Brian,

Most of the points the other individuals have made have merit but we believe it does matter and the way to verify compliance is by the supplier providing a copper foil material cert for the material used.  The thicker foils typically have higher peel values and this may be important and matter to you.  We agree it is a common practice for suppliers to perform what you call a "back etch", we call it "copper reduction", in order to start with a copper foil thickness that meets the drawing requirements and then reduce the thickness to make the product more manufacturable.  The end result is the finished copper thickness must meet the requirements of Table 3-12 per IPC-6012C (It was Table 3-8 in IPC-6012B).    

Best regards,

Scott A. Bowles
Director of Engineering and Quality
Hallmark Circuits, Inc.
13500 Danielson Street
Poway, CA 92064

858 513-2200 Office x8848
858 513-2233 Fax
858 437-7827 Cell


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-----Original Message-----
From: Brian D. Madsen [mailto:[log in to unmask]]
Sent: Friday, July 23, 2010 2:01 PM
To: [log in to unmask]
Subject: [IPC-600-6012] Fw: Outerlayer copper foil thickness for HDI designs

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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