The difference between inner and outer layer Cu resistance is easily
explained by the fact that the outer layers receive additive
electro-plating on top of the 1oz Cu foil. Inner layers are not normally
electro-plated. If you are looking for a more balanced Cu stackup you'll
probably need to go with 2oz inner layers and 1oz + plating outer
layers.
And, No... Never had to deal with this requirement before.
Denis Lefebvre, CID+
Sr. PCB Designer
Finisar Corporation
(408)542-3832
-----Original Message-----
From: DesignerCouncil [mailto:[log in to unmask]] On Behalf Of Gary
Koven
Sent: Thursday, January 22, 2009 7:25 AM
To: [log in to unmask]
Subject: [DC] Controlling DC trace resistance
We designed a board which has a number of critical connections where
"power delivery" and voltage drops can be deal breakers. The board is
FR-4 4101/21, 0.062in thick, 4 layer, 1oz Cu outer layers, 1oz Cu inner
layers.
We were looking to control the DC trace resistance on the board by
controlling the copper thickness.
In order to verify that the copper thickness was correct on all layers,
we created 4 dogbone-type test coupons, one for each layer, 2.8in long
and terminating in two plated-through holes on each end. We specified,
on drawing, a resistance of 70 milliohm nominal, 100 milliohm maximum,
measured between endpoints of the coupon using a 4-wire or Kelvin sense
ohmmeter. We calculated the expected nominal resistance using the
IPC-2221 tables and curves.
During the build, we got a phone call from the fabricator about this
requirement, and we thought we had worked it out. When the boards
arrived, we measured 40 milliohm on the outer layers and 100 milliohm on
the inner layers. This wasn't what we expected.
When we called the fabricator to find out what measurements they got, we
didn't get a straight answer. He started talking about dielectrics and
controlled impedance. We gave up, because we're going to use the boards
anyway thanks to some lucky breaks in the way the software will work.
It was slightly disturbing to find that when we specified a critical
parameter on drawing, the fabricator didn't check this parameter or log
the results before delivering product. DC resistance and controlled
impedance ain't the same animal.
Has anyone dealt with this requirement before and found a methodology to
better communicate it to a fabricator?
TIA and Best Regards,
===========================
Gary M. Koven, C.I.D.
Dynazign, Inc.
Charlotte, NC, USA
Veteran of the Marsh School
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