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. 

Here is my attempt to cover the most scenarios with the least complication:
FOR CLASS 1, NON-FUNCTIONAL INNER-LAYER PADS MAY BE REMOVED, UNLESS OTHERWISE SPECIFIED
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


 Werner

 


 

 

-----Original Message-----
From: Jack Olson <[log in to unmask]>
To: [log in to unmask]
Sent: Tue, Aug 3, 2010 12:19 pm
Subject: Re: [TN] non-functional pad removal


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



---------------------------------------------------

Technet Mail List provided as a service by IPC using LISTSERV 15.0

To unsubscribe, send a message to [log in to unmask] with following text in

the BODY (NOT the subject field): SIGNOFF Technet

To temporarily halt or (re-start) delivery of Technet send e-mail to 

[log in to unmask]: SET Technet NOMAIL or (MAIL)

To receive ONE mailing per day of all the posts: send e-mail to 

[log in to unmask]: SET Technet Digest

Search the archives of previous posts at: http://listserv.ipc.org/archives

Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for 

additional information, or contact Keach Sasamori at [log in to unmask] or 

847-615-7100 ext.2815

-----------------------------------------------------


 

______________________________________________________________________
This email has been scanned by the MessageLabs Email Security System.
For more information please contact helpdesk at x2960 or [log in to unmask] 
______________________________________________________________________

---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 15.0
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest
Search the archives of previous posts at: http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815
-----------------------------------------------------