How to set thermal profile? Blod

hello everyone, I have a PS4 affected by BLOD, at 90% due to the usual APU problem, I would have some information to ask about the thermal profile, the preheating station at what temperatures it must work, and how soon it must arrive during the desoldering while the heat gun? What apex can it reach and with what degree of scalability, IF someone already has answers to the mid questions, in reality I would need to know even during reballing, what temperatures it takes to drop the balls from the stencil to the APU slots, and I Still need precise station and gun temperatures when rewelding, Last question, why do persons add washers to the metallic backing? in logic terms it would seem to pressure ultriorly, What reason is there