PGA and SPGA are acronyms for "pin grid array" and "staggered pin grid array," respectively. Semiconductor companies apply both terms to sockets that they make to physically and electrically connect their processors, or central processing units, to the motherboards of personal computers, or PCs. SPGA is actually a variant of PGA developed to address its limitations.
CPU Socket
The CPU, or Central Processing Unit, socket is a component on the PC's motherboard on which the processor is fitted. It enables the interaction of the CPU with the motherboard, which contains many of the PC's essential components, as well as holding it in place to prevent any potential damage. The physical and electrical connection is usually enabled by a certain number of pins and pinholes for fitting the CPU, depending on the processor's interface design: A CPU with pins needs a socket with pinholes to accommodate them.
PGA
The term "pin grid array" refers to the orderly, grid-like layout of the pin contacts that accommodate the processor. Before the arrival of PGA, the PC industry used dual in-line packages for computer chip technology. However, as CPUs handled wider data and address buses, semiconductors needed more pins. As a result, DIP became inadequate. Intel Corp. introduced PGA to the semiconductor industry in 1989 when it debuted the 169-contact Socket 1.
SPGA
SPGA is a variant of PGA in which the CPU socket's contacts are staggered, rather than arranged in an orderly manner. SPGA first appeared in 1993, four years after PGA's debut, with Socket 5. It is, however, not as popular as other PGA variants such as PPGA, the Plastic Pin Grid Array, which is the most common alternate option; or FCPGA, the Flip-Chip Pin Grid Array.
PGA vs. SPGA
Intel introduced CPU sockets adhering to the SPGA form factor even as the number of pins on its processors - most notably the original Intel Pentium, which debuted in 1993 - continued to increase. Thus, the company had to respond with a variant that contained more pins than PGA that fit into the same amount of space; for example, Socket 5 has 320 pinholes, almost twice the number of contacts as Socket 1.
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