Ed

I don't interpret the values for starting thickness from Table 3-8 in IPC
6012A as "required" or "minimum" in anyway.  I interpret that column as only
the nominal foil thickness.  You'd find it very difficult to find a board
fabricator who could comply with those as minimum thickness requirements for
the foil underneath the plating.  Here's why.

The copper foil specification itself (IPC-4562) allows for a 10% variation
in electrodeposited copper foil area weight.  So even though 0.5oz foil has
a nominal thickness of 0.67 mils, it vary between 0.74 and 0.60 mils as an
incoming raw material.  In reality, it's very difficult to find anyone who
makes 0.5 oz foil that's at the nominal value.  Most foil fabricators
intentionally skimp on the copper thickness to minimize costs.  We expect
the foil receive to be below the relevant nominal thickness.  Processes such
as preclean scrubs and microetches further reduce foil thickness.  That's
why table 3-7 calls for a minimum finished 0.5 oz foil thickness (on IL) is
only 0.47 mils.  It's expected that the processing of the boards will
further reduce the copper foil thickness on both inner and outer layers well
below the nominal thickness.  It's the minimum finished copper thicknesses
from the right hand columns of Tables 3-7 and 3-8 that are relevant.  If you
really wanted a fabricator to comply with a minimum starting thickness of
0.67 mils for 0.5 oz copper, they'd have to use a thicker foil to achieve
it.

The good news is the foil thickness is relatively irrelevant.  The integrity
of the copper plating is what really determines the reliability here.
That's covered by the minimum  plated conductor and the through hole plating
thickness requirements.


> ----------
> From:         Jack Crawford[SMTP:[log in to unmask]]
> Reply To:     TechNet E-Mail Forum.
> Sent:         Wednesday, September 19, 2001 9:52 AM
> To:   [log in to unmask]
> Subject:      Re: [TN] External conductor starting thickness prior to
> plating???
>
> >>> "May William D (Dean) CNIN" <[log in to unmask]> 09/19/01 11:17AM
> >>>
>
> Reading from IPC-6012A, I noticed the required starting thickness of an
> external conductor prior to plating for 1/8 oz base copper foil is 0.00020
> inches and the starting thickness for 1/2 oz base copper is 0.00067
> inches.
> Does anyone have an idea on how minimum starting thickness is determined
> for
> each weight of base copper?  If minimum base thickness is exceeded by
> chemical milling and micro-etching processes, will it compromise board
> reliability assuming that there is sufficient electroplated copper to meet
> minimum total finished copper thickness? What possible problems may arise
> if
> these base minimums are exceeded?
>
> Thanks for your consideration!
>
> Ed McAtee
> NAVSURFWARCEN
> PHONE 812-854-3892
> FAX 812-854-1174
>
>
>
>
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