Dear technetters,

We are a military and space grade multilayer PCB manufacturing company. We have been following a comparatively longer desmear for High Tg materials and also acid etchback to achieve a 3 point contact.

As a part of an analysis, we moved a 14 layer board without desmear / etchback directly to electroless. We found a couple of opens in BBT immediately after etching. Then we did an HAL cycle to give a sort of thermal shock. We did not find any increase in the number of discontinuities. This was contrary to our expectations. We believe that desmear is critical for the reliability. We also found during microsection that copper layers to be quite shiny but we found a spec in the inner layer area too. A few queries we have are 

1. Will the smear be uniformly formed around the hole.
2. Will the presence of smear affect the board while rework

One of our customers have observed opens in reworked boards at the same points where it has been reworked. On going thru the traveller card of the lot, we find 1 PCB to have been rejected at microsection stage for smear. This was a high reliability board and we have done microsection analysis on the coupons of every panel and all other panels were found to have passed the requirements of microsection analysis. The customer feels that the reworked panels have failed due to the presence of smear. He is insisting on horizontal microsection analysis. The boards are hand soldered. Can technetters help us in analysing this problem further. 

The main question is will a presence of smear result in a failure during rework. It passes a thermal stress and even 1st or second rework too. 

Looking forward for your valuable inputs

Rgds

Pradeep Menon



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Micropack Ltd, Bangalore, India

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