Hi Edward No, Collin's statement is not wrong. He only points out, that most tests performed on the growth do not care at all about the solder volume. As usual, one does not care about when reading this stuff. But then, suddenly there is some person asking you a question about dewetted solder joints and when analysing the solderings you see that the SOP's with copper leads ( I mean these are the guys that ought to make the least trouble ) have extremely thick intermetallic layers. Phil has mentioned a lot of what happens. It is actually the case, that the thickness of the intermetallic layer is limited if a large solder volume is present like when one dips a peace of copper into a solder bath. Approx. 1um, not regarding the time the copper spends in the solder bath. The liquid solder actually eats away the intermetallics on the surface. This means that the remaining intermetallic layers do not serve as a bareer and new intermetallics are formed. This effect is accentuated with the use of ultrasonic. The intermetallic layers are thinner than again. Obviously, as mentioned by Phil, a limited solder volume, like a reflow operation, results in thicker intermetallic layers. Naturally this effect is even stronger in the thin gap between a lead and a pad. Now, I am not only looking for the effect, but for the reason of the effect. From What I learned so far - It is obvious that a movement of the solder decreases the thickness of the intermetallic layer. This makes sense since the solved intermetallics are carried away making room for fresh tin to reach the surface of the intermetallics - The solder volume surrounding the copper surface has an influence on the intermetallic's thickness. This results in the fact mentioned above. Even if Phil is not convinced that copper enrichment in the solder is a driving force for this behaviour I do think that it would explain all we see: - If you dip a piece of copper in a solder bath a certain thickness of intermetallics builds up until a equilibrium is reached. This equilibrium depends on the diffusion rate of the copper solved in the tin away from the copper surface and the growth rate of intermetallics. The thickness in the equilibrium state is independent of the time the specimen is in the solder. - Agitation of the solder promotes the transport of copper enriched tin away from the surface of the intermetallics thus reducing the thickness of the intermetallic layer - A comparable small volume of solder like fused tin, leads to a thicker intermetallic layer. The thickness does depend on the time the specimen is confronted with liquid solder. This would be, because the copper has no place to go to and the solder builds up a certain copper content until the dissolution rate of the intermetallics is so slow that only the growth rate of the intermetallic layer governs the process. One can even find Intermetallic phases in the bulk of the solder joint which might be formed out of the copper solved in the tin. Whoof, I hope I didn't bore you all with my scientific stuff. What do you folks think of these ideas. Hey Werner this could be something for us two to entertain the Technet community with a controversial scientific discussion. Best regards Guenter ################################################################ 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 To unsubscribe: SIGNOFF TechNet ################################################################ Please visit IPC's web site (http://www.ipc.org) "On-Line Services" section for additional information. For technical support contact Hugo Scaramuzza at [log in to unmask] or 847-509-9700 ext.312 ################################################################