Paul,
This is not too long of a message at all. This is an excellent message.
I worked in the industry as a consulting engineer prior to coming to
work at GD. Many of my clients are currently reporting that PWB
delamination is a major problem for them as they go to higher temps to
accomodate Pb-free BGAs and flipchips. Repeatedly, the pwb materials are
rated for Pb-free processing, and the Tg and Td ratings are purportedly
good enough to withstand the higher temps, but the boards delaminate,
even after baking to remove moisture. They are delaminating at
temperatures as low as 230 C, and that is the chamber temperature, not
the board. I brought this up in several postings to the Technet forum
last month. There has been much discussion about it.
There needs to be a time/temperature rating on pwbs for resistance to
delamination, just like there is a moisture classification on
components. The other properties of the materials are meaningless if
they pop in the reflow oven or during hot-gas rework.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of paul reid
Sent: Monday, December 12, 2005 5:10 PM
To: [log in to unmask]
Subject: Re: [TN] Qualification of the Pb-free capable manufacturers
The problem with PCB fabricators demonstrating lead-free capabilities is
really a two-fold issue. One issue is to demonstrate the proof the
product is lead-free and has been appropriately documented. The other
is to prove that PCBs are robust during lead-free assembly.
My company focuses on PWB reliability. We measure the robustness of
PWBs by testing with very specialized coupons (IST). Many of the recent
studies we have been doing have been to determine if the interconnect
structures and laminate are robust after lead-free assembly. We have
found that lead-free assembly typically reduces the cycle to failure
"entitlement" of a coupon by 50% or more, if the coupon was well made
with high reliability materials.
Poorly processed coupons, or coupons, with low end materials can expect
reliability to be dramatically reduced with lead-free assembly
temperatures.
I had the pleasure of being able to co-author a microvia paper that was
presented at IPC Work in Los Vegas in October. A part of the paper
addressed the effect of lead-free thermal excursions on coupons that had
marginal microvias. The microvias had minor amount debris and
inclusions between the base of microvia and the capture pad, which was
not visible with the established microsectioning techniques. The during
our investigations, coupons were found to be very sensitive to the
effects of lead-free thermal excursions. Tested "as received" at 150C,
the known discrepant microvia coupons achieved 788 thermal cycles before
failing. Preconditioned 6 X 230C the same coupons achieved 443 cycles.
Precondition 6 X 260C the coupons achieved a mean of 4 thermal cycles
until failure.
The data presented in the paper suggested that microvias needed to have
thermal cycle testing at 190C and that microsectioning techniques needed
to be modified for an effective evaluation of the robustness of PWBs
with microvias. We also noted that PWBs with microvias that would have
been considered acceptable after exposure to tin-lead temperatures
(230C), might catastrophically fail during lead-free (260C) thermal
excursions. Those last 30-degrees do a lot of damage.
I believe the effect of lead-free processing on the bare PWBs has not
been addressed adequately. Based on the RoHS seminars I have attended,
it appears the many of the assemble concerns; the components, materials
and methods used in assembly have addressed the effects of lead-free.
We, for example, have been contracted to do a fair bit of laminate
studies to characterize the effect of lead-free processing temperatures
on various laminates, but few OEMs, CM or FAB houses are specifying
routine lead-free reliability studies or established minimum requirement
for product acceptance, based on lead-free preconditioning.
In the good ol' tin-lead days (now) copper quality was the primary
influence in PWB reliability. With the advent of RoHS copper quality
and material quality are beginning to play equal roles in influencing
PWB reliability.
In high-end application, small holes, thick boards, material is becoming
the dominant factor.
Then there delamination...
I believe the effect of lead-free assembly on the bare PWBs is not well
understood nor is there a standard established to say a good PWB meets
some minimum requirement. I do believe that delamination will turn out
to be one of the major concerns. Delaminated PWBs do not automatically
fail during thermal cycle testing. We have found many instances where
delamination artificially extends thermal cycles to failure. If you are
not very diligent in you testing and failure analysis, it is possible to
have delaminated coupons pass thermal cycles requirements without a
failure. To address that weakness we have developed coupons that allow
us to test for delamination electronically. We have developed a method
to determine if coupons have significant delamination after thermal
excursions to lead-free temperatures. It is conceivable to process a
number of our coupons in a lead-free line (oven, wave, etc) and
determine if there was significant delamination.
I believe that you can expect delamination of the PWB to play a more
dominant role in lead-free processing. I have also noted the failure
modes of lead-free stressed PCBs have shifted, to some degree, from
barrel cracks (due to metal fatigue), to internal interconnection and
knee-crack type failures.
I am sure this response is too long for most to bother to read but there
is a lot to say on this subject. Believe it or not I have tried to be
succinct...I didn't even mention ENIG!
Sincerely,
Paul Reid
Program Coordinator
PWB Interconnect Solutions Inc.
(IST Testing)
Tel: 613-596-4244 Ext. 229
Fax: 613-596-2200
[log in to unmask]
-----Original Message-----
From: Tempea, Ioan [mailto:[log in to unmask]]
Sent: Friday, December 09, 2005 4:20 PM
To: [log in to unmask]
Subject: [TN] Qualification of the Pb-free capable manufacturers
Hi Technos,
IPC-1710A does not have anything specific for lead-free capability (or
at least I missed it). How do you qualify your PCB fabs?
Thanks,
Ioan
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