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From:
Wayne Thayer - EXT <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Wayne Thayer - EXT <[log in to unmask]>
Date:
Mon, 14 Nov 2016 20:06:34 +0000
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Moisture is unlikely culprit. Usually moisture causes delamination of some kind--typically under large pads. If that happens and a via is in the pad, the via USUALLY acts as a reinforcement preventing delam in its little area.



I betcha that if you post some pix of what the via walls look like, we'll be able to suggest a short list of which tree you should be barking up. Getting a USB camera or other rig to take those pix is cheap. Lots of people just aim a cell phone camera through the eyepiece of a microscope. Even 50X mag is usually good enough to get an idea for what is going on.



Do you have access to anyone with great hand skills? For most boards it is possible to dis-assemble the layers and see where things went wrong. But you need a decent stereo microscope and the hand skills.



Wayne



-----Original Message-----

From: TechNet [mailto:[log in to unmask]] On Behalf Of Steve Golemme

Sent: Monday, November 14, 2016 11:58 AM

To: [log in to unmask]

Subject: Re: [TN] Fixing Cracked via holes



Thanks everyone.



I'd like to summarize everything said, and answer all the questions in ways that don't reveal confidential info.



I'm looking at moisture ingress opportunities and moisture control prior to assembly since it's non-destructive and could provide a smoking (water)gun.



Wayne Thayer:



Drop a wire in the via and solder on ring or nearby pad. Not sure about reliability unless it’s a 2 layer board.





Scott Decker:



Drop a wire if it’s the same via. And wire up connections to internal planes.





Steven Kelly:



Whether or not PCBAs were pre-baked before SMT could have a factor.





jkoo@:



Need to find root cause.





Jose Rios:



isolated thin areas of plating that didn't hold up to thermal stress (requires DPA to diagnose)



plated copper properties (low elongation, less likely IMO)



it could mean there is partial plating voids (as plated)



Note sometimes ICT covers nets that aren't part of the bare board electrical test (IPC bare board net listing)





Questions:



Pre-bake? – investigating pre-assembly as well as shipping and storage conditions



TH or microvias?: TH



Two layer? multi layer (more than 2) rigid flex.



Thanks Again,



Stephen Golemme

Manufacturing Engineer, Makani

650-214-5647

solveforx.com/makani



On Fri, Nov 11, 2016 at 12:17 PM, Jose A Rios <[log in to unmask]> wrote:



> Possible root causes (seen 2 of the 3 in actuality); It could be 

> isolated thin areas of plating that didn't hold up to thermal stress 

> (requires DPA to diagnose), or plated copper properties (low 

> elongation, less likely IMO). I've also seen where ICT covers nets 

> that aren't part of the bare board electrical test (IPC bare board net 

> listing). In the case of the latter, it could mean there is partial 

> plating voids (as plated), not induced by thermal stress. I've seen 

> this (rare), on 30:1 aspect ratio boards with 10-12 mil drilled holes 

> with a 60K or so hole count so 1 hole out of hundreds of thousands in 

> a lot, can have a single partial void thats hard to detect.

>

> Sent from my iPhone

>

> > On Nov 11, 2016, at 2:14 PM, Steve Golemme <[log in to unmask]>

> wrote:

> >

> > We have a couple of expensive first time build boards that failed 

> > ICT in assembly due to what appears to be cracked vias. Adding 

> > solder to the via hole seems to fix the issue.

> >

> > What are folks thoughts on plugging vias with solder to fix them? 

> > What risks are we taking by adding solder vs another technique? 

> > Could folks recommend alternate repair techniques that may provide 

> > better thermal/vibration?

> >

> > Also, I'd love to hear up alternate possible root causes? This 

> > problem screams insufficient plating during fab or thermal shock 

> > during assembly, but I'd like to investigate all possible failure 

> > modes. This is a multi-layer double sided SMT + wave rigid-flex PCB 

> > and the cracking is on the rigid portion.

> >

> > Thanks,

> > Stephen Golemme

> > Manufacturing Engineer, Makani

> > 650-214-5647

> > solveforx.com/makani

>


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