Hi Pradeep,
I am sorry!
I do not have insight on the differences between plasma and chemical
desmear from a reliability point of view. In my mind the difference in
reliability between the two methods is negligible. Both work well when
they are well controlled and maintained. Poorly controlled both can
cause early and catastrophic (highly accelerated) interconnect failures.
Hole wall quality, types of hole prep and similar variables are a
tertiary level influence in PCB robustness most of the time. In order to
enunciate differences between hole preparation methods, copper quality
and material integrity must be high or they will dominate data and mask
the subtle differences between two well controlled desmear processes.
Have a good weekend.
Paul Reid
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of mp3
Sent: Friday, August 08, 2008 3:20 AM
To: [log in to unmask]
Subject: Re: [TN] Smear and rework failure
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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