Methods and apparatus for differential I/O (input/output) cards using compression mount technology (CMT) connectors. Assemblies include a CMT connector having an array of spring-loaded pins or contacts that are configured to contact respective CMT contact pads on a pair of printed circuit board (PCBs), such as an add-in card (AIC) and a motherboard. Stacked assemblies are also disclosed including multiple CMT AIC or PCIe modules communicatively coupled using on module CMT connectors. The connector solutions may be used for AICs without changing the overall PCB form factor outline of existing AICs employing edge connectors. Under a stacked assembly of multiple CMT PCIe modules interconnected by on module CMT connectors, wiring in the PCBs is configured to provide signaling supporting multi-lane PCIe or CXL links for each CMT PCIe module. The CMT connector approach also is scalable and can support more pins/contacts to facilitate additional I/O bandwidth.