The Free On-line Dictionary of Computing (30 December 2018):

Staggered Pin Grid Array
SPGA

    (SPGA) A style of integrated circuit socket or
   pin-out with a staggered grid of pins around the edge of the
   socket, positioned as several squares, one inside the other.

   SPGA is commonly used on motherboards for processors,
   e.g. Socket 5, Socket 7 and Socket 8.

   See also PGA.

   [Better description?]

   (1999-08-04)