Analysis of SRAM Function Failure Due to Unformed CoSi2 Using Nanoprober and Transmission Electron Microscopy

This paper analyzes the effect of sliding speed on the electrical conductivity and friction properties of the contact pair of an on-load tap changer (OLTC). Reciprocating current-carrying tribological tests were carried out on a rod–plate–copper–tin–copper contact galvanic couple at different sliding speeds in air and insulating oil media. The results show that as the sliding speed increases from 24 mm/s to 119 mm/s, the average contact resistance in air increases from 0.2 Ω to 0.276 Ω, and the average contact resistance in insulating oil also increases from 0.2 Ω to 0.267 Ω. At 119 mm/s, the maximum contact resistance in insulating oil reaches 0.3 Ω. The micro-topography images obtained by scanning electron microscopy show that with the increase in sliding speed, the wear mechanisms in the air are mainly abrasive wear and adhesive wear, and the wear mechanisms in oil are mainly layered wear and erosion craters; high sliding speed and arcing promote contact surface fatigue and crack generation. X-ray photoelectron spectroscopy was used to analyze the surface. The copper oxide in the air and the cuprous sulfide in the insulating oil cause the surface film resistance, and the total contact resistance increases accordingly. In addition, the test shows that 119 mm/s in air and 95 mm/s in insulating oil are the speed thresholds. Below these speed thresholds, the increase in contact resistance is mainly caused by mechanical wear. Above these thresholds, the increase in contact resistance is mainly caused by arc erosion and chemical oxidation processes. Non-mechanical factors exacerbate the deterioration of the contact surface and become the main factor for the increase in contact resistance.

It has been demonstrated that the formation of insulating material by mechanochemical reaction during make-break operations can lead to a rapid increase of electrical contact resistance in Pd contacts. To further examine this phenomenon the changes at contact surfaces during make-break were observed and the effects of impact and wipe, which exist at the closure of contacts, were studied separately. Observation shows the wear track spreads from a circle to an ellipse, and the frictional powder is scraped from wear track and gathers from particles to clusters. Additionally the results of impact and wipe experiments indicate that the formation of insulating material depends on wipe rather than impact. Frictional powder strongly affects the increase of contact resistance.

An increase in the contact resistance of a no-insulation (NI) high-temperature superconducting coil was observed in high-field tests, which may be related to the mechanical deformation and the separation between adjacent turns in the coil. The large electromagnetic force generated in the high magnetic field can causeseparation between adjacent turns of the NI coil, which can affect the contact resistance of the magnet. An electromagnetic–mechanical model is built to study the effect of separation on the contact resistance and field delay time of an NI layer-wound coil. The numerical results show that the large electromagnetic force generated in the high field leads to the local separation between adjacent turns and the increase in contact resistance of the NI layer-wound coil. Moreover, a higher external field or target current can result in a larger area of separation, a higher contact resistance and a shorter characteristic field delay time. An overband can restrain the mechanical deformation and separation between turns of the NI coil in the high field, which suppresses the increase of turn-to-turn contact resistance.

The reliability of ACF (Anisotropic conductive film) interconnection is a serious concern especially under thermal loading condition. This paper focuses on the online contact resistance behavior of the ACF joint for bumpless flip-chip on flex packages during different thermal cycling conditions. In this work, flip chips of 11×3 mm2 having bare aluminum pad were used. Real time contact resistance (i.e. live measurement contact resistance variation with temperature) was measured by four points probe method when the packages were inside thermal shock chamber. Tests for three different thermal cycling profiles (125°C to −55°C, 140°C to −40°C and 150°C to −65°C) were carried out. The samples bonded at temperature 180°C and pressure of 2.42Mpa was used. The initial contact resistance of the bumpless samples was 0.4Ω. Contact resistance increased with the number of thermal cycles, however the effect was severe when the temperature variation was above the glass transition temperature (Tg) of the ACF matrix (131°C). Differences in co-efficient of thermal expansion (CTE) between the chip and the substrate generated thermal stresses during temperature fluctuation, which caused the pad of the substrate to slide over the Al pad of the chip. Thus variation of the contact resistance was also observed along the interconnection position in the package, i.e. corner joint showed higher value of increase in contact resistance than the middle position. Even though flex substrate was used in this study; the sliding effect was severe at the corner Al pads of the chip, where cumulative forces generated due to the thermal stress. Results show that for thermal cycling profile 140°C to −40°C, online contact resistance increased to 1.2 Ω in corner joint, whereas for the middle joints the contact resistance just increased to 0.5 Ω. Glass transition temperature (Tg) of the ACF material plays an important role on the high temperature contact resistance. For every thermal cycling profile, there is an incubation period that would have significant impact in the application of ACF. After the incubation period the contact resistance increases rapidly and the joints are no longer reliable.

