Hi Ted, The bowing is due to the weight of the heatsink/BGA combo. By 175C, your boards are sure to have reached their glass transition temperature, which means, even though the board is still a solid, it is soft and malleable and can easily be distorted. The weight of the heatsink/BGA combo is causing the board to sag between the oven rails. Rotate the board 90 degrees through reflow and the bowing should go away. 4 inches is usually the minimum conveyor rail width, so you are in luck, your board will still fit. If you don't think rotating the board is an option, your other alternative is Titanium board stiffeners that must be attached before reflow and removed afterwards. Thanks, Ryan -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Ted Kong Sent: Monday, June 30, 2008 1:24 PM To: [log in to unmask] Subject: [TN] Bow PCAs Hi Technetters, I have a 14.5" x 4" PCB that bows after top side reflow. I haven't measure the actural bow amount yet, but it looks like it will meet IPC spec. However, I want to see what I can do to eliminate it. I run a blank board through the same reflow profile and don't see any bowing. The production boards do not have any bowing after bottom side reflow. Bowing only occur after the top side reflow. I believe the bowing is due to the two 40mm2 BGAs on the board, which is located 4" from the long edge (center from the narrow width). Both BGAs have large heat-sinks attached to them. I reflowed three additional boards on a 10 zone reflow oven (instead of a 7 zone) oven, but didn't see any improvements. Can anyone give me any other suggestion on how to eliminate the bowing? Is my assumption of the cause correct? Thanks all in advance. Ted --------------------------------------------------- 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 ----------------------------------------------------- --------------------------------------------------- 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 -----------------------------------------------------