Hi George, Actually there are papers written that describe the thermal graphs we use from the IPC as being very conservative and 50% de-rated... with numbers to back them up... But... If you are hoping to build a product that will have good reliability... that means the design performance 'insurance' is already built into the charts... That's a good thing. (No wonder boards have been surviving so well when designed to the IPC specs) I think I would stick to the values you have in those charts unless there is a really compelling reason to deviate from them... And know also that you are loosing the extra margin that is built into them if you are design to less than what they recommend. Bill Brooks PCB Design Engineer, C.I.D.+ Tel: (760)597-1500 Fax: (760)597-1510 Datron World Communications, Inc. Vista, California -----Original Message----- From: George Patrick [mailto:[log in to unmask]] Sent: Tuesday, May 08, 2007 4:18 PM To: [log in to unmask] Subject: Re: [DC] Current Capacity of Thermal Reliefs Yep, there is no thermal in the calculation. Make sure you are using the "Internal" setting when calculating the current. The value provided by a calculator is probably _quite_ conservative, considering you have a plane "heatsinking" the spokes of the thermal. I don't have any evidence, but I would bet that since the calculator formulas are based on a single trace, you would find their value derated by as much as 50-60% when applied to a thermal relief. Anybody have a better "feel" for this? -- George Patrick Tektronix, Inc. Central Engineering, EDS Applications Support P.O. Box 500, M/S 39-512 Beaverton, OR 97077-0001 * 503-627-5272 (voice) * 503-627-5587 (fax) http://www.tektronix.com http://www.pcb-designer.com "Off-Grid and Proud of it!" --------------------------------------------------------------------------------- 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 temporarily stop/(restart) delivery of DesignerCouncil send: SET DesignerCouncil NOMAIL/(MAIL) 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 847-615-7100 ext.2815 ---------------------------------------------------------------------------------