Effectively, a flux makes the oxides on the metal surface "go away", and reveal bare metal, which the solder can bond to. There really is two main ways to make oxides "go away", either reduce them to metal, or dissolve them. The rosin, and other, acids, which are in fluxes form salts with the metal in the oxide, and either dissolve the salt in the flux fluid, or cause it to ball up, and exit stage left as a sludge. There are a whole raft of different ingredients in fluxes, and you choose flux type, and thus ingredient type, depending on how aggressive you want your flux, and what sort of residue you can tolerate on the surface after soldering. This much is free...and remember, with free advice, it is often not worth what you paid for it... Rudy Sedlak RD Chemical Company --------------------------------------------------- Technet Mail List provided as a free service by IPC using LISTSERV 1.8e To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest Search the archives of previous posts at: http://listserv.ipc.org/archives Please visit IPC web site http://www.ipc.org/html/forum.htm for additional information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315 -----------------------------------------------------