What is the purpose of exposed center pads on packages such as a MLF, QFN, TSSOP, LFCSP, etc?
On each of these parts the center pad is considered a floating ground. It is recommend that it be connected to ground in order to eliminate unwanted noise. In some cases it will be necessary for this pad to be used for thermal management. Whatever the part number, it is best to look at the manufacture’s information to see what they suggest.
Also, if you don’t solder the paddle, you get very poor stress relief of the package. The leads are not sufficient to hold the package to the board. You get a lot of stress in these packages, as the thermal coefficients for the package and the PCB are different. (All packages have thermal coefficients closer to those of the die, to reduce stress on the die.) So as the board cools down after soldering, the package and the PCB shrink differently. For leaded parts like TQFP, the leads acts like springs and flex slightly, for MLF parts there is nothing that can give.