I would reply that Dave Hoover's response to his customer was right on the mark. This is a situation that we all too frequently encounter with large surface area boards and large perimeter parts. At its worst we have seen up to 12 components removed and then replaced where spot underside preheating has been used leaving a condition that for lack of a better term can only be described as "egg-crate." Once the rework/repair operator has created this situation we enter into a damage control scenerio. The dimpling or egg crate effect can leave large residual stresses in the board structure along with all the other various types of heat related damage. We have had some success with placing the board in an oven and SLOWLY bringing the board up to temperature and then back down. The actual temperature and time must be determined (based on board material / size / damage / etc.) on case-by-case basis. By the time we have become involved several rework and repair scenerios have already been attempted which severly limits the available options. Normally we will bring the board up to temperature over a 2-3 hour period; hold for a minimum of 4-6 hours; turn off the oven and allow to cool overnight. This can only be described as a salvage operation for a high value assembly. -------------------------------------- [log in to unmask] Corporate Training Administrator PACE Inc. 9893 Brewers Court Laurel MD 20723-1990 301.490.9860 x329 voice 301.604.8782 fax -------------------------------------- <!-- On 7/12/95 Dave Hoover Wrote --> Old-Return-Path: <miso!hadco.com!dhoover> Encoding: 1301 Text Resent-Message-ID: <"TGtij3.0.lSF._s31m"@ipc> Resent-From: [log in to unmask] X-Mailing-List: <[log in to unmask]> archive/latest/686 X-Loop: [log in to unmask] Precedence: list Resent-Sender: [log in to unmask] I have a customer who is experiencing excessive warpage after reworking BGAs. The PCB is a .062" thk - 10 lyr made from 140'C Tg FR-4. He uses forced hot air under the BGA device that heats only the region/area where the device is located. I discussed with him the issues (stress/TCE/expansion/contraction) that he may be encountering by heating only specific areas. I also stressed issues associated with controlled heat rise and cooldown of PWAs. His next tests will be to heat the entire board slightly below component degradation temperatures then perform his BGA rework procedure. Once rendered he was going to allow the entire PWA to cooldown at a slow rate to minimize any potential stresses that could happen during rapid cooling. Since I'm a fabricator and don't completely understand the assembly/rework issues, I thought I would throw this up for suggestions. Any responses will be greatly appreciated. Thanks in advance. Dave Hoover Senior Process Engineer Hadco Corp/Tech Center Two-Watsonville, CA [log in to unmask]