Hi, 1. The BGA lands you refer to are commonly known as 'solder-mask defined (SMD)' in contrats to non-solder-mask defined (NMSD)'. 2. I do not know where you got that " the spheres collapse during reflow the extent to which they spread latteraly is controlled by the solder resist". This is simply not true. The reason the solder ball attachment of flip chips is called C4 (Controlled Collapse Chip Connection) and now for BGAs C5 (Controlled Collapse Chip Carrier Connection) is that the solder balls collapse controlled by the surface tension of the molten solder, the weight of the component, the size and shape of the attachment pads, and the solder volume; the solder mask has nothing to do with the control of the collapse. However, for SMD pads, the solder mask etches deform the solder ball geometries to have stress concentrations. 3. Solder mask defined (SMD) solder joints show shorter cyclic lives than otherwise equal non-solder mask defined (NSMD) solder joints. The SMD solder joints contain stress concentrations which have been shown to reduce the cyclic lives to between 70 and 33% of that of the NSMD solder joints, depending on severity of the test conditions as well as the severity of the stress concentration. 4. Hence, you want the solder-mask opening to be large enough that you don not get any touching of the solder-mask and the solder balls. Typically this is accomplished with a 31 mil solder-mask opening and 25-mil-diameter copper pads. Werner Engelmaier Engelmaier Associates, Inc. Electronic Packaging, Interconnection and Reliability Consulting 23 Gunther Street Mendham, NJ 07945 USA Phone & Fax: 973-543-2747 E-mail: [log in to unmask] ############################################################## TechNet Mail List 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://www.ipc.org/html/forum.htm) for additional information. For the technical support contact Dmitriy Sklyar at [log in to unmask] or 847-509-9700 ext.311 ##############################################################