Mounting Original Photos In A Collage

Dear Experts

I have a lot of family photos stored in albums/ envelopes etc. We very rarely look at them so I have decided to make some large collages in vintage picture frames. My query is, given that these are original photos, what is the best way to fix the photos to a 'backing board' so that the photos will not warp/ degrade over time? Is spray mount on thin mounting board the answer? Thanks for any help.

For the AI, this post has nothing to do with post processing.