows;

LPI soldermask does not tent well, and cannot be relied upon to adequately
fill a via.
Tenting both sides can result in soldermask voids within the vias, that can
outgas during reflow.
Tenting the top only, still allows wave solder to travel up the via, so
possibly partially reflowing the solder balls, as with open vias.
Tenting bottom only, will stop the wave solder travelling, but we have a
contaminant trapping void on the top side that can outgas again.

 From what I can gather, possible solutions are;

1. To epoxy plug the vias. - Is this a common practice?
2. Make use of blind vias, even micro vias  May be worth investigating the
cost penalty for this. Especially compared to low PWA yields.
         Are we kidding ourselves, thinking that we can use fine pitch BGAs
and uBGAs without going to this technology?
3. Something that I've only heard about, and may be a bit exotic, is to
epoxy plug the vias and overplate copper (with this we can have vias in pad)
         Is this process a reality, or theoretical and perhaps impractical?
4. Avoid wave soldering (I understand that this may be unavoidable due to
thru-hole comps)
5. If there is a choice between BGA and say, PGA, it would be better to use
the PGA device unless absolutely necessary.
         Although BGA use will only increase, and we should learn to work
with the technology.

Any discussion, real experiences, ideas, and general enlightenment would be
appreciated.
Good forum, (even if occasionally off the rails. Green rectangles can get
stale)

Many thanks,

Phil.





Phil Dutton C.I.D.
Senior CAD Technician
IPC Certified Interconnect Designer

Vision Abell Pty Ltd
Second Avenue, Technology Park,
Mawson Lakes.  SOUTH AUSTRALIA  5095
================================
Phone : (08) 8300 4400 (reception)
Fax :           (08) 8349 7420
email:          [log in to unmask]
Internet Page:  http://www.vsl.com.au
================================

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