Hi Paul,
First, important things like telling Billy B. I said "Hi" and invite you both to stop by next time you are in Boston.
Second. This topic is design, environment, and fabricator dependant.
In most applications I have worked with, removing NFP improves the situation.
I just got done with a project with DOEs that cyclically tested a 24 layer all 2 ounce copper design.
The test samples with NFP removed were much more robust than the samples with the NFP in place.
The DOE testing also indicated that in this design, retained NFP needed to be 0.5 mm larger in diameter in order to acheive parity with the DOE testing for the samples with NFP removed.
This has usually been my experience with designs, environments, and suppliers I have worked with.
Your mileage may vary.
Brs,
Gerry
> Date: Mon, 12 Jul 2010 18:36:18 -0400
> From: [log in to unmask]
> Subject: Re: [TN] non-functional pad removal
> To: [log in to unmask]
>
> "So, under these circumstances, Paul and I 'sing from the same sheet.'
> Werner
>
> That is a great relief Werner. I am not comfortable being on the wrong
> side of a Werner technical discussion.
>
> All this is very interesting and we should consider testing NFP on some
> of those variables like hole size, grid size, low flow resin, thin vs.
> thick.
>
> In all honesty I did not expect to have such a lively discussion on NFP
> from a column written about corner cracks. I would have explained more
> of the factors affecting NFP if the column was on that aspect of
> reliability. The columns I am writing now are limited to the failure
> modes for which I have created animations.
>
> To take it one step further, there is the consideration of "glass lock"
> and "glass crunch" which is inherent to high layer counts with NFP in
> and thin dielectrics. There can be so much copper in the "pancake stack"
> of NFP that the area of the PTH is resin starved and the copper is
> resting directly on the glass fibers on the B-stage layers. Breaking
> materials is generally stress relieving extending cycles to failure. Low
> resin due to squeeze out and cracks in the dielectric between pads may
> extend thermal cycles to failure. You have to consider both copper and
> material when you talk about reliability in lead free applications.
>
> We need to study the effects of design on reliability. I met with Gary
> Ferrari and Deter Bergman when they were in Ottawa giving training
> classes last month and we had some lively discussions on reliability vs.
> design. I think the consensus is that there is less objective evidence
> and more "common sense knowledge" than we would like on the implications
> of design's effect on PWB reliability. This concerning in lead free
> applications.
>
>
>
> Sincerely,
> Paul Reid
>
> Program Coordinator
> PWB Interconnect Solutions Inc.
> 235 Stafford Rd., West, Unit 103
> Nepean, Ontario
> Canada, K2H 9C1
> 613 596 4244 ext. 229
> Skype paul_reid_pwb
> [log in to unmask] <mailto:[log in to unmask]>
>
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Werner Engelmaier
> Sent: July 12, 2010 5:42 PM
> To: [log in to unmask]
> Subject: Re: [TN] non-functional pad removal
>
> Hi Paul & All,
> that is the problem/danger with Technet responses which have to be a
> fortiori short.
> One has to assume some underlying understanding-I assumed everybody
> knows that 'small PTV diameters tend to barrel crack' and 'large PTV
> diameters will tend towards innerlayer [I did not write 'interlayer'
> anywhere] separations.'
>
> To prevent the later, remove NFPs from large diameter holes and leave
> them in place for small diameters.
> PCB thickness has also a significant influence, as do a whole number of
> other parameters.
> So, under these circumstances, Paul and I 'sing from the same sheet.'
> Werner
>
>
>
>
>
>
> -----Original Message-----
> From: Paul Reid <[log in to unmask]>
> To: [log in to unmask]
> Sent: Mon, Jul 12, 2010 5:23 pm
> Subject: Re: [TN] non-functional pad removal
>
>
> Hi All,
>
>
>
> It is very hard for me to be in disagreement with Werner but we are not
>
> as far apart as it would appear.
>
>
>
> We do agree that lead free applications increase the effect of non
>
> functional pads (NFP) in or out. The data becomes tighter, and cycles to
>
> failure reduced, if the test vehicle is exposed to lead free assembly
>
> and rework.
>
>
>
> My response reflected the findings we have on PTV (plated through vias).
>
> In smaller holes and grid sizes the degree of pad rotation is reduced
>
> due to the proximity of adjacent holes. Because smaller holes are on a
>
> smaller grid, it appears that there is less stress relieving by material
>
> squeezing out between PTV with each thermal excursion so the force of
>
> the z-axis expansion is concentrated (for lack of a better word) in the
>
> Cu barrel. In larger grid sizes the material easily stress relieves by
>
> squeeze out between the PTVs and as a result, decreases the stress in
>
> the barrel and, at the same time, increases the degree of pad rotation.
>
> With larger hole sizes the grid is larger and the amount of pad rotation
>
> is increased, in part due to the increased distance between holes. At
>
> the same time larger holes have barrels that are easier to plate and the
>
> larger hole size means the barrel are inherently more robust (even Cu
>
> distribution, small uniform crystalline structure). Interconnect
>
> failures (I think Werner miswrote interlayer separation) is exacerbated
>
> when pads are removed. Interconnect separation is not a wear out failure
>
> mode; usually interconnect failure reflects a process problem. Although
>
> larger holes on larger grids shifts the areas were stress is expressed,
>
> (from barrel to corners) I expect that if the interconnections are not
>
> compromised, then NFP out increases cycles to failure.
>
>
>
> All things being equal, NFP out increases reliability.
>
>
>
> I do not know the effect on thin coupons or low flow resin systems. It
>
> would be interesting to test those two variables.
>
>
>
> Please understand that variables like hole size, surface finish, type of
>
> material all count when you are dealing with a third level influence
>
> like NFPs in or out. It is easy to have other factors be covertly
>
> influential in a NFP In vs. Out study. Nickel protection of the PTH and
>
> degradation of material, like delamination, (in lead free applications)
>
> would be two major confounders to this type of study. It is
>
> disappointing to find, at the end of test, that the data is confounded
>
> because another factor dominated the results.
>
>
>
> Steve Kelly from PFC Inc. in Toronto read my comments in the iconnect007
>
> column and is interested in testing my contentions in a rigid flex, 12
>
> layer construction. I do not know the effect of NFP in vs. out on rigid
>
> flex or normal flex for that matter. Steve is planning to fabricate test
>
> coupons with clad polyimide and low flow resin. Our agreement is that
>
> PFC will fabricate the test vehicles and we will provide testing. If we
>
> get compelling data, confirming or refuting our historical findings, we
>
> plan to publish the results. I expect we will make this a lead free
>
> study.
>
>
>
> Sincerely,
>
> Paul Reid
>
>
>
> Program Coordinator
>
> PWB Interconnect Solutions Inc.
>
> 235 Stafford Rd., West, Unit 103
>
> Nepean, Ontario
>
> Canada, K2H 9C1
>
> 613 596 4244 ext. 229
>
> Skype paul_reid_pwb
>
> [log in to unmask] <mailto:[log in to unmask]>
>
>
>
>
>
> -----Original Message-----
>
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Jack Olson
>
> Sent: July 12, 2010 11:53 AM
>
> To: [log in to unmask]
>
> Subject: [TN] non-functional pad removal
>
>
>
> 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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