Sorry if this is a little long, but allow me to put my 2 cents in - And if anyone can find a flaw in this please correct. I'm a little rusty with units. Resistance of something is calculated using R = rho * L / A where R = resistance (in ohms) rho = resistivity of the material (in ohms-cm) L = Length (in cm, since resistivity is in ohms-cm) A = Width * Thickness of slab (in cm^2) +----------------------+ / /| / / | L / / | / / | / / | / / + / / / +----------------------+ / | W | / | | / | T | / | Area = W*T | / | | / +----------------------+ Now, "ohms per square" assumes your working with a "SQUARE" slab when viewed from above. Ohms per square doesn't care whether you are using inches, feet, or miles. The ohms-per-square of copper is going to be the same no matter how large the square. Different thicknesses of the slab will of course give you different values. But, as long as the thickness is constant and the sides are square, the ohms-per-square will be the same for no matter what size square slab you have. Anyway, since we're talking about a square slab, we have to assume that the Width is the same as the Length, i.e. square looking down from above. The equation given above reduces to: Given: R = rho * L / A = rho * L / (W*T), and since L = W, R = rho / T Now, I have rho for copper as Cu = 1.724*10^-6 ohm-cm I must convert the cm to whatever units I'm using for thickness units. Let's say mils. So, it just happens that 0.000001724 centimeters = 0.0006787 mils Now we say rho Cu (mils) = 6.787*10^-4 ohm-mils So our ohms-per-square formula for the units of thickness in mils is Rsq Cu (mils) = 6.787*10^-4 ohm-mils / Thickness (mils) Thickness in mils Ohms-per-Square 0.7 (1/2 oz Cu) 9.696*10^-4 ohms 1.4 (1 oz Cu) 4.848*10^-4 ohms 2.8 (2 oz Cu) 2.424*10^-4 ohms Roughly, 1/2 oz Cu is 1 milli-ohm per square 1 oz Cu is 1/2 milli-ohm per square 2 oz Cu is 1/4 milli-ohm per square I believe this is correct. But I would really like to know if I'm wrong. Regards, Doug ---------- > From: [log in to unmask] > To: [log in to unmask] > Subject: [DESIGNERCOUNCIL] Resistivity of copper > Date: Thursday, September 25, 1997 3:08 PM > > Is there a chart or formula out there somewhere that will allow us to > figure the resistivity of copper / sq. I recall seeing one at one > time, and am not sure if it was part of an IPC Standard. > > Any help is appreciated. > > John Glowacki > Sr. PC Designer > Vicor Corp. > Andover, MA ############################################################## IPC Designers Council Mail List 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 DESIGNERCOUNCIL <your full name> To unsubscribe: SIGNOFF DESIGNERCOUNCIL ############################################################## Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional information. For the technical support contact Dmitriy Sklyar at [log in to unmask] or 847-509-9700 ext.311 ##############################################################