If my goal is doing a Phd, which wud be better : Btech from NIT Trichy and then Phdor Msc integ- from Centre for excellence in Basic Sciences(CBS) in Bombay and then a Phd?Since the degree from CBS is awarded by the University of Mumbai, how much weight does it carry abroad?

If you are interested in doing a PhD, then first of all you should decide your area of intrest or the research field and then accordingly you should carry out your career plan.

Coming to B.tech from NIT Trichy, NIT Trichy is a very prestagious institution and B.tech degree from such a institution will be enhancing your CV. If you want to go for a PhD then you should be selecting one of these branches CSE, EEE, ECE or Mechanical as these branches wide variety of options for your PhD in many US universities. But inorder to get a PhD directly after you completing your B.tech from a very good university in US which is very essential, you should have an excellent academic aggregate, a very good score in GRE and good recommondations from professors. Coming to acdemic aggregate and score in GRE it depends on your hard work, but good recommondations depends on how reputed and publications professors have and institutes like NIT trichy sure will be having very good professors who can serve your need for very good recos.

CBS Bombay is set up by the Department of Atomic Energy and the University of Mumbai. So a Integrated MSc. course in basic sciences is very much useful for you inorder to get into PhD. Many good US universities have very good openings for PhD. in Physics. So a Msc. course in physics would be defnitely a very good option. And coming to faculty, as the faculty is from TIFR, BARC and IIT Bombay, the recos from these professors would immensely help you. And sometimes the professors of such caliber have very good contacts abroad so if you can maintain  very good relationships with your professors then they can recommend you to that Phd positions abroad directly.

Finally i would give a suggestion that if you are intrested in basic sciences you should go for CBS but if you are intrested in technical sciences you should definetely choose NIT Trichy as the b.tech degree has wide variety of options and it is safer to complete a B.tech degree rather than MSc. and with B.tech degree you can switch to any field you want may it be research or corporate

Naveen, thanks a lot for answering my question in detail.