Hi Ken,
Bow and twist can be caused by so many different scenarios. But, if your vendor is "dewarping"
product, there is something astray!!!.... Some questions I would ask from an outsiders perspective:
1) Is the "dewarping" process common on ALL product built by your vendor?.. Or just this particular part
number. If it is a common process, then either the requirements are too stringent or a definite process
problem is present. If it is just this design, look at the symmetry of dielectric and layer function (sig,
plane, mixed). and the balance of copper pattern.
2) Is the problem warp OR Twist. Warpage typically (not always) infers a bake induced issue. I.E. the
racking technique. and may be relieved by "flat baking" under weight. Twist may lead to a "memory" type
condition induced by design, build symmetry, "cross ply construction", excessive cool down rates during
lamination or reflow at assembly. (PEERS>>. be gentle.. I realize a very fine line of cause is present
here, but.... )
3) Are the boards "flat" at time of receipt.? and does your vendor hold boards for 24 hours AFTER
"dewarping" and then remeasure. If a memory induced warp/twist is present, it typically will start
returning when left unrestrained for 24 hours...(not ALWAYS the case, but is a good safety window).
4) have you tried running a board before assembly through your IR reflow.. It is possible that component
layout could affect this as well.
Sorry to layout so many variables, (and not all are noted here), but warp/twist can be introduced from
design, manf, or assembly. If you would like to call me, I would be more than happy to offer any other
information I can. The product type (I.E. multilayer vs. 2 sided, smt vs. mixed technology, etc) may offer
some clues to help reduce the effort to analyze and determine the root cause of your dilemma. My phone
number is 804-237-6391 ext. 126. Good luck.. JOHN WAITE
Ken Nevius wrote:
> We have been seeing boards come off our reflow oven with bow and twist
> that exceeds IPC specs. Our vendor insists that we are the only
> customer that complains about this. We had our reflow oven calibrated
> and it looked fine. The only thing about the profile that may be
> suspect is the cooling rate may be a little too fast. According to the
> vendor, they put all of their boards through a de-warping process.
> The vendor has tweaked their de-warping process to cool slower in an
> effort to make the problem go away.
>
> My question is: This is our first bout with warping, what should we be
> looking at for a cause? Is it normal for a pc board vendor to have to
> de-warp boards?
>
> Regards,
>
> Ken Nevius
> Veeder-Root
> Altoona, Pennsylvania
>
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