CAF is hard to detect using X-ray.  cross section is also a hit and  
missing game.  Unless you have thermal imaging data.
not easy.
    jk
On Apr 11, 2016, at 10:04 AM, Stadem, Richard D. wrote:

> If you have determined that the via in the picture is the culprit,  
> or is very likely the issue, I would start by having a set of  
> really good X-rays performed with a machine such as an Yxlon.
> Iff those X-rays prove the internal short condition, then you have  
> justification for microsectional analysis by a testing house that  
> has experience in this.
> Concurrently, electrical analysis using PWB layer artwork should be  
> able to determine if there is +12V and -12V both in the same area  
> with close approximation to each other. I am assuming the via in  
> the picture is either one or the other.
> If that is the case, it may be a situation of CAF internal to the  
> PWB, which is the only scenario that I can see that would develop  
> over time. But that is speculation at this point.
> To answer your question, the more samples you can provide for the  
> initial X-ray analysis, the better. Two may be enough, but perhaps  
> 5 or 6 will provide more data; at least enough to justify  
> sacrificing one for sectioning. Try to find one that displays gross  
> shorting.
> An initial X-ray survey should cost less, perhaps 2-3K.
> If the X-Rays provide justification, then microsectioning would go  
> to another $3K, depending on the number of cuts/mounts required.
> A good analysis house can give you a good estimate if you were to  
> forward the pictures to them and explain what you are experiencing.  
> They may have other ideas for confirming that the via is the  
> culprit, and in determining the cause. Be prepared to provide the  
> layer artwork. I would get approval from the customer first; there  
> may be security or ITAR concerns.
> And I am assuming the large probe mark in the via is from  
> troubleshooting to locate the short. The problem is, the short  
> could be anywhere. How was it determined that that particular via  
> is the source?
> dean
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Steve Gregory
> Sent: Friday, April 08, 2016 11:00 AM
> To: [log in to unmask]
> Subject: [TN] Recommendations for Failure Analysis / Root Cause of  
> PCBA Failure...
>
> Good Morning All,
>
> We have an assembly that we build here for a customer that we built  
> for some time, over 2,000 assemblies. There have been some failures  
> that our customer has had field returns with that are beginning to  
> escalate. There have been 12 field failures that we know of. The  
> failures are avionic assemblies that have been in the field ranging  
> from 111 days to 472 days.
> The actual failures are internal +12V to -12V shorts and we need to  
> know why. I have two failed assemblies here. Here are a couple of  
> external photos of a via that shows the short:
>
> http://stevezeva.homestead.com/Internal_Short.jpg
>
> http://stevezeva.homestead.com/Internal_Short_Close.jpg
>
> So my question is, can a lab take these boards and examine them to  
> determine what the root cause actually is from the two assemblies  
> that I have, or does there need to be more samples? What is the  
> range we should expect to pay for this type of analysis?
>
> We're trying to work with our PCB vendor on this issue, but I have  
> also been asked by my boss to pursue this with a third party.
>
> Thanks!
>
> Steve
>
>
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