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
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-----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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