Leoncio D. Lopez

Preparation of loading information for reliability simulation

Many decisions regarding the design with a system using sockets are made using a point estimate (often from a standard or accepted practice) of the compressive load. The components of a CLGA assembly were analyzed using Monte Carlo simulation methods to determine the distribution of the compressive loads that are applied to the component socket. The components of the assembly that impact the resulting load were identified and their individual distributions defined. The relationship between the height of the components and the compression provided by the springs was determined and used to estimate a load distribution. The load distribution thus estimated can be used as an input to further virtual qualification efforts. For example, the estimated load distribution from this analysis was used with other manufacturing parameters and FEA models to estimate the IC socket contact compression. The results showed that the minimum compression required for a good electrical contact was achieved over 99.9% of the time. Therefore, information that is obtained from virtual qualification was shown to be of great value in the determination of proper test conditions and design decision making.

Assessing the Reliability of Elastomer Sockets in Temperature Environments

This paper presents a study on the contact resistance behavior of elastomer sockets used to interconnect microprocessors and printed circuit boards in enterprise servers. The integrated circuit sockets, installed in production representative assemblies, were evaluated at 25degC, 55degC, and 75degC for 2000 h. A sample subset was evaluated up to 16 500 h at 25degC and up to 4500 h at 55degC. The test results show that contact resistance decreases over time for all test conditions, as much as 50% from their initial values. Elastomer contact behavior is strongly dependent on temperature and time. The resistance behavior over temperature is modeled with multiple statistical distributions. The mean contact resistance is represented with a physics-of-failure model, and the elastomer contact reliability is estimated using a log-normal distribution.

Maxima-SPRT methodology for health monitoring of contact resistance in IC sockets

The evaluation of integrated circuit sockets for quality, reliability, and/or prognostic applications is performed with accelerated tests and stress conditions that are defined by industry standards. These methods typically use threshold values to detect degradation and failures of the electrical interconnect. However, this approach does not consider the physics of failure of the component under test and is not sensitive enough to detect small changes in behavior. This paper presents a physics of failure based methodology that, combined with a sequential probability ratio test, overcomes the issues of the traditional approach. This new approach, called Maxima-SPRT, enables early detection of degradation and provides the information needed for the successful implementation of prognostic tools. The method is validated using an elastomer socket, a daisy chain package, and a test board assembly.

VCSEL reliability: a user's perspective

VCSEL arrays are being considered for use in interconnect applications that require high speed, high bandwidth, high density, and high reliability. In order to better understand the reliability of VCSEL arrays, we initiated an internal project at SUN Microsystems, Inc. In this paper, we present preliminary results of an ongoing accelerated temperature-humidity-bias stress test on VCSEL arrays from several manufacturers. This test revealed no significant differences between the reliability of AlGaAs, oxide confined VCSEL arrays constructed with a trench oxide and mesa for isolation. This test did find that the reliability of arrays needs to be measured on arrays and not be estimated with the data from singulated VCSELs as is a common practice.

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

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 behavior of the socket.

Modeling of IC Socket Contact Resistance for Reliability and Health Monitoring Applications

We present a methodology based on the physics of failure, and the sequential probability ratio test, for modeling and monitoring electrical interconnects in health monitoring, and electronic prognostic applications. The resistance behavior of an electrical contact was characterized as a function of temperature. The physics of failure of the contact technology were analysed. A contact resistance model was selected, and its parameters were fitted using the temperature characterization data. The physics of failure model was evaluated with a reliability application (temperature cycle test), and was found to produce estimation errors of < 1 mOmega of during a training period. The temperature and resistance of ten sample contacts were continuously monitored during the temperature cycle test, identifying the maximum temperature and resistances for each cycle. Using the physics of failure model, maximum resistance estimates were generated for each test sample. The residual between the monitored and estimated resistance values was evaluated with the sequential probability ratio test. The method was shown to overcome the issues of traditional threshold-based monitoring approaches, providing accurate resistance estimates, and allowing the detection of abnormal resistance behavior with low false alarm and missed alarm probabilities.

Assessing the operating temperature and relative humidity environment of IC sockets in enterprise servers

Typical reliability evaluations of integrated circuit (IC) sockets are performed with stress loads and load levels that are defined by industry standards, without consideration to component properties, reliability requirements, or target operating environments. The problem with this approach is evident when evaluations are completed. If the standard requirements are met, then the test results provide no assurance of adequate product life for the customer. If the standard requirements are not met, then the test results provide no clear indication as to which failures are relevant and which are not. An alternative approach for the estimation of product reliability is found in health monitoring and electronic prognostics, where the reliability of an IC socket can be estimated from operating condition monitors, and physics of failure analysis and modeling. The results obtained in this manner can provide greater assurance that reliability requirements will be met and that the incipience of failures will be detected, with corresponding root causes identified. In this paper the operating environment of an IC socket inside an enterprise server was experimentally assessed by means of a sensor network, providing vast information for the definition of reliability models and future implementations of health monitoring and electronic prognostic methods.

Reverse-bias emission sheds light on the failure mechanism of degraded vertical-cavity surface-emitting lasers

The phenomenon of reverse-bias electroluminescence (EL) has been exploited extensively to elucidate the physics of failure of Si-based semiconductor devices. In this article, we study the reverse-bias emission of vertical-cavity surface-emitting lasers (VCSELs) that have been stressed under accelerated temperature, humidity, and bias. Using quantitative emission microscopy in conjunction with other electrical and optical measurements, we show that the dark line defect dislocation network responsible for VCSEL degradation can be imaged directly as reverse-bias EL and that the spectra of the reverse-bias EL provide further insight into the failure mechanism.

Stress relaxation in a commercial stamped metal land grid array socket

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 sockets electrical behavior was demonstrated, without the need for system level testing.