Improved micro-contact resistance model that considers material deformation, electron transport and thin film characteristics

This paper reports on an improved analytic model forpredicting micro-contact resistance needed for designing microelectro-mechanical systems (MEMS) switches. The originalmodel had two primary considerations: 1) contact materialdeformation (i.e. elastic, plastic, or elastic-plastic) and 2) effectivecontact area radius. The model also assumed that individual aspotswere close together and that their interactions weredependent on each other which led to using the single effective aspotcontact area model. This single effective area model wasused to determine specific electron transport regions (i.e. ballistic,quasi-ballistic, or diffusive) by comparing the effective radius andthe mean free path of an electron. Using this model required thatmicro-switch contact materials be deposited, during devicefabrication, with processes ensuring low surface roughness values(i.e. sputtered films). Sputtered thin film electric contacts,however, do not behave like bulk materials and the effects of thinfilm contacts and spreading resistance must be considered. Theimproved micro-contact resistance model accounts for the twoprimary considerations above, as well as, using thin film,sputtered, electric contact

ELECTRICAL AND MECHANICAL CHARACTERIZATION OF MWNT FILLED CONDUCTIVE ADHESIVE FOR ELECTRONICS PACKAGING

Lead-tin solder has been widely used as interconnection material in electronics packaging for a long time. In response to environmental legislation, the lead-tin alloys are being replaced with lead-free alloys and electrically conductive adhesives in consumer electronics. Lead-free solder usually require higher reflow temperatures than the traditional lead-tin alloys, which can cause die crack and board warpage in assembly process, thereby impacting the assembly yields. The high tin content in lead-free solder forms tin whiskers, which has the potential to cause short circuits failure. Conductive adhesives are an alternative to solder reflow processing, however, conductive adhesives require up to 80 wt% metal filler to ensure electrical and thermal conductivity. The high loading content degrades the mechanical properties of the polymer matrix and reduces the reliability and assembly yields when compared to soldered assemblies. Carbon nanotubes (CNTs) have ultra high aspect ratio as well as many novel properties. The high aspect ratio of CNTs makes them easy to form percolation at low loading and together with other novel properties make it possible to provide electrical and thermal conductivity for the polymer matrix while maintaining or even reinforcing the mechanical properties. Replacing the metal particles with CNTs in conductive adhesive compositions has the potential benefits of being lead free, low process temperature, corrosion resistant, electrically/thermally conductive, high mechanical strength and lightweight. In this paper, multiwall nanotubes (MWNTs) with different dimensions are mixed with epoxy. The relationships among MWNTs dimension, volume resistivity and thermal conductivity of the composite are characterized. Different loadings of CNTs, additives and mixing methods were used to achieve satisfying electrical and mechanical properties and pot life. Different assembly technologies such as pressure dispensing, screen and stencil printing are used to simplify the processing method and raise the assembly yields. Contact resistance, volume resistivity, high frequency performance, thermal conductivity and mechanical properties were measured and compared with metal filled conductive adhesive and traditional solder paste

A Review of Micro-Contact Physics for Microelectromechanical Systems (MEMS) Metal Contact Switches

Innovations in relevant micro-contact areas are highlighted, these include, design, contact resistance modeling, contact materials, performance and reliability. For each area the basic theory and relevant innovations are explored. A brief comparison of actuation methods is provided to show why electrostatic actuation is most commonly used by radio frequency microelectromechanical systems designers. An examination of the important characteristics of the contact interface such as modeling and material choice is discussed. Micro-contact resistance models based on plastic, elastic-plastic and elastic deformations are reviewed. Much of the modeling for metal contact micro-switches centers around contact area and surface roughness. Surface roughness and its effect on contact area is stressed when considering micro-contact resistance modeling. Finite element models and various approaches for describing surface roughness are compared. Different contact materials to include gold, gold alloys, carbon nanotubes, composite gold-carbon nanotubes, ruthenium, ruthenium oxide, as well as tungsten have been shown to enhance contact performance and reliability with distinct trade offs for each. Finally, a review of physical and electrical failure modes witnessed by researchers are detailed and examined

Investigation of the contact resistance as a function of the temperature for connectors and wire terminals

The hardness of coating materials such as tin or gold is temperature-dependent, so the contact area and thus the contact resistance change depending on the temperature. Contact resistance measurements are carried out on hard gold- and tin-coated connector contacts at elevated temperatures. It is shown that the contact resistance decreases significantly with increasing temperature. Tests are also being carried out with solid and stranded copper wires. In addition to the hardness, foreign layers on the copper conductors have a further influence on the contact resistance

Measurements and Prediction of Thermal Contact Resistance across Coated Joints

An integrated experimental and numerical investigation of the thermal contact resistance across two nominally flat, coated metallic engineering surfaces in contact is presented. The model consists of a surface deformation computation, which determines the actual contact area and number of contacting asperities at a joint, and a constriction resistance analysis, which determines the constriction resistance through each individual contacting asperity. Predictions from the model are validated against experiments conducted for the purpose. The experiments are performed according to a “design of experiments” approach and evaluated using statistical regression. Three substrates (copper, brass, and aluminum) and three coatings (silver, nickel, and tin) are considered with a variety of coating thicknesses and substrate roughnesses. The contact load is also varied. The experimental measurements show that the best choice of a coating for contact resistance mitigation depends on the substrate material and roughness, and it cannot be prescribed in general. A regression equation developed for the experimental results offers a useful tool for the design of coated contacts. The measured results agree well with predicted values from the numerical model, especially in cases of a rough substrate or hard coating

