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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