Hi, When discussing solders and soldering, we should get our definitions and terminology straight, so we do not talk two differtent 'languages' and miss any points made. Tin-lead solder does NOT reflow at 179-183oC. 183oC is the Solidus for these solders; that is the temperature below which the solder is completely solid. For the pure eutectic solder 183oC is also the Liquidus (not Liquidous); that is the temperature above which all the solder is in liqud form. Any other combination of tin and lead (as well as contaminations from disolved metals, e.g. copper) will have a Liquidus higher than the Solidus, which remains constant (except for the mentioned contaminations). The more the combination differs from the eutectic, the higher the Liquidus. At temperatures between the Solidus and the Liquidus the solder is pasty. Reflow does not take place at practical rates just above Liquidus. It takes thermal energy to affect the eflow process. The reflow process consists of (1) the liquification of the solder, (2) the flowing of the solder along the surfaces to be wetted, and (3) the wetting of the surfaces, which involves the disolution of some of the base metal into,tin to form intermetallic compounds (IMCs). No IMCs equals no wetting. Therefore, for practical reflow processes there is a minimum reflow temperature that is about 20oC above the Liquidus for the solder used when soldering to copper (soldering to nickel or alloy 42 requires more thermal energy and thus a minimum reflow temperture of 25 to 30oC above Liquidus; or a longer dwell at peak reflow temperature). The peak reflow temperatures reached will vary across the assembly; solder joints for small chip components will reach the highest peak reflow temperatures, whereas solder joints for massive components, acting as heat sinks, or shadowed components (BGAs) will reach much lower peak reflow temperatures. The reflow profile will have to be set for these solder joints to reach minimum reflow temperature. Werner Engelmaier Engelmaier Associates, L.C. Electronic Packaging, Interconnection and Reliability Consulting 7 Jasmine Run Ormond Beach, FL 32174 USA Phone: 904-437-8747, Fax: 904-437-8737 E-mail: [log in to unmask] ############################################################## TechNet Mail List provided as a free service by IPC using LISTSERV 1.8c ############################################################## 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. For the technical support contact Dmitriy Sklyar at [log in to unmask] or 847-509-9700 ext.311 ##############################################################