While all that is said below is true, it goes further than the question
which was simply how to *specify* the metric equivalent of the oz./sq.ft.
designation and the answer is as shown in IPC 6012 (which also includes
the min, max, and ref thicknesses). No one is saying that 1 oz always
means you get 35 microns; only that to convert a specification from the 1
oz./sq. ft. requirement, substitute 35 microns for the requirement as the
designation for the starting foil thickness. This original question was
not about how to figure out what the thickness of the copper is or is
likely to be; only how to call it out on a print.
notice that I top posted and clipped ..
Valerie
David Greig <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
11/05/2004 02:18 PM
Please respond to TechNet E-Mail Forum; Please respond to David Greig
To: [log in to unmask]
cc:
Subject: Re: [TN] copper weight
I would have to agree with Werner, and add that ED Cu foil from the
material
suppliers tends to be at the lower end of the tolerance range - if the
process control is accurate enough then why would they want to supply more
than the least amount of expensive copper. Never come across anything at
the
higher end yet, but maybe one day...
Other considerations are the removal of copper from the inner layer foils
that are a natural and necessary result of the fabrication processes.
Usually best to ask your target fabs for the expected final weight, and
they
should be sentient of this for impedance controlled jobs.
At least you can have some control over the finished outer layer copper
weights by forcing you fabricator to supply to your tolerance requirements
on this.
Best Regards
David Greig
-----Original Message-----
Hi Jeffrey and all,
Things are not quite as simple as has been portraied--the density of ED
copper is not a fixed number, it varies depending on the plating details
of
a given batch.
This is the reasopn ED Cu foil is given as a weight per area, not as a
fixed
thickness. The situation is further muddies, because many of these foils
have a variety of adhesion "tooth" geometries.
Thus, to say "copper weights in oz - 1/4, 3/8, 1/2, 1.0, 2.0, 3.0, 4.0 are
as follows in microns - 9, 12, 18, 35, 71, 107, 142" is not correct. The
micron numbers are only the metric equivalents of the mil numbers, BUT the
mil-thicknesses are only NOMINAL values with significant +/- ranges.
Regards,
Werner Engelmaier
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