It is common to do MBA after btech course. Many students choose MBA after their Btech course. One of the factor why one chooses MBA after Btech is good salary pacakages which initially a student with an engineering degree is less likely to get (Depends on certain variables). Also there are less of openings in core and technical levels so students shift to management field.

To get in the field of management, you will be required to clear various management entrances like CAT, MAT, XAT, SNAP, NMAT etc..

Definitely there is growth in management field, if your interest and hard work keeps you motivated or else you will step down in any field.

If you are good in designing then management skills can help you to enhance those skills too.

You can work for 2-3 years before you plan to do MBA so that you get a add on preference but preferably in the relevant field. 

Good Luck!!