What is Fuzz button?

“A Fuzz Button is a high performance electrical connection material used to connect two parts of an electrical circuit together, for example an IC to a PCB or two PCBs to each other. Fuzz Buttons consist of a single strand of gold-plated beryllium copper wire compressed into a dense, sponge-like cylindrical shape. Their diameter can range from a few tenths of a millimetre to a millimetre.” From HandWiki

Application:

Fuzz Buttons are commonly utilized in satellite, phased array radar and space applications, but are also broadly used in military and commercial applications.

Practical uses:

"This kind of connection is used in cases where permanent connection techniques like soldering or non-permanent ones like pogo pins or elastomeric "Zebra" connectors are not suitable, for example due to extreme vibrations or temperatures. They can also be used where low-distortion signal transmission is needed, like on PCB (board-to-board) interconnects, or in IC testing sockets where additionally many mating cycles are required. They can be used discretely as a signal/ground contact or for Electromagnetic interference (EMI) protection. When a mating array for multiple contacts is needed, like for BGA or LGA IC packages, Fuzz Buttons can be placed in thru-holes in an insulated piece of material, such as dielectric plastic. " From HandWiki

Advantage: 

Fuzz Buttons offer low distortion, high frequency capabilities, superior grounding, low insertion force, cost effectiveness, shock/vibration resistance, low weight and extended life cycles.