To Al Slagle;

Coupons are used to evaluate what you cannot see by examining
the surface of boards.  They are used to verify that the appropriate
weight of copper called for in the drawing was used on specific 
inner layers.  They are used to determine if dielectric thickness
between inner layers meets drawings or specification.  They are used 
to assess how the board will perform during assembly operations
by the thermal stress test (10 seconds/550F).  They are also used
to assess numerous other requirements found in  industry and military 
specifications and drawings.  

All of the above are things that a board manufacturer should verify
prior to shipment of the boards.  The use of coupons enables such
testing to be performed without sacrificing actual boards.  In some
cases where boards are produced one/panel this is the only way.

Additional coupons sent to customers are used as cross checks
of the board manufacturer.  Some are tested by third parties and 
some are only tested if there is a problem during assembly or after
the boards are in use.

In process testing by a board shop can supplement or be used in 
lieu of coupon testing, with proper correlation studies.  However,
there is always going to be the need to see inside a finished hole and
most would much rather sacrifice coupons than boards.

Susan Mansilla
Robisan Laboratory