Hi Scott, You have a legitimate question, but the reponses you got....well. The intermetallic compounds (IMCs) of concern for solder joints are all tin based; thus we have AuSn, AuSn2, AuSn4, Ag3Sn, Cu3Sn, Cu6Sn5, Ni3Sn2, Ni3Sn4, Ni3Sn7. All of these IMCs are brittle, the important ones CuxSny and NixSny are strong, wheras AuxSny and AgxSny IMCs are weak, leading to what has been termed 'gold embrittlement' when concentrations get too high. The dissolution rates into Sn are for Au:Ag:Cu:Ni about1000:200:30:1. Originally, Ni was used as a barrier layer for Ag-metallizations to prevent the formation of Ag3Sn IMC. On PCBs the Ni layer provide the possibility to provide a single surface treatment that is both solderable (albeit significantly less so than Cu) and wirebondable. I do not no where the notion of "Cu diffuses into the Au" or that"Ni acts as a barrier layer to prevent the formation of CuAu intermetallics" comes from, but that is not the case. The Au on the Ni has the primary function of preventing the oxydation of the Ni (much the same as the solder coverage of Cu pads); it also helps the spreading —not the wetting—of the liquid solder over the Ni surface. To properly wet to Ni you need higher reflow temperatures (about 15C) than for Cu. If you do not need wirebonding, you are definitely better of with solder coated Cu. 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 ################################################################