Hi Gary ,
We mark on our drawings a simple note :
Total resistance of each line must not exceed xx ohms.
However , we usually design the board with this in mind and we
know in advance what the limits are, so there are fewer surprises this way.
It's normal that the resistance is lower on the outer layers since the
plating adds another once of copper.
If your fabricator can't check DC trace resistance ,then I suggest you find one who can.
Best Regards,
Andre Demers C.I.D.
Senior PCB Designer
cmrsummit Technologies
310-733 boulevard St-Joseph
Gatineau (Québec) J8Y 4B6
Canada
Telephone: 819.205.1185
Toll Free: 877.428.1229
Fax: 514.428.5953
[log in to unmask]
www.cmrsummit.com
-----Original Message-----
From: DesignerCouncil [mailto:[log in to unmask]] On Behalf Of Gary Koven
Sent: Thursday, January 22, 2009 10:25 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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