Tuan, this is to be expected. You are adding a dielectric layer on top of your outerlayer conductors. The impedance calculator program I got from Polar Instruments can take this into account. You need to know the thickness over the traces and the dielectric constant of the cured mask. Timothy Reeves ECD Circuit Board Division 13626 South Freeman Road Mulino, OR 97042 [log in to unmask] (503) 829-9108 (800) 228-8198 FAX (503) 829-5482 > ---------- > From: Tuan Le > Sent: Thursday, February 17, 2000 11:12 > Subject: Soldermask effect on impedance > > Hi everyone, > > I have done some tests recently by measuring impedance (on traces from the > coupons) before and after soldermask. I found that there are quite a > large drop in impedance after soldermask: 4 to 6 ohms for single ended and > 8 to 10 ohms for differential impedance. Has anyone experienced this? > Could anyone offer me an explanation of what happened? We use the same > Taiyo product for a long while. > > Your help is greatly appreciated. > > Tuan Le > [log in to unmask] > > ############################################################## TechNet 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 TECHNET <your full name> To unsubscribe: SIGNOFF TECHNET ############################################################## Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional information. If you need assistance - contact Gayatri Sardeshpande at [log in to unmask] or 847-509-9700 ext.5365 ##############################################################