In our experience, SIG Boards are not required or necessarily needed.
If you're SIG has one, don't worry, that's ok. That's great in fact and means that a lot of folks are interested in getting involved.
We find that often a looser leadership construction can work well too. Hours of getting an election off the ground, assigning official roles and duties, are saved by not having an "official" Board.
Instead, we suggest a volunteer Leader/Chair, or co-leader/co-chair (as many as you like/need), and then additional volunteers to help.
Also when you figure out what you want to do as a SIG, creating small committees (long-term, or better yet, short-term) to do small, specific tasks, often work well to get things done.