This paper explores contact heating in microelectromechanical systems (MEMS) switches with contact spot sizes less than 100 nm in diameter. Experiments are conducted to demonstrate that contact heating causes a drop in contact resistance. However, existing theory is shown to over-predict heating for MEMS switch contacts because it does not consider ballistic transport of electrons in the contact. Therefore, we extend the theory and develop a predictive model that shows excellent agreement with the experimental results. It is also observed that mechanical cycling causes an increase in contact resistance. We identify this effect as related to the build-up of an insulating film and demonstrate operational conditions to prevent an increase in contact resistance. The improved understanding of contact behavior gained through our modeling and experiments allows switch performance to be improved.

There has been a great upsurge in research and failure analysis on automotive interconnections over the past 10 years. Failures are often attributed to a combination of loss of normal force, fretting corrosion, general corrosion, overheating, plating degradation, and others. Much of the research effort has been to establish a reproducible link between material system and electrical performance reliability. Metal alloy producers and material platers have attempted to contribute to this effort by performing specific coupon studies which characterize the electrical performance of their products. Generally, these coupon studies do not address the critical differences between the performance of a flat coupon sample and a formed connector. Within the automotive industry, there are various performance tests applied to interconnection components which evaluate electrical performance results after simulated or accelerated service conditions. This paper describes the development of a test method which is capable of simulating the key performance characteristics of an automotive interconnection. Recommendations for future improvements are included. The goal of this development was to provide a testing system which evaluates the degradation of contact resistance in samples which closely represent formed components, without the encumbrance of complete component fabrication. The following criteria were selected as minimum test system requirements : Samples must be easily fabricated from the material systems of interest; The environment must provide for thermal cycling (Delta T=125/spl deg/C, min.); Provide for stress relaxation of a formed contact spring to manifest in the performance result; Provide for the motion required for fretting corrosion at the contact surface; The change in contact resistance with time must be measured at intervals close enough to identify critical phenomena; Samples must be easily retrieved and prepared for contact surface analysis after cycle testing is completed. Initial results comparing heavy hot tin coatings on both brass and copper beryllium show an extremely high difference in the rate of increase in contact resistance.

PurposeAnisotropic conductive film (ACF) is now an attractive technology for direct mounting of chips onto the substrate as an alternative to lead‐free solders. However, despite its various advantages over other technologies, it also has many unresolved reliability issues. For instance, the performance of ACF assembly in high temperature applications is questionable. The purpose of this paper is to study the effect of bonding temperatures on the curing of ACFs, and their mechanical and electrical performance after high temperature ageing.Design/methodology/approachIn the work presented in this paper, the curing degree of an ACF at different bonding temperatures was measured using a differential scanning calorimeter. The adhesion strength and the contact resistance of ACF bonded chip‐on‐flex assembly were measured before and after thermal ageing and the results were correlated with the curing degree of ACF. The ACF was an epoxy‐based adhesive in which Au‐Ni coated polymer particles were randomly dispersed.FindingsThe results showed that higher bonding temperatures had resulted in better ACF curing and stronger adhesion. After ageing, the adhesion strength increased for the samples bonded at lower temperatures and decreased for the samples bonded at higher temperatures. ACF assemblies with higher degrees of curing showed smaller increases in contact resistance after ageing. Conduction gaps at the bump‐particle and/or particle‐pad interfaces were found with the help of scanning electron microscopy and are thought to be the root cause of the increase in contact resistance.Originality/valueThe present study focuses on the effect of bonding temperatures on the curing of ACFs, and their adhesion strength and electrical performances after high temperature ageing. The results of this study may help the development of ACFs with higher heat resistance, so that ACFs can be considered as an alternative to lead‐free solders.

This paper deals with a problem encountered during the development of a new gate electrode structure for MOS VLSI (Metal-Oxide-Semiconductor Very Large Scale Integrated Circuits), namely, the increase in contact resistance between the two layers in a Mo/poly Si gate electrode structure after annealing at around 1273 K. This study was carried out by using ESCA (Electron Spectroscopy for Chemical Analysis) to understand the above phenomenon and find the means to prevent this contact resistance increase.It was found that (1) the formation of Si oxide at Mo/poly Si interface was the cause of the increase in contact resistance, (2) the O (oxygen) necessary for this oxide formation was supplied from the Mo in contact with the poly Si, and (3) the oxide formation can be prevented by the addition of Si to the Mo, allowing the Si atoms to bond with the O atoms inside the Mo layer but not at the Mo/poly Si interface.

