PCB temps in the max. 70 c area are good.
PCB temps over 80 c, are not so good and the miner might overheat in 85-95 c area.
The s9 is good to a minimum of 105c on chip temperatures. Earlier models would not shut down due to heat until the chips reached 115c
The s9 will run for months at a time at 100c chip temps, its not an issue.