The Mother Board is the Hard Mode version of the Brain of Cthulhu. It is summoned either naturally (which can only happen in a Crimson world) or by using the Mechanical Brain which is crafted with 6 Vertebrae, 6 Souls of Night, and 5 Iron/Lead Bars at a Mythril/Orichalcum Anvil. When summoned naturally, a message appears saying: "Your entire body goes numb..." It has 25,000/40,000/50,000 , 30/49/80 melee damage, 45 ranged damage and 16/21/70 defense.
In the first stage, the Mother Board will float and teleport like the Brain of Cthulhu. It will also spawn a Signal Drone every 6 seconds until there are 25 of them. (eg: if you kill 3 then it will make 3 to replace them.) Finally, every few seconds it will fire a laser beam much like the ones that are shot by the Necromancer, the difference being that the laser connects to each of the Signal Drones.
The Mother Board goes into stage two when when it reaches 12,500/20,000/25,000 . Its stats change to 40 melee damage, 50 ranged damage, and 20 defense. In the second stage, it no longer spawns Signal Drones. However, 4 clamps appear (one out of each of the 4 black cord things on the bottom of its head), which act much like Plantera's tentacles. Due to the fact that there are no Drones, the lasers it shoots just go straight like the Heat Ray, however, it shoots the laser much more frequently and in random directions. As for how Mother Board moves; it will still teleport near the player, but instead of rushing at them, it will move away.
Credit for this idea goes to zoomo.
His profile can be viewed here: http://www.terrariaonline.com/members/zoomo.71392/