Hi Pradeep,
As you know it is imprudent to comment on specific materials so what I am writing is an overview of things that influence materials as you described.
My experience is that what you have described is not unusual. Some materials have reduced reliability after three thermal excursions to 260°C. In this case you are increasing the temperature to 288°C although it is only for 10 seconds. If you have a sequential lamination process you may be at the materials limit particularly if this is for a lead-free application.
I believe that the number of lamination cycles can degrade the dielectric. I doubt that plating, hole prep or drilling will be significantly influential. HASL or fused surface finishes will degrade material.
The expected failure modes are adhesive delamination or cohesive failure for the dielectric and barrel cracks or knee cracks in copper. You may not find the copper cracks as they will be small and randomly distributed in the sample particularly if you are reviewing a few PTHs.
If you are going to IPC I will be proposing a test method called Cyclic T260 that would help shed light on this problem. It is a TMA based method that measures the degradation of material after thermal cycles to simulate assembly and rework. I say degradation of material but it would be better to describe this failure as the degradation of material that has been through fabrication including the influence of construction, fabrication and design. You see I find that the copper quality, material robustness, design influences and fabrication variables (like lamination, oxide coating, surface finish) conspire to provide a given level of reliability. I will be presenting this at the C-10C PTV Reliability-Accelerated Test Methods Task Group on Wednesday April 7th at 10:15 am. I hope that those available will join us at the meeting and support investigating this test method. Anyone interested in PWB reliability is encouraged to join us at the Wednesday Meeting. I am looking forward to critical comment and hopefully support.
I do not know if there is a conference call capability planned for that meeting. You could contact IPC is see if that could be arranged. Some times the size of the room makes conference calling problematic.
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 Pradeep Menon
Sent: April 2, 2010 2:11 AM
To: [log in to unmask]
Subject: [TN] Outer layer separation
Importance: High
Dear all,
Wanted some inputs on a problem which we are facing currently
We observe a separation, rather tendency for separation, on 2.5mm thick
MLB boards when subjected to 3X 288 deg 10 sec thermal stress. This is
mostly found only on one side ie in one of the outerlayers, and very few
on both outer layers. The inner layers are intact.
We use N4000-6 as well as 4000-13EP laminates for fabricating PCBs and we
use laminate construction for the stack up. We do Plasma desmear followed
by a run thru permanganate too. We tried various parameters in drilling as
well as reviewed all the basic electroless / electroplating parameters ,
and all were found as desired.
We use 1-1.2 microns thick electroless copper followed by a flash
electrolplated copper of about 12 microns before pattern printing. The
board is then pattern plated to achieve a minimum PTH copper of 30
microns.
Looking forward fro ur inputs
Rgds
Pradeep
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