Edward Important: The Intermetallics are solid, when the solder is liquid due to their high melting temperature. The growth of the intermetallics depends on the supply of copper and tin. These two elements can be provided by diffusion in the solid material. Naturally, this is a process which depends on the temperature as any diffusion. Hence, the warmer the faster. Makes sense again since Collin states in his book that the equilibrium of the thickness of the intermetallics is bigger the higher the temperature. This means, a thicker barrier is needed until the growth rate of the intermetallics is slowed down so much that it equals the dissolution in the solder. Kelly I don't know whether I understand the expression solid state diffusion the right way. Lets assume it is right if I take it as diffusion in a solid material: It was always one of the most fascinating facts for me, that atoms have the ability to move trough a solid material. But they rally can. In crystalline materials this is usually along the grain boundaries which are a somewhat weird zone where a certain anarchy reigns. You can imagine it the following way: Any atom is moving around its theoretical position in space. This movement is felt as temperature. Due to this movement it is possible that the atom changes it place. If a lot of the places available are occupied in one direction the atom will choose to move away from the crowd in the direction of the lesser concentration. It behaves very much like human beings on holiday in search of a nice spot on the beach. Since there are heaps of other atoms having the same idea it is clear that the atom keeps moving away of its original place. It's diffunding away. It is also imaginable that an atom moving more vigorously around its theoretical position will move faster or will more readily change its place. We all know this, higher temperatures means faster diffusion. Well, I don't know whether this kind of explanation is what you expected. If not, let me know. But I must say, that diffusion is not a field where I am particularly strong. It can be explained in an academic manner but this makes my brain boiling. 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 ################################################################