The original post was inquiring about the use of conformal coating and/or potting on high-speed boards. Since the Er of the coating material is >1 (air) the resulting impedance of the coated lines will be lower than the same uncoated lines. The effect is most easily evaluated with a field solver. The exact Er of the coating material and the thickness of the coating will determine how much the line impedance is changed. An example: Surface Microstrip: 6 mil conductor width, 8 mil Er=4.2 dielectric, 1.2 mil thick conductor ==> Z ~ 73.2 ohms Coated Microstrip: Same stackup but include 2 mils of Er 4.2 coating on top of the trace ==> Z~ 66.2 ohms Moral of the story: Be sure to evaluate what the coating will do to the impedance of your PCB traces. If you intend to coat the board, design to account for that effect. Adding conformal coating to a properly operating board _may_ cause improper operation of the circuitry if you are depending on a particular impedance. -Ray Anderson ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu