With respect to the existing boards - I'm skeptical that they can be stress relieved at 150deg F. It appears that whomever recommended the procedure is hoping for some creep in the resin over time to fix the problem. You don't mention whether or not the boards are FR4; however, if they are, the temperature suggested is about half of the lowest FR4 Tg normally found in service (120deg C). It is unlikely that any significant creep will occur so far below the glass transition temperature.
Even if the boards themselves can be relieved (which I doubt), the components - and the pads to which they are soldered, will be under stress if the board shape is changed after the components have been mounted. Premature failure is almost certain as a result of component and/or joint stress fractures.
The cause of the board distortion should be examined carefully. Some common causes are asymmetric stackup, improper lamination cooldown, improper reflow or wave soldering heating/cooling programs, and mechanical stress when mounting large or heavy components. Any one of the above, or a combination, can cause the board to warp because of internal stresses. A properly designed, properly laminated, and properly handled board doesn't warp.
At 08:40 AM 7/20/04, you wrote:
snipThis email is not a Protel-specific question but instead concerns a generic problem with some boards I'm getting, so I apologize for being off-topic, but this forum has more PCB experience than any other I know of, so I would like to get your advice.
A partner in a manufacturing effort is sending us full populated boards (ICs and passive components) that are badly warped. We sent them back and rather than get the boards remade and repopulate them, they want to heat the boards up (fully populated) for three days at 150 degrees Fahrenheit in order to straighten them out. These boards perform an extremely important job and I'm worried about the stress on both the electrical components and the boards themselves. Should I allow them to do this or should I demand the boards be remade?
Thank you, Michael Robison
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