StackableUSB 104 form factor plug on I/O boards now come in Â¼-size and Â½-size. The current 104 form factor only supports I/O boards of the same size (3.55″ x 3.775″). The Â¼-size and Â½-size I/O boards more closely match the space needed to implement simple I/O onto stackable boards today than the older and larger 104 form factor. The introduction of Â¼-size and Â½-size StackableUSB boards solves the CPU cooling dilemma. The challenge with higher power Pentium-class single board computers (SBC) on the 104 form factor has been providing cooling to the CPU when an I/O board is plugged on top and completely covers the CPU chip, trapping heat and seriously reducing air flow. Extending the CPU board outside the 104 footprint has been one approach that has lead to form factor introductions such as the EPIC form factor and EBX form factor. Now, however, between the spaced gained with the smaller StackableUSB connector and the greater exposed board space with Â½- and Â¼-size I/O boards, the 104 form factor becomes more viable for Pentium-class SBCs.