Wee Mei, Without being offensive, again, there is so much in this forum's archives on the BGA subject. My book is based on all my years experience doing board fabrication to keep bow and twist easily within IPC specified limits and, of course, IPC clearly indicates .75$ as the primary requirement for SMT in general. Exceeding that limit invites trouble as it does with the BGA substrate. As Phil Zarrow, others, and I have said so many times BGA's are no big friggin deal when processes are well managed and "good" components are assembled on "good" boards within a "good" process The good process must consist first of using the right solder paste in the right stensil printing machine with the right stencil apertures (20% reduction in ball size as an example), etc. The reflow profile must be right as well. Then, the device types must be right as plastic, ceramic, flip chip, etc. I know you know most of this stuff. Therefore I must ask what you are doing we don't know about? You must be up to something special, Earl --------------------------------------------------------------------------------- Technet Mail List provided as a free 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/html/forum.htm for additional information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315 ---------------------------------------------------------------------------------