Hi Marcello, Unfortunately, your solder paste vendor is giving you a rule-of-thumb (30 seconds from the time the solder becomes liquidus, 183 deg. C), which works well enough in most cases, but is technically insufficient and inaccurate. [By the way, for eutectic Sn/Pb solder the Liquidus (the temperature above which the solder is liquid or in a liquidous state) is 183*C, but for most solders the Liquidus is higher because of non-eutectic compositions or other contaminations, such as Cu or Au]. To properly reflow solder, which includes the wetting of the surface to be soldered as well as forming some intermetallic compound (IMC) layer with a constituent of that surface, e.g., Cu, Ni, you not only need to have liquid solder and a solderable surface, but enough heat input to bring that surface constituent into solution with Sn. At Liquidus that will take quite a long time. A better rule-of thumb is to have every solder joint at least 10 seconds at or above Liquidus+20*C for a Cu surface and Liquidus+35*C for Ni and Alloy 42 surfaces (Ni disolves much more slowly into Sn than does Cu). That is the reason, why reflow profiles should be measured at the solder joint that will heat up the slowest, e.g. near the center of BGAs. In general, the solder joints of larger components (more thermal mass) will heat up slower than small component or the PCB; differences of as much as 20*C have been measured with 'Forced Convection' relow ovens, more with 'IR' reflow ovens. The thickness of the IMC layer is unimportant (you do not want it too thick, but that is outside of normal processing), as long as you have some thickness. Your Au coating will let the solder spread more easily over the surface, but it does nothing to enhance or improve wetting other than presewrve the solderability of the underlying surface. 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 E-Mail Forum 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://jefry.ipc.org/forum.htm) for additional information. For the technical support contact Dmitriy Sklyar at [log in to unmask] or 847-509-9700 ext.311 ################################################################