The first clue I would look for is what metal is showing on the surface
when you turn over your tape and look at it. You say your tape is pulling 
off the tin and exposing bare copper on the pcb, but you need to determine
exactly what metallic junction is being broken. If you see tin on the
underside of
the tape, you have a failed tin/copper bond. If you see copper under the
tape you have a failed copper/copper bond...which is something controlled
by the board fabricators process (ie. electroless copper/flash copper
adhesion, flash copper/pattern plate copper adhesion).

Bill Parlin
Parlin Industries, Inc.
www.parlin.com

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