Ive never heard of a chip running at 90c being ok. Nvidea 1080 with Asus aftermarket heavy duty heat sinks would have thermal breakdown at 90c. You should contact Bitmain and ask them they usually reply within 3 days,but I would not risk 90c on any device. Weird thing is on D3 i tested the fans kick in 100% when chip reaches 71c,but you say on yours it kicks in at 90C ?