Matthew,

I don't have any specific design rules.  First, I check with the fab house to see if they have a sheet resistance table or chart based on their capabilities.  Then, I look to see what resistance is acceptable and then caculate what will work.  Or have the engineer do it.  I use a table for the resistivity (p greek symbol "rho") of various materials in both ohms-centimeters or CM-ohms/ft  out of the "Boylestad, Introduction to Circuit Analysis, 5th edition" and the chapter on resistance.  

Basically, R=p(l/A) is used for round wire. For thin-film or sheet resitance, Rs=p/d (=ohms) where d=depth of film. Get this from the fab house.

Then the overall resistance R=Rs(l/w)  where l=length and w=width of the area you want to determine the resistance.  

FYI: p for Carbon is:   21,000.0 if using an inch based system
                                3500 x 10-6 ohm-centimeters if using the metric system

Hope this helps.

Cherie Litson, CID+
PCB Designer/Librarian
Sonosite, Inc.
425-951-1306 wk
206-619-8844 cell
425-952-1201 fax


-----Original Message-----
From: DesignerCouncil [mailto:[log in to unmask]]On Behalf Of
Matthew Lamkin
Sent: Tuesday, July 13, 2004 2:25 AM
To: [log in to unmask]
Subject: [DC] Using carbon ink on pads?


Hello, I am searching for some design rules etc for using carbon inks on
pads?

Is there an IPC standard that relates to PCB design for Carbon pads etc?

Thankyou,
Matthew Lamkin.

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