It's interesting the way you word this inquiry. Not EVERYONE uses
non-functional pads. I AM a customer and PREFER that non-functional
pads NOT be used. I have multiple manufacturers approved and some add
non-functional pads to my boards and some don't (even on the same
designs). My opinion is that PCB suppliers who add non-functional
pads have inferior plating processes. I don't mind too much when the
supplier asks to add these pads but I do require approval on an
individual basis and I do like to get cheaper boards because the
manufacturer gets higher yields. However, I do harbor second thoughts
about the quality of the suppliers plating process.
NOTE: Japanese PCB manufacturers don't use non-functional pads.
______________________________ Reply Separator _________________________________
Subject: Non-Functional Pads
Author: [log in to unmask] at Internet-HICAM-OK
Date: 1/31/96 11:52 AM
I would like to perform a sanity check regarding the use of
non-functional pads on high layer count multilayer designs. First I'd
like to present a small scenario.
You're at your desk, doing whatever you do. The phone rings. Behold,
it's your customer and the conversation goes something like this. " I've
got these ___(insert a large quantity) assemblies and we have opens at
functional test." " When I re- test, there seem to be additional
failures." " I was just wondering, do you know what might be
happening?"
After the phrase "Value Added" flashes through your brain, you respond
with the usual. "I'll check it out and get back to you." You reach for
the antacid and think. "There's another afternoon shot to hell!" Of
course, it's either a high visibility product or something built and
shipped six months ago, and we all know anything that old can't be
researched.
We've all seen .187 thick panels built with little or no consideration
for thermal abuse. They survive just fine. We've also seen .062 PCB's
that look so poor, they must belong to somebody else..or do they?
Back to the subject.....
This is not a specific problem solving request and does not relate to any
particular processing issue. I am looking for an updated compromise
strategy that will minimize the impact to all or most areas of
manufacturing. It seems that today's board designs and manufacturing
yield considerations have made the addition of non-functional pads more
of a curse than a cure. Gone are the padmasters you just superimpose and
be done with it! Following are some of the mfg. issues:
1) The addition of non functional pads increase the opportunities for
innerlayer shorting defects, along with escapes from AOI.
2) Drilling is impacted, you can't just stack three high with #80 drills
and let 'er rip.
3) Adding non functional pads to CAD data that originally did not
incorporate them requires careful DRC and netlist retesting. As often as
not, the designer has encroached upon those areas and at best you can only
add pads of a smaller diameter. This presents a new set of problems such
as pad spin etc.
We currently have specific criteria for incorporation. Of course, we
also have additional options and material upgrades from 140 to 170 Tg
laminate. Like our competition, we use the best materials, chemistry
etc. and apply what appear to be very good process controls. We are
acutely aware of material constructions, aspect ratios, drill
de-optimization etc. and all the "good stuff".
Several questions regarding criteria that come to mind;
1) Is a tented via less prone to failure than an exposed one? Could non-
functionals be omitted?
2) Should non- functionals only be added to extremely dense areas such
as PGA, BGA etc.
3) Should I really compromise and just add them to the outermost
innerlayer signals?
Your opinions and comments are appreciated.
PS: If you never use non-functional pads, don't worry about their use
or just don't care, one of the following is probably true.
1) All you manufacture is Polyimide!
2) Your in-coming phones are out of order!
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