Seems to me (and I should think anyone qualified to be in that position), the first step is to investigate how the "units" are failing.
Are the pcbs failing? Open or shorted tracks?
[How could you tell? If the board itself is the problem, why and how do you remedy the problem?]
Is there an assembly issue?
[Same question as above.]
Is there a component issue?
[Defective, counterfeit, use of different parts than specified?]
Is there a flaw in the circuit design?
Is there a board layout issue?
After that, the possible causes are endless.....
I suggest searching for "edn tales from the cube" and reading every one of them to gain a tiny bit of understanding about what you've asked.