Hey 

The eligibility criteria for IIT JAM is a bachelor's degree with minimum 55% for OBC and general candidates and a minimum of 50% for SC or St candidates there is no eligibility criteria for class 12th so it does not matter whether you have done your class 12th with PCB or PCM .

You are eligible for IIT JAM if and only if you have passed your Bachelors Degree with the minimum qualified percentage as mentioned above

To know more , follow the given link :

https://university.careers360.com/articles/jam