Stress relaxation in a commercial stamped metal land grid array socket

Abstract

Land grid array (LGA) sockets provide a solderless printed circuit board (PCB) attachment method for microprocessors that require high interconnect density. Stamped metal LGA sockets consist of metal contacts embedded in a plastic housing, that form the electrical and mechanical connection between the component and PCB by means of compression.To achieve a stable contact interface between the socket and component/PCB, it is necessary to maintain a minimum normal force. Stress relaxation results in a loss of normal force, and could cause an increase in contact resistance, potentially leading to a failure. Contact alloy manufacturers report stress relaxation data for the alloy in strip form. This data may not be an appropriate measure of the stress relaxation in an LGA socket because it does not take into account the geometry of the increasingly miniaturized contacts or the effect of the surrounding polymer. This paper therefore aims to study the stress relaxation of stamped metal LGA sockets, by testing the actual socket in its final processed form and capturing the effect of the polymer on stress relaxation. Furthermore, by testing small pieces of the socket, independent of the loading assembly, a method of studying the temperature and load dependence of stress relaxation and the effect on the socket's electrical behavior was demonstrated, without the need for system level testing.