IEEE Transactions on Device and Materials Reliability, Vol.10, No.1, pp.55-61, March 2010

Stress Relaxation Testing of Stamped Metal Land-Grid-Array Sockets

Vidyu Challa and Michael Osterman, Member, IEEE
University of Maryland
College Park, MD 20742 USA

Leoncio D. Lopez
RAS Computer Analysis Laboratory
Sun Microsystems
Inc., San Diego, CA 92121 USA

Michael G. Pecht, Fellow, IEEE
City University in Hong Kong
Kowloon, Hong Kong
University of Maryland
College Park, MD 20742 USA



Stress relaxation in stamped metal LGA sockets can result in a loss of normal force and an increase in contact resistance, potentially leading to a failure. This paper describes an approach for determining the risks from stress relaxation in stamped metal LGA sockets by taking into account the effect of Joule heating of the socket contacts. Stress relaxation data were obtained at different temperatures and strain values, representative of both operating and overload conditions. Contact resistance measurements were conducted as the force was varied to determine the minimum force below which stress relaxation is likely to cause failure. Joule heating of the socket contacts was found to cause a measurable rise in temperature with a typical value of ~ 40°C above the unpowered state of 90°C. at the maximum rated current of 3 A for the socket being studied. This temperature rise was determined to be a significant factor in stress relaxation and was found to cause an average reduction in normal force by ~26%. The properties of the polymer housing were found to be sensitive to Joule heating effects and to have a significant influence on the stress relaxation behaviour of the socket.

Index Terms — Contact resistance (Rc), land-grid-array(LGA) socket, stamped metal socket, stress relaxation.

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