I am a PCB design engineer, and we have been getting conflicting requests from our assembly houses regarding thermal pad styles. I am hoping some technetter assembly folks can give some guidance as to good practices for reliable mounting. This part is a good example of the type of part in question: http://www.linear.com/designtools/packaging/qfn/UHF38.pdf In general, for most thermal pads we create our footprint matching the recommended solder pad layout, and the stencil house windows the paste appropriately for correct mounting. (the vias connecting the pad to planes are outside of the thermal pad boundary of the part) Our PCB assembly company we use for production in Japan has not had any problems with this, and our QFN-style parts have mounted fine on many different boards. We have been using a local (US) assembly company to do our prototypes, and we have been getting some conflicting requests from them. They have requested that we add vias to the thermal pad, not for thermal sinking reasons, but to "wick" away extra solder to make the parts settle properly. In the spectrum of modifications to a thermal pad, this sort of sounds like the worst case. Some web evidence seems to agree with that: http://blog.screamingcircuits.com/2009/01/speaking-of-common-qfn-issues.html Our understanding has been that in the best case for vias in the thermal pad they should be plated shut or otherwise plugged (coplanar with the pad). Another option would be to window the paste as to not overlap the vias (we have seen various app notes from IC vendors suggesting this strategy) but that is still risky. What is the correct answer? We have been going on the standard of trusting the assembly company, and doing what they have qualified as good practice, but we are at a point where the recommendations conflict. Thanks for any suggestions, Tom --------------------------------------------------- Technet Mail List provided as a service by IPC using LISTSERV 15.0 To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest Search the archives of previous posts at: http://listserv.ipc.org/archives Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815 -----------------------------------------------------