Yes, you can do b.tech after B.Sc. Moreover you can save 1 year as some colleges allow B.Sc students to the lateral entry directly into second year. There will be no problem in placement because most of the companies eligibility criteria is that you have to get atleast 60% in your 10th, 12th in graduation  (means your CGPA).

Dear Rahul, there will be no issue as such if you join B.Tech after B.Sc. And we would not have such details of Candidates who have pursued B.Tech after B.Sc. Your placement depends on your Academic profile over the past 4 years in B.Tech, performance in the Interview and the companies that might come to your Campus for hiring. Your past Academic profile doesnt come into picture. As you are studying B.Sc, you might as well do good so that your aggregate is good so that it is convincing enough for the recruiters.