2015年：用公司A的140拿到I 485
2016年：換到公司B，重新開始辦140
2017年11月：拿到公司B 140 approval
2017年12月：離開公司
2018年2月：可以向移民局申請link 485到公司B的140嗎？-No. The I-1`40 is void due to your leaving B company.
還是需要等到180填後（2018年5月）才能申請link？
還是完全不能link？-Right. But because your I-485 had been pending for > 180 days during your working for A company, your I-485 is still valid. So, as long as you can find a new job now in the same/similar field as your work for A company, your I-485 will be approved. You just need to ask your new employer to file Form I-485 Supplement J when you file a new job, not need to file a new PERM/I-140.
I had answered your same type of questions many times before, so please check my previous answers and also please do not ask the same type questions again.
謝謝
