Hello Peter, Symmetrical stack-up is crucial and I've seen a few designs where it just couldn't be helped (impedance). Rapid or uncontrolled cool down during lamination can also contribute to bow/twist in a balanced construction. I've successfully used post lamination dewarp procedures similar to Jeff's suggestion but that was for Class 2 stuff. The trick is making the dewarp last through subsequent high temp. assembly steps (soldering). Hans Integrity First - Service Before Self - Excellence in All We Do ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Hans M. Hinners Electronics Engineer Warner Robins - Air Logistics Center (WR-ALC/LUGE) Special Operations Forces System Program Office (SOF - SPO) Gunship Team 226 Cochran Street Robins AFB GA 31098-1622 mailto:[log in to unmask] Com: (478) 926 - 5224 Fax: (478) 926 - 4911 DSN Prefix: 468 -----Original Message----- From: <Peter George Duncan> [mailto:[log in to unmask]] Sent: Thursday, November 15, 2001 9:11 PM To: [log in to unmask] Subject: [TN] Board Warp Hi, All, We have received a number of 12 layer ENIG-finished MLB's with blind via hole groups, and which are destined to have BGA's (1.27mm pitch) populated onto them. The boards measure 8.2" x 5.7" and they are warped to the extent of one corner being raised by between 1.5mm and about 3mm. The axis of the warp is the long side of a triangle whose other two sides measure about 5.7 x 5.7. In this age of sensitive boards, does a technology or technique exist for straightening the boards back into spec - they are class 3 boards - or is this an area where angels still fear to tread? The boards are desperately urgent, which is why I'm asking this rather than simply rejecting them and having them re-made (which takes about a month). TIA for any help. Peter Duncan [This e-mail is confidential and may also be privileged. If you are not the intended recipient, please delete it and notify us immediately; you should not copy or use it for any purpose, nor disclose its contents to any other person. Thank you.] --------------------------------------------------------------------------------- Technet Mail List provided as a free service by IPC using LISTSERV 1.8d 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 delivery of Technet send the following message: SET Technet NOMAIL Search previous postings at: www.ipc.org > On-Line Resources & Databases > E-mail 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 ---------------------------------------------------------------------------------