According to Nomura between 90 and 100 percent of the world's supply of bismaleimide triazine is made in northeast Japan. Two Mitsubishi Gas & Chemical plants are responsible for most of the production and they are both closed.
As you say, the question is how quickly other vendors can produce suitable resin and get it tested and approved.
What the industry does NOT want is "nearly identical" materials pressed into service which then lead to chip failures in the field.
@PeterClarke: timely article. There have been mentions made about looming shortage of BT substrates in about 45 days.
I have used BT substrates (common for many multichip packages using BGA & LGA) in many projects that were sourced from China & Taiwan. I do not know the exact % of Japan's share in this market but there are many vendors in the world that can fill the gap. Japan's include Mitsubishi Gas and Chemical, Hitachi Chemical, Sumitomo Bakelite, Nippon CMK, etc.
More than the loss of one country's (Japan's) source of BT, I suspect the supply chain logistics & disruption as the main reason for the predicted shortage of BT. I believe it will take some time for other countries to step up the production and also to get setup in the supply chain. The QA departments will need to speed up vendor audits and placing qualified ones on the approved vendor lists (AVL). Japanese companies have a long reputation of good quality component sources, so the alternates will have to match that.
Dr. MP Divakar