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Subject:
From:
Paul Reid <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Paul Reid <[log in to unmask]>
Date:
Mon, 12 Jul 2010 18:36:18 -0400
Content-Type:
text/plain
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text/plain (450 lines)
"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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