There are quite a few possible causes. I will try to list some, others will also.
1. Cathodic-Anodic Filament (CAF) caused by a combination of power-to-ground voltage differential coupled with moisture/organic residues inside the PWB.
2. Dendritic growth between two conductors caused by power-to-ground voltage differential coupled with moisture/organic residues trapped on the surface of the PWB. (Look under components with good X-ray)
3. Conductive FOD such as lead clippings, solder fines, etc, coming to rest between two conductors. (Inspect with good X-ray).
4. Water retention inside a component.
5. Connector or component insulator that was laser-cut instead of stamped/milled. The carbolized edges allow current to pass, and the more current that passes, the worse the carbolization gets, and the worse the high resistance short becomes. Eventually it is no longer a high resistance short and catastrophic failure occurs.


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Pradeep Menon
Sent: Tuesday, March 27, 2012 6:33 AM
To: [log in to unmask]
Subject: [TN] Short after assembly

We had supplied a PCb to one of our high reliability customers 2-3 years back. Now we understand that in one of the PCB they have observed a low resistance short after thermal cycling. The bare PCB was BBT tested at our end and was also cross verified as a bare board by the customer. They had populated the board and the tests were all found compliant. After some time they say they observed a low resistance short. We are a bit perplexed on this. Can anyone share what can be the possible reasons for the same. We feel it may be even something to do with assembly as well. Can you pls provide some inputs

Rgds

Pradeep

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