Effect of Joule Heating on the Reliability of Stamped Metal Land Grid Array Sockets
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Performance requirements in high end microprocessors have increased tremendously in the last several years, leading to higher I/O counts and interconnect densities. As greater currents pass through the microprocessor interconnect, higher temperatures driven by Joule heating are expected to pose reliability risks to high pin count microprocessor sockets. In this study Joule heating and its effect on the reliability of stamped metal land grid array (LGA) sockets was investigated using a combination of experimental and numerical methods. A methodology to determine socket temperature environments under electrical loading was developed. Knowledge of socket operating temperatures can allow original equipment manufacturers (OEMs) and socket manufacturers to test for and mitigate failure mechanisms under thermal aging. The factors that influence Joule heating and contribute to premature socket failure were examined. Processor temperature, contact alloy and contact pitch were all found to significantly influence socket temperatures driven by Joule heating, with the contact alloy and processor temperature having the most significant effects. The resulting temperatures at higher currents were found to significantly influence the mechanical properties of the polymer housing and adversely affect socket stress relaxation behavior. The properties of the polymer housing were found to be sensitive to temperature owing to its visco-elastic nature. Polymer housing relaxation was therefore identified as a principle contributor to failure in stamped metal sockets under high temperature environments. In the latter part of the study, numerical modeling was used to develop a methodology for assessing socket life expectancy under temperature and deformation loads. A full visco-elastic characterization of the polymer housing was conducted and the measured properties were subsequently used to model socket stress relaxation time to failure. The results of this study indicate that socket temperatures under electrical loading can be significantly higher than those called for by EIA test specifications for LGA sockets. Passing tests that are not stringent enough to account for worst case scenarios can pave the way for field failures. The methodology outlined in this dissertation may be used to determine socket temperature environments and their effect on socket life expectancy.