WLCSP unit cost is very low, but there is a very high fixed cost. Also WLCSP packaging must be done a wafer-at-a-time. With an MPW, you don't get a whole wafe

WLCSP unit cost is very low, but there is a very high fixed cost. Also WLCSP packaging must be done a wafer-at-a-time. With an MPW, you don't get a whole wafer of your chips -- they cut up the wafer and send the chips with your design to you, and the chips with other peoples' design to the other people. So ordinarily on an MPW if you want WLCSP, you have to pay the fab to run a whole second "boat" of wafers just for you, so those wafers can be run through the packaging process before they are cut up. This often costs more than the MPW booking does!