There are 3 significant things I can think of that affect the impedance of trace features on 2 sided micro strip boards. Substrate dielectric constant, substrate thickness, and copper conductor trace width. These types of boards are typically single conductor traces over a ground plane structure. Variation in dielectric thickness and trace width are typically the manufacturing process related variables. A thousandth of an inch difference in trace width or in dielectric thickness can have a large affect on the impedance of a very thin line at higher frequencies... and getting more and more sensitive as frequency increases. We had to use special shops that had very good etching controls in their processes in order to use printed elements in our Microwave boards, like printed mixers, couplers, power splitters, edge coupled filters, etc. A small change in trace width could move the performance of the board out of tolerance electrically. We chose trace widths that would be less affected by variation in the geometries. The percentage change in trace width was the key factor in repeatable performance from board to board. At 6GHz we used 22 mil traces on .040 thick material (if I remember correctly) and chose this width because manufacturers can keep the trace width to within a small percentage of the nominal size. Bill Brooks PCB Design Engineer , C.I.D., C.I.I. Tel: (760)597-1500 Ext 3772 Fax: (760)597-1510 --------------------------------------------------------------------------------- DesignerCouncil Mail List provided as a free service by IPC using LISTSERV 1.8d To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF DesignerCouncil. To set a vacation stop for delivery of DesignerCouncil send: SET DesignerCouncil NOMAIL Search previous postings at: www.ipc.org > On-Line Resources & Databases > E-mail 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 ---------------------------------------------------------------------------------