Cliff,

I understand your point, and basically agree. However, I believe the 
original intent focused on normal copper processing, such as surface 
prep prior to applying an imaging resist. I do not believe that it 
adequately addresses the needs for sequential blind via construction and 
any associated fine line technology.  This is probably something we 
should be looking at in the Fall meeting.

Regards,

Gary

On 7/26/2010 5:50 PM, Maddox, Clifford R wrote:
> Brian,
>
> Unless otherwise specified I do not believe that copper foils may be reduced by thinning or other special processing beyond the limits established by the specification.
>
> IPC-6012C Section 3.6.2.13 defines "Minimum External Conductors Thickness" as "a + b - c"  where a = "Absolute copper foil minimum", c = "Minimum copper plating thickness" and c = "Maximum variable processing allowance".  Therefore, external conductors are comprised of both plating and foil less some MAXIMUM processing allowance.
>
> This is repeated within Table 3-12 where a "Maximum variable processing allowance reduction" is specified for all listed copper foil weights. These variables are defined as representing one standard plus one rework cycle for weights>/= half-ounce and are reduced to a single standard loss for foils below half-ounce. As I understand it these variables represent the maximum amount of copper foil that may be removed during processing and the fabricator must preserve the original foil weight within these specified limits. Should a processor remove greater than these maximum values from the foil, this would NOT be in accordance with the specification.
>
> Also note that footnotes "2" and "3" to Table 3-12 appear to be reversed.
>
> Best regards,
>
>
> Cliff
>
>
>
> -----Original Message-----
> From: IPC-600-6012 [mailto:[log in to unmask]] On Behalf Of Brian D. Madsen
> 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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