Hello Colleagues,

I recently received a question for which I need an interpretation of IPC 
6012.

The background:
A product with a HDI design specified 3/8 oz (12um) outerlayer (external) 
copper foil on the drawing.  We think the PCB supplier started with foil 
that was within the 6012 specified foil thickness (+/- 10%), but the 
supplier next performs a back etch process on the copper prior to laser 
drilling.  The etch process reduced the foil thickness to ~4 um.  The PCB 
supplier then plated copper up to our specification limits, which resulted 
in a total copper thickness that meets the end copper requirement per 
6012.  I did not find any reference in 6016 to copper foil/layer 
thickness, so I am still referring to 6012 table 3-8 for this topic.

The questions:
1) Does the second column in 6012 table 3-8 ("Absolute Cu Min.") apply 
only to the starting foil thickness, or does it apply to the thickness of 
the copper foil layer in the end PCB (visible in cross-section)?
2) If this back etch process is standard prior to laser drilling, how can 
I verify the original foil thickness is within specification?

Thanks and best regards,

Brian Madsen

Continental Automotive

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