Socket 478 is the current mainboard interface for Pentium 4 processors. Unlike previous Intel desktop socket interfaces, Socket 478 is a microPGA connector rather than a full-size PGA connector, meaning that the pins are shorter and denser than full-size PGA sockets like Socket 423. A Socket 478 package is less than two-thirds the size of a Socket 473 package. As new microprocessors begin to require larger and larger numbers of pins, this change was almost inevitable.
Due to the small size of Socket 478, it is impossible to clip a heatsink of sufficient size to the sides of the socket. Thus, a new heatsink retention mechanism was devised. For a Socket 478 chip, the heatsink clips to a frame that is mounted around the socket. To maintain rigidity, the frame is attached through the mainboard to either a metal plate on the back of the mainboard or to the case. This permits more efficient insertion and removal of the heatsink than was possible with a traditional heatsink clip.
Socket 478 was introduced in August 2001 with the Pentium 4 2.0 GHz. All Pentium 4 models available at that time were reintroduced in Socket 478 packaging. These new models quickly replaced the older Socket 423 models in new systems, particularly since the new, inexpensive i845 chipset was used almost exclusively with Socket 478. This left early Pentium 4 adopters in the lurch, because all Pentium 4 models introduced after the 2.0 GHz model were available only in Socket 478 packaging.
Over the years, Socket 478 has hosted processors on 400 MHz, 533 MHz, and 800 MHz buses, based on the Willamette, Northwood and Prescott cores, in both Pentium and Celeron branding, and in clock speeds ranging from 1.6 GHz to 3.4 GHz. In 2004 Intel introduced the new LGA775 socket which transfers the pins from the processor package to the motherboard socket. Soon, it will have replaced Socket 478 in all but the lowest-end systems.
This writeup is copyright 2002,2004 D.G. Roberge and is released under the Creative Commons Attribution-NoDerivs-NonCommercial licence. Details can be found at http://creativecommons.org/licenses/by-nd-nc/2.0/ .