Hi Phil,

 

I am not surprise when tests on boards and IPC coupons provide different results. The AB coupons are usually a poor reflection of the PWB's construction and attributes and consequently also a poor anticipator of reliability. Although the coupons provide QC insight and demonstrate meeting a contractual requirement that does not always reflect PWB robustness through assembly and in the field.  

 

The AB coupons, for example, frequently do not have the same hole density (grid size) or interconnection structure (blind, buried etc.) as the board. The coupon's very low hole count (dozens of holes) raises doubt about the statistical relevance to the PWB (thousands of holes).  Lead free assembly has given us insight on the effect of design and construction on reliability and now more than ever the coupons need to be physically the same and a statistically valid representation of the PWB. If the coupons are not a good reflection of the PWB attributes, false positive results may occur.

 

That being said we find reliability improvement of representative coupons after a month or two of storage. Unless the storage was compromised, it is odd that the boards degraded significantly in storage. I know this is an unpopular thought but I would expect the first solder float and sections on coupons may have produced a false positive result and the PWB samples are a better reflection of the product.

 

For these reasons there is a growing push for performance testing to become part of PWB acceptance throughout the world. I do not consider the "solder float" test an accurate performance test based on it's checkered history of producing false poistive and false negative results.

 

Best regards,

 

Paul Reid


________________________________

From: TechNet on behalf of Phil Bavaro
Sent: Mon 20/06/2011 1:12 PM
To: [log in to unmask]
Subject: [TN] Using PWBs instead of test coupons to meet IPC-6012 Class 3



I recently came across a situation where PWBs were used in lieu of test
coupons to meet a contractual 3rd party micro-section test requirement
for IPC-6012 Class 3 hardware.  

This decision was made after having the PWBs stored in the stockroom for
two years. 

I was asked to get involved when the PWB "coupon" failed for
Solderability testing by the 3rd party test house.

I checked and the original PWB micro-section report from the fabricator
indicated that the test coupons all passed at the time of manufacture.

So I am left wondering about the requirement that is intended by 6012 to
use test coupons.

I would have thought at the very least, the PTH hole locations of PWB
which would correspond to the test coupon sections would have to have
been identified and approved. 

I can rationalize why the stored board would not meet Solderability
requirements after two years in a stockroom, but I cannot get past the
requirement to use actual test coupons for acceptance testing.

Am I missing something or do we have a choice whether to test the
coupons vs the actual PWB? 




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