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I've seen lots of photos printed on foamboard getting warped - looks like the side where the printing is done contracts (and the opposite side does not) and so the print becomes slightly U-shaped instead of flat.
Is this a common problem? How is it resolved?
Photos aren't often printed on foam board, but they are mounted on them.
One of the major commandments when producing works of art on a rigid substrate like wood, masonite or heavy board is "whatsoever thou doest to one side, that shalt thou also do unto the other side". Essentially, that means making sure that however one modifies the stresses upon or the absorbency of one side of the ground, you have to match on the other side. In the case of mounting photos to foam boards, you are covering the paper on one side of the board with glue and another piece of paper having at least a sizing and probably a glossier ink-receptive coating to one side of the board. That will significantly affect the rate at which moisture migrates into the paper component on that side of the board, while the other side is free to absorb humidity and expand. So it isn't the printed side contracting that's the problem, it's the back expanding.
You can mount another piece of the same type of paper on the back of the board, making sure that any varnishes, etc., used on the front are also used on the back. (Commonly, one would use a lighter/cheaper weight of paper on the back -- it's the glue and coatings that matter more than the paper itself.) Or you could step up to a more rigid support (Gatorfoam, painter's panel with bracing, gessoed hardboard).
Mounting on foam board is, though, at best a short-term display solution. It isn't archival, and isn't meant to be. It isn't a substitute for proper framing.
As Stan points out ""whatsoever thou doest to one side, that shalt thou also do unto the other side". There is actually no other method, unless your shots are kept in an environment with perfectly controlled humidity, and temperature. Many types of glue contains small amount of water, which makes the photo expand as you apply it to the foam core, thus greatly affecting the amount of warping.
Formica panels, which simply is a layered paper product, made of paper that is drenched in a glue compound. It is often used as a hard-wearing surface, on benches, floors, and so on, glued to a substrate, which often is plywood, fibre board, or a similar material. I've seen table tops made up of 3/4" finest plywood bending amazingly, just because the manufacturer didn't apply any similar paper on the bottom, so 'Stan's Rule' certainly should be obeyed, always!
