Mail*Link(r) SMTP FWD>warped completed assemblies Chet, You asked, "1. Does anyone know a way to straighten a completely stuffed board without damaging the components?" In the IPC's-R-700, the basic method to "flatten" out printed boards and their assemblies is to place the board/assembly in some form of a restraint and heat the board up to some temperature (ideally above the Tg of the resin), and allow the board/assembly to cool back down to ambient while still in restraint. We've done this for many years with good results and no major end product reliability problems. Most "flattenings" have been for printed board assemblies, though we have flattened some printed boards. Results of flattening printed boards has been mixed, they're flat for assembly, but due to design, some will "re-pretzel" in flow soldering or convective IR reflow, so sometimes we will need to "re-flatten" the assembly. 1) We design a 25 mm high, "egg-crate" x-y grid-like structural support system of 0.7 mm thick CRES (corrosion resistant (Stainless TM) steel) (other metals could be used, but we designed them for long term general use). The centerline spacing of the egg-crate is determined by the arrangements of components on the board/assembly and should not be greater than about 50 mm pitch. The metal spacer/straightners to run the full length and width of the board/assembly in full contact with the surface of the board/assembly between components. 2) If an assembly, we examine all the components to determine the one(s) with the lowest (non-use) storage temperature. Generally aluminum/tantalum electrolytic capacitors and plastic cased components, such as audible transducers, speakers, some meters, liquid crystal displays, etc. This is used to establish the bake temperature, which is generally in the range of 100-120 deg C. (except for some plastic cased components). Some times we removed some of the parts to increase the bake temperature. 3) We then make a "stack-up" up boards/assemblies to be flattened. The stack-up consists of a heavy base plate (13 mm thick aluminum tooling plate (they're flat)), then one of the egg-crate supports, then one board assembly, then repeat the alternating layers of board/assembly and egg-crate support as required or limited by the size (height) of the oven, lastly another egg-crate support, then another heavy base plate (13 mm aluminum tooling plate), and lastly some mass (weights) about 20 kg per 0.1 square meters (40 lbf per sq.ft.) 4) Place the stack in a "cold" forced air oven with known good temperature control or assembly the stack in the oven ( (;-) and cook until done and ready to be served (;-) ). 5) We do our "flattening" overnight. At the end of the work day, we place the stack in a cold oven, have it slowly ramp up to temperature (about an hour) hold temperature for 4-hours, turn off the heat, allow to cool with the forced air blower on until morning (the over has a small make-up air vent capability). Remove from the oven and 95+% of the time we have acceptably flattened boards/assemblies. Then you asked, "2. How much of warp can be reliably corrected?" Most boards/assemblies have ranged in size from 150 mm to over 450 mm in length/width. Board thicknesses have been up to 6 mm, though most are 1.5-3 mm in thickness. We've done up to 12-layer multilayer's with not problems. As the ol'memory bank recalls, we have routinely "flattened" most assemblies with about 5% out of flatness. Results, some to the assemblies were edge-board connector assemblies were the ol' "quad-wide" DEC M/R-Series computer boards of our design which consisted of a series of four edge-board connectors that totalled about 300 mm wide. All "use-environments" were relatively benign indoor/outdoor like operating enviroments. Hope this helps, Ralph Hersey [log in to unmask] -------------------------------------- Date: 8/27/96 7:16 AM From: Chet_Meddles_at_Micro__Industr 1. Does anyone know a way to straighten a completely stuffed board without damaging the components? 2. How much of warp can be reliably corrected? From: [log in to unmask] ------------------ RFC822 Header Follows ------------------ Received: by quickmail.llnl.gov with SMTP;27 Aug 1996 07:14:52 -0700 Received: from ipc.org by simon.ipc.org via SMTP (940816.SGI.8.6.9/940406.SGI) id JAA27761; Tue, 27 Aug 1996 09:01:10 -0700 Resent-Date: Tue, 27 Aug 1996 09:01:10 -0700 Received: by ipc.org (Smail3.1.28.1 #2) id m0uvOKc-0000OYC; Tue, 27 Aug 96 08:37 CDT Resent-Sender: [log in to unmask] Old-Return-Path: <[log in to unmask]> Date: Tue, 27 Aug 96 09:36:09 EST Message-Id: <[log in to unmask]> To: [log in to unmask] Subject: warped completed assemblies Resent-Message-ID: <"E9H4S3.0.wqI.Fel8o"@ipc> Resent-From: [log in to unmask] X-Mailing-List: <[log in to unmask]> archive/latest/5970 X-Loop: [log in to unmask] Precedence: list Resent-Sender: [log in to unmask] *************************************************************************** * TechNet mail list is provided as a service by IPC using SmartList v3.05 * *************************************************************************** * To unsubscribe from this list at any time, send a message to: * * [log in to unmask] with <subject: unsubscribe> and no text. * ***************************************************************************