I can think of a few good reasons for having coupons. The most striking one is that it allows for process/product checks without destroying a perfectly good board. A cross-section of a coupon designed to the same parameters of the circuit board and placed on the panel should closely reflect what a cross-section of live product would look like. Another good application is with measuring controlled impedance. Today's designs make it very difficult to do a good TDR test. Ideally, a 4" to 6" trace with no treeing or branching makes for the best test. These are not always evident on board designs, but a coupon can be designed that way. I hope this helps. Regards, Tom Coyle Field Services Engineer HADCO Corporation