Ken, Here's how to calculate the theoretical (read ideal) resistance of a copper trace. The formula for resistance of a material is: R = r(l/a) where R = resistance in ohms r = resistivity (Resistivity (r) of copper at 20 degrees C = 8.15 * 0.083 [(square mils * ohms) divided by inches] = 0.679 [(square mils * ohms) per inch]) l = length in inches a = cross sectional area of conductor (trace) in square mils So: If an internal trace on a circuit board is 0.003 inch wide, is 22 inches long and was etched on 1/2 oz/sqft (0.0007 inch thick) copper clad laminate, then its' resistance is : a = 3 * 0.7 = 2.1 sq_mils l = 22 R = 0.679 sq_mil-ohms per inch ( 22 in/ 2.1 sq_mils ) = 0.679 sq_mil-ohms per inch ( 10.48 in/sq_mils ) = 7.11 ohms Keep in mind that all test equipment will add some internal resistance of its own, but this is usually in the 25-50 milli-ohm (0.025 - 0.050 ohm) range. If they're getting values in the 13 ohm range (twice as high) then something has increased the ideal resistance by a factor of two. This means (obviously) that something in that formula has changed by a factor of two. I don't think the trace was etched to be twice as long. Therefore, it was either etched in 1/4 oz. copper (14.22 ohms) or was etched too thinly. Michael Hiteshew Lockheed Martin Launching Systems [log in to unmask] (410) 682-1259 > ---------- > From: Ken Patel[SMTP:[log in to unmask]] > > This is urgent!!! > Our procurement spec is calling 100% continuity at maximum 10 ohms. One of > our fab has 3 mil line on 1/2 ounce copper. Fab house is saying that they > are measuring it around 13 ohms on 22" line. May be to be safe, he is > asking > for deviation which allow him to pass up to 20 ohms. > > The questions are: > (1) Is it OK to accept fabs with above condition? > (2) Can someone explain me why it's measuring higher than normal - does it > has to do with the trace width. (I do know for sure that resistance is > higher with higher length but less with bigger dia. Is it because of > that?) > (3) Anything should I know more about the situation. > > re, > ken patel > ______________________________________________________ > Ken Patel Phone: (408) 490-6804 > 1708 McCarthy Blvd. Fax: (408) 490-6859 > Milpitas, CA 95035 Beeper: (888) 769-1808 > > ################################################################ TechNet E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c ################################################################ To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body: To subscribe: SUBSCRIBE TechNet <your full name> To unsubscribe: SIGNOFF TechNet ################################################################ Please visit IPC's web site (http://www.ipc.org) "On-Line Services" section for additional information. For technical support contact Hugo Scaramuzza at [log in to unmask] or 847-509-9700 ext.312 ################################################################ ################################################################ TechNet E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c ################################################################ To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body: To subscribe: SUBSCRIBE TechNet <your full name> To unsubscribe: SIGNOFF TechNet ################################################################ Please visit IPC's web site (http://www.ipc.org) "On-Line Services" section for additional information. For technical support contact Hugo Scaramuzza at [log in to unmask] or 847-509-9700 ext.312 ################################################################