Quality and Reliability of Elastomer Sockets
Lopez, Leoncio D.
Pecht, Michael G.
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Integrated Circuit (IC) sockets provide hundreds to thousands of electrical interconnects in enterprise servers, where quality and reliability are critical for customer applications. The evaluation of IC sockets, according to current industry practices, relies on the execution of stress loads and stress levels that are defined by standards, having no consideration to the physics of failure (PoF), target operating environment, or contact resistance behavior over time. In a similar manner, monitoring of contact resistance during system operation has no considerations to the PoF or environmental conditions. In this dissertation a physics of failure approach was developed to model the reliability of elastomer sockets that are used in an enterprise server application. The temperature and relative humidity environment, at the IC socket contact interface, were characterized as a function of external environmental conditions and microprocessor activity. The study applied state-of-the-art health monitoring techniques to assess thermal gradients on the IC socket assembly and to establish an operating profile that could be used for the development of a PoF model. A methodology was developed for modeling and monitoring contact resistance of electrical interconnects. The technique combined a PoF model with the Sequential Probability Ratio Test (SPRT). In the methodology the resistance behavior is characterized as a function of temperature. The effective a-spot radius was extracted from the characterization data and modeled with a power-law. A PoF model was developed to estimate the resistance of an elastomer contact, based on the effective a-spot radius and the ambient temperature. The methodology was experimentally demonstrated with a temperature cycle test of the elastomer socket. During the evaluation the difference between estimated and observed resistance values were tested with the SPRT. The technique was shown to be very accurate for modeling contact resistance and to be highly sensitive for the detection of resistance degradation. A qualitative reliability model was developed for the mean contact resistance of an elastomer socket, using fundamental material properties and user defined failure criteria. To derive the model, the resistance behavior of contacts under nominal mechanical load was studied as a function of time and temperature. The elastomer contact was shown to have a very complex resistance behavior, which was modeled by multiple statistical distributions. It was shown that elastomer sockets, in spite of experiencing stress relaxation at the macroscale (elastomer), can exhibit decreases in contact resistance, a result of stress redistribution at the microscale (Ag particles), which increases Ag-Ag particle stress and the effective contact area.