All:

Our company recently received some boards that, after thermal cycling,
started showing barrel cracks.  The boards were ordered and inspected to
IPC-6012 (class 3) with one after-thermal stress coupon on each panel
evaluated.  According to the manufacturer they saw no evidence of cracks in
these coupons.

One thing we discovered was that the supplier used a full pad stack in the
coupon but the boards themselves had all non-functional pads removed.
Typically there were three connections per hole. Layer 1 and 10 had pads
plus one internal connection per hole - either signal or plane layer.

I think this is one of those gray areas that are not fully defined in
IPC-2221, 6012.  2221 states that the coupon should represent the board
but...

The supplier is using the coupon to detect smear, (etchback if required),
annular ring, foil thickness, laminate voids etc.  If they don't put a full
pad stack they can't evaluate everything.  On the other hand, by putting a
full pad stack they are re-enforcing the barrel of the hole which may very
well hide plating defects such as barrel cracks.

This scenerio brings up several questions.

How are you handling coupon design?  Do you leave it up to the fabricator?
Do you specify to the supplier how they should design the coupons.

If we specified something like 'determine the average number of connections
on a plated through hole and make the coupon look like the average hole' -
how would would the board house evaluate the attributes where no pads exist?
(or should we care - if one pad is good they're all good)  If there was only
one internal pad represented on the coupon is that enough to complete an
evaluation?  Do we ignore annular ring?  I know that IPC-6012 specifies to
use the "F" coupon for a-ring evaluation but the inspection requirement is
somewhat unbelievable - "3.4.2 Annular ring and break out (internal) The F
coupon may be used to assess annular ring...by external visual
inspection..."

I'd really appreciate comments from both the fabricators and OEM's.

Thank you -
Pete

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