Hi Tuan, Just real briefly; Zo is = to L/C (inductance over capacitance) and capacitance is proportional to Er (dielectric constant), so C= Er A/d (area) over d (distance/separation). It is important to understand that solder resist is a dielectric and acts as insulation to the surface traces, thus reducing the circulating currents. I hope this helps. Mike Beckman [log in to unmask] Packaging Engineer Assembly Technology Division CH5-157 Ofc. (602) 554-4232 5000 W. Chandler Blvd. Fax (602) 554-7615 Chandler, Arizona 85226 Pgr. (602) 340-3983 -----Original Message----- From: Tuan Le [mailto:[log in to unmask]] Sent: Thursday, February 17, 2000 12:13 PM To: [log in to unmask] Subject: [TN] 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 ##############################################################