In a message dated 10/2/98 11:44:31, you wrote: >Mr Engelmaier, > >Are you sure about some of the statements made, with regards to Ag and gold >being similarly solderable, and that the nickel is a means of gaining a >solderable surface? > >Nickel underplate is simply a diffusion barrier for copper, upon which the >gold is plated to prevent the nickel from tarnishing. A copper surface (not >sure about brass) is very solderable, hence the development of OSPs, and >the like, however, gold cannot be used as a solderability preservative on >top of the copper because the copper will diffuse through the porous gold, >hence, nickel is first plated, followed by a typical, 8 microinches gold >layer. Too thick a gold deposit will lead to solderability issues with the >Au-Sn intermetallic, hence the ratio of solder thickness to gold thickness >must be controlled closely, to achieve a good bond. Thermosonic wire >bonding has different parameters. > >LK Hi 'LK,' I think we pretty much said the same thing, but came from different perspectives or emphases. Certainly, Au or Ag should not be plated on Cu; there is not any need for it and OSP or Sn/Pb are much better. The nickel is not just a diffusion barrier, but also the surface to which the actual solder connection is made. That means the Ni has to be kept solderable, hence the Au-overplate, but it also means that it has a good bond to whatever is underlying the Ni or you could get adhesive failure of the Ni from the underlying metal. Alternatively, the Ni could be thick enough to form a quasi-exoskeleton that is selfsupporting. And the problem with thick gold overplates is not so much solderability, but too much Au-Sn IMCs, which are not only brittle, as are all IMCs, but also weak (most IMCs are rather strong). 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 ################################################################