Same inputs as before. We can't find the short for you. Time to get out the precision ohmmeter and start probing.
Lay down a sheet of white shelving paper. Pick up the assembly, turn it over, and shake vigorously over the paper. See what falls out. Then re-test. 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Pradeep Menon
Sent: Thursday, March 29, 2012 6:49 AM
To: [log in to unmask]
Subject: [TN] Short after assembly
Importance: High

Dear all,

Thanks for the inputs on my last query. In the meanwhile we have got some more details on the same issue which we would like to share.

This is a 6 layer board manufactured with Nelco 4000-6 (Tg175deg) material with laminate construction. We had fabricated this board 2-3 years back. We had done the isolation test in our BBT at 10Mohm threshold. The board was assembled and was working perfectly.

The customer does a continuity and isolation test after assembly. Then they have done a  thermal cycling for the package in powered OFF condition. Ie 3 cycles at 13 deg C and 60 deg C with 45 minutes duration at both temperatures. Then with the powered ON condition 2 cycles at 13 degC and 60deg C with 45 minutes duration at both temperatures. Functional check and performance was confirmed after that. This is the standard procedure.

But for this package, they have carried out an additional thermal cycling over and above the normal thermal cycling procedure indicated above. This time they have done a  thermal cycling for the package in powered OFF condition. Ie 3 cycles at 13 deg C and 60 deg C now with 100 minutes duration at both temperatures. Then with the powered ON condition 2 cycles at 13 degC and 60deg C with 45 minutes duration at both temperatures. Functional check and performance was verified and this time they found a short between  2 points. And this is not a low resistance short as per the customer, it is a 0ohm dead short.

Now we have been asked to analyse the same. Can you pls give some inputs for this

Rgds

Pradeep

From: Pradeep Menon
Sent: Tuesday, March 27, 2012 5:03 PM
To: [log in to unmask]
Subject: 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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