Tim, I don't think there are many "rules of thumb" for thermodynamics. It's a relationship between power dissipation of that part and the surrounding parts, ambient temperature, thermal resistivity of that particular layout, and rate of airflow or convection. It seems to me you have two choices. Either try adding copper to the design until the problem goes away or have someone do a thermal analysis. I'd recommend number two. If you're unable to do a thermal analysis, try providing as many paths for heat to flow out of the system, that is, to the chassis and out into the air surrounding the unit, as possible. If the board has a ground plane, add an array of vias under and around the part connected to the plane. Add copper ground plane on the top and bottom of the board also connected to those vias. If the board is held by metal card guides, place the part adjacent to the card guides. That'll reduce the thermal resistivity between the part and the outside world (the infinite heat sink). So that's the approach. Either disperse the heat into the unit by raising the ambient of the entire unit, or (preferably) find a way to conduct it out of the unit altogether. Michael Hiteshew Lockheed Martin NE&SS-Baltimore [log in to unmask] (410) 682-1259 > ---------- > From: Tim Cables[SMTP:[log in to unmask]] > Subject: [TN] 2512 Chip Resistors > > Hi, > > We are using these 1W devices at about half their rated capacity > (.5W) on > FR4 board material and are seeing discoloration of the FR4. The part is > capable of reaching about 175 degrees C. FR4 would probably start measling > well before it reaches that temperature. Has anyone seen any rules of > thumb > for adding additional copper to the PCB footprint to spread the heat out > so > discoloration or worse would not occur. > > Tim > [log in to unmask] > > ############################################################## TechNet Mail List provided as a free service by IPC using LISTSERV 1.8d ############################################################## 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 Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315 ##############################################################