Dear Paul,
Thanks a lot for the feedback. These were valuable inputs. Can you also give some feedback on a comparison of Plasma desmear to the chemical desmear. Can we again have a rating including plasma also.
Rgds
Pradeep
Paul Reid wrote at Thu Aug 07 23:45:14 IST 2008:
>Hi Pradeep,
>
>We specifically test the robustness of interconnects as an inherent part
>of our reliability testing of PWBs (using representative IST coupons).
>Our experience in three point contact is that it is less reliable than a
>moderate desmear or negative etch back. Our experience is base on
>testing interconnections on hundreds of thousand of coupons.
>
>Aggressive desmear can cause two failure modes.
>
>1. Most common, with three point contact, is foil cracks at a distance
>of about 1/2 the thickness of the copper foil in from the edge of the
>barrel of the hole. There is a bending moment that occurs at that
>location and it appears that three point contact focuses stress there.
>This is most common on layer 2 or N-1. Lead free thermal excursions
>greatly increase this risk.
>
>2. With aggressive desmear (without three point contact), interconnect
>separation (post sep) is most common. This is cause by the roughness of
>the barrel of the hole. Aggressive desmear produces a rough hole wall
>which makes the PTH rigid much like a corrugated can. The hole cannot
>flex during thermal cycles so the internal interconnections are work
>harder. The result is interconnect separation.
>
>I expect a 20% reduction in reliability with three point contact or
>aggressive desmear.
>
>The ranking of internal interconnect robustness: A flat interconnection,
>produced by moderate desmear and a good microetch, is most reliable.
>Negative etch back is rank number two followed by three point contact.
>
>A simple test can be run using reliability testing of representative
>coupons, to confirm or refute these contentions with your design and
>construction.
>
>
>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
>[log in to unmask] <mailto:[log in to unmask]>
>
>
>-----Original Message-----
>From: TechNet [mailto:[log in to unmask]] On Behalf Of mp3
>Sent: Thursday, August 07, 2008 9:44 AM
>To: [log in to unmask]
>Subject: [TN] Smear and rework failure
>
>Dear technetters,
>
>We are a military and space grade multilayer PCB manufacturing company.
>We have been following a comparatively longer desmear for High Tg
>materials and also acid etchback to achieve a 3 point contact.
>
>As a part of an analysis, we moved a 14 layer board without desmear /
>etchback directly to electroless. We found a couple of opens in BBT
>immediately after etching. Then we did an HAL cycle to give a sort of
>thermal shock. We did not find any increase in the number of
>discontinuities. This was contrary to our expectations. We believe that
>desmear is critical for the reliability. We also found during
>microsection that copper layers to be quite shiny but we found a spec in
>the inner layer area too. A few queries we have are
>
>1. Will the smear be uniformly formed around the hole.
>2. Will the presence of smear affect the board while rework
>
>One of our customers have observed opens in reworked boards at the same
>points where it has been reworked. On going thru the traveller card of
>the lot, we find 1 PCB to have been rejected at microsection stage for
>smear. This was a high reliability board and we have done microsection
>analysis on the coupons of every panel and all other panels were found
>to have passed the requirements of microsection analysis. The customer
>feels that the reworked panels have failed due to the presence of smear.
>He is insisting on horizontal microsection analysis. The boards are hand
>soldered. Can technetters help us in analysing this problem further.
>
>The main question is will a presence of smear result in a failure during
>rework. It passes a thermal stress and even 1st or second rework too.
>
>Looking forward for your valuable inputs
>
>Rgds
>
>Pradeep Menon
>
>
>
>-------------------------------
>Micropack Ltd, Bangalore, India
>
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-------------------------------
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