Can extreme temperatures damage stored boards?

Storage temperature and process temperature are two different questions, and mixing them up is how a rack gets destroyed on its first bake.

The storeroom is the easy part

For boards sitting on a shelf, stable and dry beats cold and swinging. The damage in an unheated warehouse is not the low temperature, it is the cycle: air that cools past its dew point overnight puts water on the board, and it does that every night. A conditioned room at an ordinary working temperature is the whole requirement for most bare stock.

The rack is where temperature gets expensive

Racks do not stay in the storeroom. They go into bake ovens, they sit next to reflow, they come out of a process still hot and go straight back on a shelf. That is where the rating matters, and it is a property of the rack, not of the room.

Ours are rated at four levels, and the difference between them is the material and the frame, not the badge:

• 80 C. Handling, storage, transport between benches. The default for a rack that never sees a process.
• 120 C. Warm processes and light drying.
• 200 C. The bake rating. This is the one that matters if you follow IPC/JEDEC J-STD-033 for moisture-sensitive assemblies, because the rack has to go into the oven with the boards still in it.
• 300 C. The high-temperature series, for the hottest work.

Choose from the hottest process, not the average

A rack rated exactly at your bake temperature is a rack running at its limit for the whole cycle, every cycle. Plastics do not fail politely at their rating: they soften, the slot geometry moves, and the rack that was holding boards apart starts letting them touch. Margin is the reason to buy 200 C for a 130 C bake rather than something rated 130 C.

The reasoning in full, including why the bake schedule belongs to the component and not to the board, is in baking MSD boards per J-STD-033.

What heat does to the boards themselves

Bare boards tolerate storage temperature swings better than assemblies do, but they are not indifferent: the surface finish oxidises faster warm than cold, and solderability is what you lose. Assemblies add moisture-sensitive parts, and those are governed by the floor-life clock rather than by the shelf.

The short version: control the room, and buy the rack for the oven.