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August 2010

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Subject:
From:
Gary Ferrari <[log in to unmask]>
Reply To:
Date:
Wed, 4 Aug 2010 14:04:02 -0400
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Jack,

The IPC-2221 committee has addressed this issue in the working draft. 
They have discussed removal of NFP's and made a recommendation similar 
to what you and Werner have summarized. The designer may then choose the 
best note path for his/her application requirements. As has been 
mentioned, I do not believe that one could develop a single generic 
note, that would cover every application issue.

Gary


On 8/3/2010 8:06 PM, Werner Engelmaier 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.
>
> 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
>
>
>
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