Board to board mezzanine connector and modular connector system

A modular board to board ball grid array BGA connector includes a plug (12), a receptacle (13) and if needed an adapter (110). The plug (12) and the receptacle (13) can be made from the same base pieces to accommodate different stack heights. If a greater stack height is needed, spacers (20) can be used in the plug (12) and the receptacle (13) to accommodate a greater selected stack height. The plug (12) and the receptacle (13) both include a base (14) having interstitial diamond recesses (22) in which the solder balls (29) are disposed and in which one end of a contact (59, 61) is inserted. The plug may further include a plug cover (18) that can be connected to the base (14), and the receptacle may include a receptacle cover (70) that fits over its base (14). The plug can have a plug contact assembly (16), and the receptacle can have a receptacle contact assembly (72). The plug and the receptacle can be mated by mating the plug cover (18) to the receptacle cover (70) and the receptacle contacts (84, 86) to the plug contacts (59, 61). If a larger stack height is desired, a spacer (20) can be attached to the base of either or both the plug or the receptacle to achieve a larger stack height.