Hi Earl, You quote a 'definition' of the soldering process straight out of Manko's book, which represents 1962 thinking and understanding (on the macro-level, you may actually see on IMC layers, but they are there on the micro-level). If you consult more recent works (Klein-Wassink, Lea, Frear, etc.), you will find that in soldering, solubility of the metals involved is important and that some intermetallic compound formation (it may be only a couple of atomic monolayers thick) is required to form a metallurgical bond. It is unfortunate, that the term 'wetting' has taken on the definition of forming a metallurgical bond, because I can wet (spread/cover with solder) a metal surface with liquid solder without forming a metallurgical bond by means such as a gold flash on nickel but inadequate temperatures and time for the process of metallic solution formation between nickel and tin (this, of course would be not possible with a metal more readily soluble in tin, e.g. copper). Intermetallic compounds are typically not involved in the failure of solder joints (Morris, Frear, etc.), but the visible lack of an IMC layer which often is an indication of a failure to 'wet', is involved in 'solder joint failures' (in quotation marks because no real solder joint had been formed) showing interfacial separations (no indication of solder on the metal surface). Intermetallic compounds are essentially all brittle, but most are brittle and much stronger than solder. The important exceptions are gold/tin and silver/tin IMCs, which are both brittle and weak—thus, the observed problems with them in sufficient concentrations. IMC layers grow with time and temperature following an Arrhenius relationship with activation energies ranging from about 0.5 to 1.8 eV. As they grow, they 'consume' tin from the adjacent solder region and form a lead-rich region ahead of them, which has lower strength than the nominal solder composition. Thus, solder joint failures frequently show fractures close to, but not in the IMC layers (but, frequently there are other reasons for this fracture location, as well). For a thin HASL surface prior to the formation of solder joints, the growth of the IMC layer can deteriorate the solderability of that surface; re-processing these boards will cause a degradation of quality and reliability. Also solder joints, unless they first have been subjected to thermal cyclic fatigue, do not fail as the result of mechanical shock and/or vibration. Many studies have shown this; other parts of the assembly will invariably fail first. The very property of solder that makes solder joint fatigue such an important issue—the readiness to creep because of the use temperatures close to the material's melting temperatures, makes solder joints much less susceptible to mechanical shock and/or vibration as well as non-uniform geometries creating stress concentrations than would be the more common structural metals (steel, copper, aluminum, etc.). 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'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 ################################################################