Hi Li Yi,
We perform reliability testing on representative coupons from back drilled printed wire boards frequently. If the back drilling is performed correctly, it will significantly extend the reliability of the board. It is rare that reliability failures occur in PTHs that have back drill. The only failure I remember on a back drilled hole was when the drill touched the layer that was part of the test circuit. The coupon passed electrical test but failed almost immediately when thermal cycled.
It appears that by removing the barrel and connection to the surface pad and some internal interconnections, the PTH is stress relieved. When we test coupons that have PTHs with and without back drilling in the same coupon, failures occur first on the holes that are not back drilled. Base on testing we have done hole size does not appear to be influential on back drill reliability.
If back drilling is performed to the correct depth, and with good registration, then I expect that the problem you are seeing is coincidental to the back drilling.
I would not have non-functional pads present. Generally that does not improve reliability based on thermal cycle testing.
Can you post a microsection of the failure mode?
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 ??
Sent: July 1, 2010 12:01 AM
To: [log in to unmask]
Subject: [TN] Single PCB backdrilling
Dear Technos,
Anyone use backdrilling tech in a single pcb? We backdrilled a lot of backplanes, but none of them should survive a second reflow process and maybe a wave process.
Here is the questions:
Does the high temperature stress affect the backdrill holes reliability? Two reflow processes and a wave process, Consider the PTHs' drilled edge are rough, maybe they will easy to be damaged by thermal stress.
Will the the rework process of the connector damage the backdrilled holes?
Anything we can do? No HALS? More inner layer connection? Other suggestions?
Thank you so much.
B.R.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Li Yi
H3C R&D DFM Engineer312 Room, Oriental Electronics Bldg., Shangdi Information
Industry Base, Haidian District, Beijing, PRC(100085)
Tel: 8610 82774692
Fax:8610 82774214
www.H3C.com
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