can i do M.Tech after BCA with 'computer science' or 'electronic and communication' .What type of exam i should have to face ?

You can pursue M.tech after BCA, no doubt as a few universities allow this technical shift. But usually it so happens that students face problems completing the prgram successfully as they are not used to such technical orientation. Hence some universities seek B.tech students.

Its widely advised that one should go in for MCA after BCA as theres no guarantee how the placements happen with a combination of degrees.

Hope this helps.