Placement Games

I would really like to know how placement games work and determine where you will be ranked. Last season I was placed directly into Silver 2. I played around 150-200 ranked games and stayed in silver 2-3 the entire season. This season, despite going 6-4 in placements and getting a B+ once and the rest A- or higher, I get placed into Bronze 2. Now I am trapped in the cancer that is bronze with no end in sight.