How do I replace the CPU?

Hey!

A few weeks ago I tried to repair my friend's iPhone 5S with a new housing and LCD. When I tried to start it the first time it didn't even respond so I thought I might try to charge it. When I connected the charger the "Charging sound" kept on going every 5th second and it was still dead. This went on for like an hour or two and then I realized it was not going to start at all.

So I brought the iPhone to a local phone repair store and after a day of testing what's wrong with it, they said it probably was the CPU. (whatsoever they told me it was nicely done except the matter of fact that I didn't manage to fix it, hehe)

So it seems that I've accidentally short circuited the CPU.

How do I fix this? I mean one option is to just buy a used one and take the logic board out. But is there any way you could possibly replace only the CPU?

If not, where can I get a "cheap" logic board?