David, Salt or sugar for that matter in water is a solid to liquid diffusion process. It like all diffusion processes is affected by time, temperature and quantity of materials in the diffusion source. Soldering is nothing more than liquid to solid diffusion with tin being the main diffusion species into a base metal, typically copper or nickel. Before this diffusion takes place, surface coating being solder (most common in past), tin (probably most common in future), gold (i.e. an ENIG surface) and PdAu over nickel on some semi lead frames must be dissolved. Then we have solder joint formation. But if we just solder to the surface finish we have not achieved tin diffusion into the base metal and that diffusion results is solid crystalline structures called intermetallic layers creating the solid bond between solder and component lead or pad surface.. Also I have observed in the past if a reflow process is marginal then we see ENIG "brittle fractures" and what has happened is the the gold has not been completely dissolved restricting tin diffusion into the nickel layer and there is soldering to the finish. There are two stable tin gold eutectic alloys than can form and freeze out during solder joint formation. One alloy is 80/20 Au/Sn with a eutectic temperature of 280C. The other is Au/Sn 20/80 and has an eutectic temp of 240C. These frozen zones are readily dissolved into the solder but if the time and temp are not adequate the there are solder joint fractures in these zones. The resulting fractured surface can be very smooth and frustrating. In the last couple of year there was an excellent paper by some lads from Taiwan who were able to capture solder joints with these zones frozen and had some excellent photo in their paper. I believe it is available thru the SMTA web site if one is a member. John Maxwell David Tremmel wrote: > Hello, > > When someone equated soldering to that of salt dissoving in water, it > threw > me for a loop. Now I am wondering if someone willing to explain how an > inter-metallic bond is formed when one of the metals (Ni, in the example > below) is not in a liquid state? > > > Thank you in advance, > > David > > --------------------------------------------------- Technet Mail List provided as a 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/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815 -----------------------------------------------------