This doesn't seem like an active place to disuss technical issues anymore,
but in case anyone is still here that is interested, I'm trying to summarize
some various comments on the topic of non-functional pads, in an attempt
to create a comprehensive fab dwg note.

Sorry for cross-posting from TechNet....

Jack

---------- Forwarded message ----------
From: Jack Olson <[log in to unmask]>
Date: Tue, Aug 3, 2010 at 11:19 AM
Subject: Re: non-functional pad removal
To: [log in to unmask], Jack Olson <[log in to unmask]>


I have re-read all of the responses related to removing non-functional inner
layer pads (or not), and this will be my final attempt to condense what you
guys have taught me. I usually get in trouble trying to summarize other
people's posts, but its important because somehow we have to tell the
fabricators what we want.
How can we neatly describe a broad set of considerations into one simple
note
on a drawing?

So here is what I have been told about the subject of non-functional pads
(NFPs):

- Drill bits last longer if NFPs are removed
- In some cases NFPs create anchor points for stress,
 and barrel cracks occur
- Reliability is more relevant to hole diameters less
 than .020", remove NFPs from larger hole diameters
- Yield may increase by reducing possibility of inner
 layer shorts by removing NFPs
- NFPs only play a role with higher aspect ratios
- Divide the hole length into three zones, and remove
 NFPs from the middle zone
- 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
- NFPs reduce voids in thin low-flow prepregs
- Don't remove NFPs from layers 1,2,3,n-2,n-1,n

In additon, there was a direct conflict between two comments, one
recommended removing NFPs for thick power planes (BEllis), the other said a
DOE testing 24 layers of 2oz was more reliable with NFPs removed (GGagnon).

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.

Here is my attempt to cover the most scenarios with the least complication:
FOR CLASS 1, NON-FUNCTIONAL INNER-LAYER PADS MAY BE REMOVED
FOR CLASS 2 AND 3, NFPs MAY BE REMOVED FROM DESIGNS WITH LESS THAN
5:1 ASPECT RATIO
OTHERWISE,
NFPs MAY NOT BE REMOVED FROM LAYERS 1,2,3,N-2,N-1,N OR FROM HOLES
GREATER THAN .5MM

The IPC is interested in eventually developing a NOTE GENERATOR as a
companion to the IPC-2610 documentation series, and this seems to be a
perfect example of the type of subject that designers would want help with.

Do you have a better suggestion for a good note, or care to present a
note "framework" for non-functional pads?

so close, and yet so far...
Jack


.
On Mon, 12 Jul 2010 10:52:51 -0500, Jack Olson <[log in to unmask]>
wrote:

>I know this topic has been discussed here before, but an article I just
read
>by Paul Reid put a different twist on it
>http://www.pcb007.com/pages/zone.cgi?a=69569&_pf_=1
>
>I wanted to ask about a paragraph where he says:
>
>"We know from many years of reliability testing that a board with
>non-functional pads removed tends to be more robust than the same board
with
>non-functions at every layer. Occasionally, designers will have
>non-functional pads at every internal layer. In most applications this
>produces a reduction in reliability with an increase in barrel cracks in
the
>central zone of the PTH. It appears that this produces a number of anchor
>points along the PTH and failure occurs in the barrel. Customers who remove
>non-functional pads for increased PTH reliability reduce the "anchor" point
>and stress is transferred to the knee of the hole."
>
>This is a very important point for me, because I have always heard it
>explained a different way. My (unfounded unscientific) understanding was
>that fabricators wanted to remove them to save drill bit wear (especially
>for high-volume boards in benign environments), but designers often want to
>keep them in because the extra ribs provide more support (especially for
>harsh environments).
>This article suggests that keeping inner layer pads is LESS reliable.
>The reason it is important to me is that our boards are expected to survive
>20 years in an automotive environment, we have been allowing unused pads
to
>be removed, but some have suggested we retain them for lead-free
processing
>temperatures.
>We haven't cared about inner-layer pad removal until now, but soon we will
>be required to design for RoHS compatibility, and we were about to start
>specifying that they be retained. Am I misunderstanding these results?
>
>Jack


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