I made a solid padauk pickguard for my strat. I noticed that there is a thin layer of metal on the back of the old pickguard. Do i need to put some kind of metal on the bacj of this new wood pickguard? would foil work?
That foil is there to shield the electronics from RF. It would be advisable to put it on your new pickguard. HOWEVER, properly shielding a guitar is more than just that foil. Companies putting it there is an effort but it doesn't really do the job. To properly keep your guitar from hums and radio waves, etc you need to really have your electronics shielded front to back, top to bottom. Read this:

I'll be honest, I have not read this myself but I'm going to bet it puts you in the right direction.