Hi Steve,

+12V & -12V is 24 volts with a little prepreg layer in between.  Sounds like trouble.

------------------------
Ed Hare
[log in to unmask]




> On Apr 8, 2016, at 9:44 AM, Steve Gregory <[log in to unmask]> wrote:
> 
> Hi again Dave,
> 
> So the PCB is a .090" thick, 8-layer board, the stack-up is this:
> 
> Soldermask
> Layer 1
> ----------
> Ground
> ----------Pre-preg
> Layer 3
> ----------
> +12V
> ----------Pre-preg
> -12V
> ----------
> Layer 6
> ----------Pre-preg
> Ground
> ----------
> Layer 8
> Soldermask
> 
> Steve
> 
> 
> On Fri, Apr 8, 2016 at 10:18 AM, David Hillman <
> [log in to unmask]> wrote:
> 
>> Hi Steve - yes, you have enough physical details that a lab that routinely
>> conducts FA work on electronic assemblies (there are several who are part
>> of Technet) would have a reasonable chance of finding the root cause. At a
>> very minimum with the details you have on this issue, the laboratory should
>> be able to rule out possible root causes even if they can't find a specific
>> root cause. I didn't see any dendritic shorting - is the belief that the
>> shorting is internal to the pwb?
>> 
>> Dave Hillman
>> Rockwell Collins
>> [log in to unmask]
>> 
>> 
>> On Fri, Apr 8, 2016 at 11:00 AM, Steve Gregory <[log in to unmask]> wrote:
>> 
>>> 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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>> 
> 
> 
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