Hey Jack,
The problem is that it depends. There are several other (maybe fabricator specific) factors that may be well understood, but you need to de-correlate them before you could make a decision.
that's why many times design activities simply "allow NFP removal". And "engineering punt" to the fabricator.
A relatively simple way that some OEMs handle it is to include an empirical study during fabricator qualification. Delphi is probably the best known example. It works.
Chris
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Jack Olson
Sent: Wednesday, August 04, 2010 10:15 AM
To: [log in to unmask]
Subject: Re: [TN] non-functional pad removal
Well, maybe I should have known better than to try and summarize all the
previous responses, both public and private.
The goal I was pushing towards is related to the fact that the committee that
just finished balloting on IPC-2614 (Board Fabrication Documentation) included
an appendix with "sample notes".
On the subject of non-functional pads, here are the three sample notes
provided by IPC-2614:
1) Non-functional pads may be removed from inner layers at vendor's
discretion
2)Manufacturers shall not remove non-functional pads
3)Removal of non-functional pads from internal layers is permissible
Yes, those are notes. and maybe they are representative samples in
someone's opinion, but in my opinion they don't provide any guidance to the
reader and they gloss over what could be a significant reliability issue.
It seems like there is enough technical knowledge on this forum, and enough
people have already replied with interesting related points about the subject,
that we could write at least one other note that might be more useful.
Can't we come up with a note that fits MOST applications, and leave it to the
user to modify it for his specific circumstances?
onward thru the fog,
Jack
.
On Tue, 3 Aug 2010 20:06:01 -0400, Werner Engelmaier
<[log in to unmask]> wrote:
> Hi Jack,
>Here are my comments
>
>
>So here is what I have been told about the subject of non-functional pads
(NFPs):
>
>- Drill bits last longer if NFPs are removed - "YES
>
>- In some cases NFPs create anchor points for stress, and barrel cracks
occur - "I THINK YOU MISUNDERSTOOD THIS.
>
>- Reliability is more relevant to hole diameters less than .020", remove NFPs
from larger hole diameters - "RELIABILITY IS
>OF COURSE EQUALLY RELEVANT, YOU JUST GET DIFFERENT FAILURE MODES
DEPENDING ON DRILLED DIAMETERS:
>BARREL CRACKING FOR SMALLER DIAMETERS, INNERLAYER
SEPARATION/SHOULDER CRACKS FOR LARGER DIAMETERS.
>REMOVING NFPs FROM SMALLER DIAMETER HOLES HAS LESS RELIABILITY
CONSEQUENCES.
>
>- Yield may increase by reducing possibility of inner layer shorts by removing
NFPs - "???
>
>- NFPs only play a role with higher aspect ratios - "NO
>
>- Divide the hole length into three zones, and remove NFPs from the middle
zone - "YES
>
>- High layer count with thin dielectric creates resin-starved pancake stack,
remove some NFPs - "???
>
>- Keep the stack symmetrical, if a connection is made on layer 8 of a 16
layer, leave NFP on layer 9 to balance the stress - "
>A SYMMETRICAL STACK IS A GOOD IDEA TO REDUCE WARPAGE DURING
SOLDERING; YOU CANNOT "BALANCE" [WHATEVER THAT MEANS] STRESSES
THIS WAY
>
>- NFPs reduce voids in thin low-flow prepregs - "POSSIBLY
>
>- Don't remove NFPs from layers 1,2,3,n-2,n-1,n - "FOR LARGER DIAMETER
HOLES
>
>
>I assume that those working on high-tech high-reliability designs know what
>they want to do and have customized their fabrication drawing to get what
>they want. On the other hand, I'm concerned about the average designer
who
>wants to know best practices, but isn't sure how to form the note.
>
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