Hi R!

You should be able to see the effects of the noted problem on the cross sections.  Of course, polishing and etching the sections so they can be correctly interpreted is an art form.

Determining assembly reliability beyond cross section evaluation is an expensive proposition.  Tests need to be designed which compress a lifetime into as short a timespan as possible without sacrificing the validity of the extension of the test results to real time.  The most common technique to use for this kind of potential defect is thermal cycling:  Fast thermal cycles over a temperature range much larger than the expected condition the assembly is operating in.  The engineering discipline is HALT design, for Highly Accelerated Life Testing.

OR

You can probably sleep pretty well at night if you take some of the boards and mechanically tear through bunches of vias to convince yourself that the plating to the inner layers is reliable.  A skilled Failure Analysis engineer can discern the malleability of the plating, attachment to inner layers, quality of adhesion between dielectric layers, quality of adhesion from pre-preg to metal, etc.  A very thorough analysis of even a small number of vias will reveal most of the weakpoints possible.

Other "feel good" testing includes good old IPC solder float testing.  Nice to have a dense via array coupon for that.

Good Luck,

Wayne Thayer

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ramakrishnan Saravanan
Sent: Thursday, January 17, 2013 11:16 AM
To: [log in to unmask]
Subject: [TN] reliability of PCBs in wired assembly

Dear all ,

We received few  PCBs from a fabricator in 2007. The PCB coupon's were micro-sectioned and accepted. Then the  PCBs were wired and conformally coated. Now we came to know that the PCBs were from defective batch. The fabricator did not use sweller before permanganate de smear. 

How to access the reliability of wired PCBs. The PCB assembly is working fine. 


regards,

R.Saravanan

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