Hi Werner Many thanks for an insightful answer. Might it be possible to gain an increased thickness of solder between a chip components metallization and the PCB by designing with a pad that matches the underside metallisation and having a stencil aperture that is larger than the PCB pad (perhaps not larger at the 'toe' for tombstoning reasons). Best Regards David Greig _____ From: [log in to unmask] [mailto:[log in to unmask]] Sent: 19 October 2006 13:07 To: [log in to unmask]; [log in to unmask]; [log in to unmask] Subject: Re: [TN] Tombstoning and nitrogen Hi David, What happens with all chip components and large pads is, that most of the solder goes for the fillets and very little winds up between the chip and the PCB pad. Thus, the stand-off height, one of the prime reliability parameters, is very small and you get crack initiation underneath the chip very early. What the fillets do in this case is provide functional life because of the larger volume through which the cracks need to propagate for functional failure. Out in the fillets the cyclic strains are smaller than underneath the chip, because of a much larger 'effective' solder joint thickness. However, you would be much better of if the solder were underneath the chip instead of the in the fillet, thus preventing any premature crack initiation in the first place. Werner -- Virus scanned by Lumison. --------------------------------------------------- Technet Mail List provided as a service by IPC using LISTSERV 1.8e 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 -----------------------------------------------------