FR4 material is not homogenous, it (more or less) consists of epoxy, glass fibers, gas pockets and contaminants. That means that between copper layers it looks like a number of different capacitors in series. Capacitors with different dielectric constants resulting in different fields across each. And that means one of these capacitors may be the beginning of the breakdown. Also, I expect that the 740V/mil is for DC, while you are applying AC. I would start with applying a variable DC voltage to one conductor and connect the other one through a resistor to ground. Look across that resistor with a scope and see when corona starts to occur as you increase the DC voltage (noise on the scope trace). Once you have corona, breakdown will follow, either quickly or after a longer time after the ozone eats away the epoxy. But at least the test will tell you whether the 750V/mil is real. To do the same with AC you will need to either filter the 500kHz out, or use a microphone that is sensitive well beyond our hearing and "listen" for corona (feed the microphone signal to the scope). Once upon a time HP made an HF converter that did exactly that, it listened to high frequency noise and down-converted it so you could hear it. I did use it to solve your kind of problem. Sorry for not being able to give you rules to live and design by. Ahne. -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Bill Higinbotham Sent: Tuesday, August 24, 2004 9:25 To: [log in to unmask] Subject: [TN] 500 KHz breakdown of FR4 We are using a multilayer FR4 board with 12 mils between layers and getting breakdown between traces on different layers. The voltage is about 3500 V peak. THis board should be good for 740 V/mil x 12 or 9 KV. I wonder if it is excessive leakage due to moisture absorption? Bill Bill Higinbotham RFL Electronics Inc. E-mail : [log in to unmask] Voice: 973.334.3100 Ext. 386 Fax: 973.334.3863 --------------------------------------------------- Technet Mail List provided as a service by IPC using LISTSERV 1.8e To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest Search the archives of previous posts at: http://listserv.ipc.org/archives Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315 ----------------------------------------------------- --------------------------------------------------- Technet Mail List provided as a service by IPC using LISTSERV 1.8e To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest Search the archives of previous posts at: http://listserv.ipc.org/archives Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315 -----------------------------------------------------