I have a question regarding the clamshell type socket. What is the 'center ground pin'? What is it's use? How do I decide whether or not to purchase a socket with this pin?
In most IC technologies, the exposed pad is electrically connected to the chip substrate, acting both as electrical ground and heatsink. If it's necessary to connect it in the text fixture depends solely on your design, e.g. how many GND pins are provided besides exposed pad? Is ground inductance a critical parameter in your test?
If your design is low power and moderate speed, it can probably use a package without exposed pad.