Tribological and arc erosion behaviors of copper-refractory metal in situ composites

Tribological and arc erosion behaviors of Cu-Nb and Cu-Cr in situ composites were investigated in this dissertation. Dry sliding tests were performed in a pin-on-disk wear tester with the composites rubbing against a rotating tool steel disk under ambient conditions with a pressure of 0.68 MPa, sliding speeds up to 2.5 m/s, and electrical current densities up to 2.89 MA/m[superscript]2. Electrical arc erosion tests were performed in a make-and-break test set-up and a high-energy stationary arc gap test facility;Sliding friction and wear behaviors of Cu-Nb composites with and without electrical loads were studied in terms of the effects of composition, filament orientation, true deformation strain, sliding speed and annealing temperature. The Cu-20 vol.%Nb composite had the best wear resistance among the compositions studied in both cases. Subsurface deformation was revealed by the presence of filaments and was one of the wear mechanisms for the composites. No debonding was observed in the composites during sliding. The presence of electrical current increased the temperature and promoted oxidation on wear surfaces;The contact behavior of Cu-Nb composites against tool steel was studied in terms of the contact resistance and temperature rise. Surface oxide film development, wear particle accumulation, and unsteady contact caused by sliding were found to be the major factors governing electrical contact resistance and, therefore, temperature rise. Arc erosion behavior of the composites was also investigated. It was concluded that, in low-energy make-and-break contacts, oxidation was the major mode of surface deterioration; and, in high-energy contact situation, melting was the major cause of surface damage. The Cu-refractory metal in situ composites had better arc erosion resistance in high-energy contacts than the commercially used Cu-W composite

Mechanics of Bolted Electrical Splices

Localized heating of bolted electrical splices in the power distributing bus is a primary concern in the industrial automation industry. While localized heat generation problems are commonly reported in the field, it is not entirely clear what the root causes are. A methodology is presented for development of a tool to measure in-situ the influence of clamping load on the thermo-electric behavior of the splice joint. Applied research and reasoning used to identify probable root causes for failures reported in the field are also presented. Experiments were conducted to characterize the mechanical properties of the bolt and nut system used in service. A bolt was modified and retrofitted with strain gauges. This system was calibrated as a load cell and experiments were conducted to develop a sample specific model for determining the bolt pretension as a function of torque applied to the nut. The methods herein described can be implemented for applications in which optimal performance of bolted connections is required. Measurements were made of the electrical contact resistance and an idealized finite elements simulation of the contacting bus materials was studied alongside real samples. Based on this study, the author presents potential root causes for the onset of problematic localized heat generation

Effect of Joule Heating on the Reliability of Stamped Metal Land Grid Array Sockets

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

Carbon nanotube surfaces for low force contact application

This thesis focuses on developing a testing method to estimate the mechanical and electrical characteristics of CNT-based contact surfaces. In particular the use of gold thin films deposited on a CNT forest has potential to offer a very effective contact surface. Two different pieces of experimental apparatus were used in this research to determine the mechanical and electrical properties of gold/multi-walled carbon nanotube (Au/MWCNT) composites: 1) a modified nano-indentation; and 2) a PZT actuator test rig. These apparatuses were used to mimic force-displacement and contact behaviour of the MEMS relay?s contact at a maximum load of 1 mN with dry-circuit and hot-switched conditions. The surfaces were compared with reference Au-Au and Au-MWCNT contact pairs studied under the same experimental conditions. In the modified nano-indentation experiment, tests of up to 10 cycles were performed. The results showed that the Au-MWCNT pair electrical contact resistance improved when the Au-Au/MWCNT pair was used. Additionally the Au-Au/MWCNT pair electrical contact resistance (Rc) was comparable with the Au-Au contact pair. When the Au-Au/MWCNT composite surface is in contact with the Au hemispherical probe it provides a compliant surface. It conforms to the shape of the Au hemispherical probe. For a higher number of cycles, a PZT actuator was used to support Au/MWCNT planar coated surfaces. This surface makes electrical contact with a gold coated hemispherical probe to mimic the actuation of a MEMS relay?s contact at higher actuation frequencies. This apparatus allows the performance of the contact materials to be investigated over large numbers of switching cycles. Different current loads were used in this experiment, 1mA, 10mA, 20mA and 50mA at 4V supply. The Rc of these surfaces was investigated as a function of the applied force under repeated cycles. Under current loads of 1mA and 10mA the Au/MWCNT composite surface provides a stable contact resistance of up to more than a million cycles and no degradation was observed on the surface. Compared with Au-Au contact pair, degradation occurred after 220 cycles. The Au-Au contact pair shows delamination of the Au surface on the probe. The possible reason is the softening or melting of the Au surface. Furthermore, under higher current loads of 20mA to 50mA, degradation had occurred after 50 million cycles (at 20mA) and degradation had occurred at around 45 to 150 cycles for 50mA to 30mA respectively. This is because of the softening or melting of the Au and Au fatigue after a large number of cycles. This study is the first step to show the potential of CNT surfaces as an interface in low force electrical contact applications. With this research, current trends in materials used on contacts and fabrication methods can be explored and even modified or adopted. The use of CNT?s and their composites for contacts can be tested using the available apparatus to look at their performance and reliability in terms of mechanical and electrical properties. This is useful for MEMS contacts that form part of MEMS relay devices