I doubt it's rocket science that these guys are up to.
A large percentage of electronics failures are due to dry joints - soldered connections which have become detached over time due to heat, vibration and movement. 2 boiler PCBs, a car ECU, a Hi-Fi, and a TV all got years of ongoing service following attention of these. The other failures are typically due to failed passive components such as electrolytic capacitors, which are industry standard and have the sizes etc written on the side so you can replace them with exactly the same spec component.
There's very little chance of changing the logic of the component by doing the above, most of that is undertaken by 'chips' which will take 40+ years to degrade in most applications.
To answer those questioning modern PCB assembly, look up capacitor plague on wikipedia. A typical example of how volatile electronics can be even from the factory. One failed copy of a recipe doubtless caused countless premature failure of assembled board, including those in otherwise working boilers.