When fabricating a printed wiring board, if the amount of copper per unit area differs from one side to anther, undesirable things can happen. One is, the action of etchant in one area will be less than in another,as etchant is used up fastest there, than in areas with less copper. This causes pads and traces whose width varies from place to place on the board. If you already are using minimum width traces, some may not be able to carry the design current, or even be etched open. And of course impedance depends on width and distance from a plane.
Another is, since copper has a different thermal coefficient of expansion than the substrate, one part of the board will be subjected to a force (as the copper expands) that another part is not, and the board can actually warp during soldering. It may cool warped to beyond usable limits. To prevent these effects, it is common practice to leave copper to steal etchant away from other parts of the board. It's also common, for boards whose outer layers are planes, to add holes in the plane to equalize chemical and thermal effects much as above. And this scan be a real annoyance to an EMI engineer! Cortland ====================== Original Message Follows ==================== >> Date: 18-Jan-01 06:20:01 MsgID: 1077-22149 ToID: 72146,373 From: [email protected] >INTERNET:[email protected] Subj: Copper Thieving Chrg: $0.00 Imp: Norm Sens: Std Receipt: No Parts: 1 From: [email protected] Subject: Copper Thieving List-Post: [email protected] Date: Thu, 18 Jan 2001 08:15:12 -0600 Reply-To: [email protected] Please excuse my lack of knowledge..........................what is "copper thieving"? ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: [email protected] Michael Garretson: [email protected] For policy questions, send mail to: Richard Nute: [email protected] ====================== End of Original Message ===================== ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: [email protected] Michael Garretson: [email protected] For policy questions, send mail to: Richard Nute: [email protected]
