To fix a cracked or broken circuit board, first adhere any broken pieces together using a plastic-bonding epoxy glue. Using a precision knife, scrape any conformal coding from the traces that may meet through cracks on the circuit board. Avoid damaging traces by not applying excessive pressure.
To prevent overuse of the epoxy glue, use a small tool such as a Popsicle stick instead of directly applying the glue on the circuit board; this helps regulate the amount of adhesive, since over-application may cause more damage. For areas on the circuit board that are missing traces, find a component lead that is coming through the board, and solder a jumper board from a component lead to the trace affiliated with it.
Using a magnifying glass, verify that the green coding is scraped off near any cracks on the board; even a minor remaining piece can cause the solder not to stick properly. Apply liquid rosin flux or no-clean liquid flux to any bare copper joints that need to be soldered, and use a small amount of solder on the circuit board to connect the joints. No-clean flux doesn't require additional sanitizing; however, if using rosin flux, clean the area with rubbing alcohol. To prevent any minor cracks from expanding, drill a hole at the end of the crack using a small drill bit.