Backlogs are to be cleared before you complete the entire session of your engineering course. Backlogs create unnecessary pressure to the candidates because of their constant thinking on that issue. But you should keep a cool mind and devote some time to clear the backlog. You may even meet your professor of the subject and discuss how best these papers can be cleared in the next time. Session of each semester is of six month duration and hence in a given year, the examination will be conducted twice and you must avail of such oppurtunities to clear the back logs.
Failure to clear the backlogs in time would produce severe strain within your mind and may affect your performance for the overhaul tests. The remaining backlog papers will multiply your load in respect of preparations of the remaining papers included in a given semester.
In case of your campus interview attended by the expert of the companies, they would see your performance of each semester and non clearance of backlogs will be viewed seriously from their ends and it would create a bad impression in relation to your performance. Hence it is always better to give utmost priority to clear the backlogs in time.
Normally the campus interview is held when you are in the sixth semester and by that time, it should have been cleared. Your selection in the campus interview would depend upon your final performance in the interview before the Board, impressive scorings in each semester and free from backlogs.
In regard to your query regarding your questions, answers are as follows -
1) Semester - system examinations are held twice ie in the month of January and July every year. Hence you may appear for your backlogs during such sessions.
2) Your mark sheet would indicate the month and year of passing such backlogs.
3) It will not affect your placement- oppurtunities provided you clear the same before your final semester of your engineering session. If you clear the backlogs before you enter sixth semester, your recruiter would be pleased to see that your mark - sheets are free from backlogs.