David,
I did extensive testing of ISR with a customer trying to increase these
values.  When SEM/EDX was done on test coupons which passed SIR with flying
colors vs. those failing SIR a cohesive pattern of chrome and other metals
was found on the laminate surface of those which failed SIR testing.  This
indicated that the chromate conversion coating on the copper cladding had
embedded in the laminate and was not removed by alkaline etchant.  Product
which passed SIR had this pattern of chrome broken up sufficiently.
SIR testing measures surface conductivity and detected this metallic pattern.
 Chrome would not be detected by ionic testing because this test removes
conductive material from the surface, then measures the conductivity of the
solution.
If you want to discuss this more, call me.  You have my card.

Dave Sullivan
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