Chris,
This has a bit unclear through the years. When working in a
manufacturing facility, I noticed most specified a finished
thickness. Unknown to them, that wasn't what most finished at.
I address that in my book.
Many don't know or realize how the hole plating rules the external
plating. As you stated in a later email, plating is normally 1oz (.0014")
Therefore, you will never have a 2oz external either 1.5oz (.0021")
or 2oz, and up from there. There are some manufacturers that have
many different foil thicknesses, but using that limits the designer/board
to a smaller amount of manufacturers.
I now specify external and plating, so when external values become
critical, they are easily changed.
Also...I have found some manufacturers are plating smaller values and
using odd foil thickness. It takes a lot of guess work out, with
specifying this and is fairly simple.
Chris Robertson
[log in to unmask] <mailto:[log in to unmask]>
Author "PCB Designer's Reference"
to be released in Summer 2002
Greetings Guru's-
I've got a double-sided board. Some traces on the board run 15A
continuous.
Due to space constraints, I have to size the traces for 2oz. copper.
Historically, on the drawing notes I've specified 2oz FINISHED copper
weight and not called out a starting weight (also specified
.001" min/avg.
plating in holes). My thoughts were that I didn't care what the
base copper
was as long as the end product met the callouts. Our good-old supplier
started with 1oz. and plated up to 2oz.
We are re-vamping all our standard notes to try to better
reference IPC and
other specs. Global market, more outsourcing, all that. It
seems I should
specify a starting copper thickness on the drawing, as well as
a finished
copper thickness.
I'm reading IPC-6012 (page 14, table 3-9) and trying to
understand why 2oz.
copper AFTER plating has a min. thickness of 76uM (.003"). Seems like it
ought to be .0038' (0.097uM). Is that because it's a MIN instead of an
average?
What do/would you put on the fab notes for starting and
finished Cu thk.?
Quick responses would be appreciated.
Thanks,
-Chris
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