On your fabrication drawing, place a note stating "certificate of
conformance required" pertaining to your critical items.
John Babcock, C.I.D.
PCB Designer III
www.spraycool.com
-----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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