D. Rooke wrote:
> From personal experience, removal of inner layer non functional pads has
> resulted in pad lifting rejects as per MIL-P-55110. The same designs, with
> non functional pads replaced, exhibited no pad lifting.
>
> Just out of curiosity, can anyone out there explain why pad lifting is
> rejectable? I understand that it's origin was based on solder iron touch up
> of leaded components, IE boards that had excessive pad lifting resulted in
> pad removal during post assembly rework. Is this still valid?
I SAVED A "THREAD" ON THAT SUBJECT, AND RATHER THAN TRY TO SUMMARIZE,
HERE IT IS:
Subject: ARE LIFTED LANDS ACCEPTABLE?
From: Gregg Klawson <[log in to unmask]>
Hello! We're having a discussion here regarding the inspection criteria for
PWB coupon microsection lifted lands after thermal stress. We're just
cutting over from military procurements to more "commercial" and are trying
to decide what makes sense. The specifications state:
Mil-P-55110E allows a maximum lift of 0.001 inch.
Mil-P-55110D before ammendment 4 allowed a lift of 0.003 inch maximum.
IPC-RB-276 states "lifting allowed" for all classes (no limit).
IPC-A-600D allows a maximum lift of 0.003 inch.
IPC-A-600E allows a maximum lift of ?????
What is the purpose of this requirement? Do we need a limit? If we need a
limit what is the "industry standard" limit from board house and OEM
viewpoints. We've been calling out IPC-RB-276 class 2 with a 0.001 inch
limit. A couple of our board houses tell us 0.003 inch is the "industry
standard". Any and all comments appreciated.
-Gregg Klawson
GTE Government Systems COrp
+1.508.880.1822
mailto:[log in to unmask]
-=x=-
From: David Bergman <[log in to unmask]>
This looks like a fun one and I'm sure the opinions will run strong.
IPC's specifications for lifted lands unfortunately were wagged by the
55110 untill they went to the 0.001 requirement. As my buddy the late
George Smith told me once, lifted lands could roll up like window shades
as far as he was concerned. Of much more importance was the integrity of
the knee and whether it was cracked. The rest was irrelavent.
I would stand by the latest 276 requirements as the industry position on
lifted lands.
David W. Bergman, Technical Director
IPC
2215 Sanders Road
Northbrook, IL 60062-6135
847-509-9700 x340 Phone
847-509-9798 Fax
email [log in to unmask]
www http://www.ipc.org
faxback support 800-646-0089
-=x=-
From: "Ralph Hersey" <[log in to unmask]>
For the IPC, the RB-276 is the controlling requirements, therefore, there is
no specified limit.
The RB-276 requirement for "no specified limit" is that there is no published
data supporting that lifted lands have led to function failure(s).
In general, from most of the discussions that have been held in the A-600
committee meetings have centered around two issues:
1) IMO, one of the general feelings is that the lifted land should not be
greater than "one metal thickness" or about 25-35 micrometers. This is
because there should be a number because if the land were "lifted" at an angle
of 45 degrees to the plane of the printed board's surface "it just wouldn't
look right", and most people would feel uncomfortable accepting such a
condition.
2) The second discussion point has been that if the assembly is to be
conformally coated, there may be some contaminants left in the space may lead
to either insulation resistance problems; or the residual contaminants or the
"air void" will lead to possible outgassing or fracturing of the conformal
coating, epecially if subjected to low ambient pressures (aero-space).
Ralph Hersey, e-mail: [log in to unmask]
-=x=-
From: [log in to unmask]
The subject is the reliability of lifted Lands
Lifted lands are caused by a fracture of the copper-epoxy bond at high
temperature. The mechanism is somewhat complex. Below Tg, epoxy is rigid
with a high modulus; and when a board is heated below Tg, the TCE mismatch
between the copper barrel and epoxy substrate produces elastic (and plastic)
strain in the barrel. However, when the temperature rises above Tg, the epoxy
modulus drops and the elastic stress in the barrel begins to compress the
substrate. The result is that the barrel contracts and the epoxy in the
neighborhood of the barrel is deformed. The board can be viewed as a soft
substrate with rivets that compress local areas causing the surface to puff up
between the rivets. Under these conditions, the lands which are connected to
the substrate bend up. This "hinge" effect is the source of the fatigue
failure that leads to interface cracks during thermal cycling above Tg. When
the board cools, the epoxy contracts and tries to pull the land back down.
However if the barrel to land-barrel hinge is too rigid, the epoxy-copper
adhesion may be inadequate and a lifted land results. By decoupling the land
from the substrate, a lifted land reduces land bending in subsequent thermal
cycles and may actually improve reliability during multiple solder transients.
According to this explanation, lifted lands can be caused by poor copper-epoxy
adhesion, a strong hinge (ie thick plating), or small lands (ie not enough
area for the epoxy to hold onto). Only poor adhesion is a serious threat, and
this can be detected in other ways.
I agree with David Bergman. Lifted lands are a cosmetic problem and there
really needs to be no limit in a cross section examination. I do support a
visual look without magnification after a solder transient (either the board
or a coupon). If lifting can be seen under these conditions, it is a
cosmetic defect and grounds for rejection.
Robert R. Holmes PhD
Lucent Technologies
[log in to unmask]
-=x=-
From: [log in to unmask]
Lifted lands!!!!:
Those who do not like the lifted land criteria in IPC-RB-276 will probablay
not like the requirement in IPC-6012 (interim final) which allows lifted
lands if not visually seen in the unstressed board inspection. We can find
many reasons why lifted lands are not desirable ( i.e. possible entrapped
fluxes, loosened holes. cracked knees etc, but for product function, I have
never seen a functional failure resulting from lifted lands. We allow
breakouts, 0.002 inch annular ring and the processing solutions to be in
contact with the bare-hole wall for an hour or so l during
desmear/electroless process; these could all result in the same condition
that may result from a lifted land. I feel, as do many others, that lifted
lands are process indicator or a cosmetic defect and not related to the
performance of the board.
Phil Hinton
[log in to unmask]
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