Reed switches fail in one of two ways: the switch will fail to open when expected (stick) or it will fail to close when expected (miss). During life testing at rated power it was found that switches with plated contacts exhibited variable and gradually increasing contact resistance up to the first stick followed by very stable contact resistance after the first stick. For reed switches with sputtered contacts, variable and gradually decreasing contact resistance was observed up to the first stick followed by an abrupt increase in contact resistance which then stabilized. End of life failure analysis is used to explain these different behaviors.

The effect of a 10 nm Ce coating layer on long term oxidation behaviour (up to 3000 hours) of a Fe–22%Cr ferritic stainless steel for solid oxide fuel cell (SOFC) interconnect application is investigated. Scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) analysis showed segregation of Ce to the grain boundaries of Cr,Mn spinel layer adjacent to the scale-gas interface. The findings in this study are in line with the grain boundary blocking model for the Ce effect mechanism. However, segregation of Ce was observed at the grain boundaries of the (Cr,Mn) spinel in the vicinity of the scale-gas interface. No evidence of Ce segregation was found in the grain boundaries of the chromia layer, neither any Ce rich particle was observed in the chromia layer after longer exposure times.

Polyaniline and polypyrrole coatings on aluminum for PEM fuel cell bipolar plates

EELS elemental mapping with unconventional methods II. Applications to biological specimens

<span lang="EN-GB">Polymer electrolyte Membrane (PEM) fuel cell is an electrochemical device producing electricity by the reaction of hydrogen and oxygen without combustion. PEM fuel cell stack is provided with an appropriate clamping torque to prevent leakage of reactant gases and to minimize the contact resistance between gas diffusion media (GDL) and bipolar plates. GDL porous structure and gas permeability is directly affected by the compaction pressure which, consequently, drastically change the fuel cell performance. Various efforts were made to determine the optimal compaction pressure and pressure distributions through simulations and experimentation. Lower compaction pressure results in increase of contact resistance and also chances of leakage. On the other hand, higher compaction pressure decreases the contact resistance but also narrows down the diffusion path for mass transfer from gas channels to the catalyst layers, consequently, lowering cell performance. The optimal cell performance is related to the gasket thickness and compression pressure on GDL. Every stack has a unique assembly pressure due to differences in fuel cell components material and stack design. Therefore, there is still need to determine the optimal torque value for getting the optimal cell performance. This study has been carried out in continuation of deve­lopment of Air breathing PEM fuel cell for small Unmanned Aerial Vehicle (UAV) application. Compaction pressure at minimum contact resistance was determined and clamping torque value was calcu­la­ted accordingly. Single cell performance tests were performed at five different clamping torque values i.e 0.5, 1.0, 1.5, 2.0 and 2.5 N m, for achieving optimal cell per­formance. Clamping pressure distribution tests were also performed at these torque values to verify uniform pressure distribution at optimal torque value. Experimental and theoretical results were compared for making inferences about optimal cell perfor­man­ce. A clamp­ing torque value of 1.5 N m was determined experimentally to be the best for getting optimal performance as well as uniform pressure distribution for this specific fuel cell.</span>

Normal and tangential forces coexist between rough surfaces in engineering components under most operating conditions. Accurate measurement of contact forces (both normal and tangential forces) on rough surfaces is critical for the safety and stability of engineering equipment, as interfaces are typically discontinuous regions within mechanical systems. However, existing contact mechanics and electrical contact models mostly neglect tangential force effects, hindering their application to shearing behavior research and precluding the development of a contact force measurement methodology applicable to simultaneous normal and tangential force quantification. Inspired by the yield criterion for material damage, a contact mechanics model was developed that simultaneously accounts for the effects of normal and tangential forces. Then, a new principle of contact forces measurement is developed by correlating the contact resistance with the real contact area, which enables the simultaneous measurement of normal and tangential forces between rough surfaces based on the single contact resistance under steady-state contact conditions. By proposing a “static friction surface”, the static and dynamic friction stage is effectively differentiated, and the reasons for the sudden drop in friction force and the sudden increase in contact resistance during the static and dynamic transition stages are given. This work proposes a novel explanation for the friction mechanism in terms of mechanical deformation and electrical resistance changes.

The atmospheric corrosion resistance of equiatomic tin‐nickel alloy having the weight percent composition, 65Sn‐35Ni, increases with time due to the growth of a protective oxide. Tin‐nickel is used as a material for wiping electric contacts, usually as an underplate for gold, and during use can become exposed as the gold is worn through. In this study, the contact resistance of tin‐nickel was found to be initially low and to change on aging as the oxide forms at a rate which depends on relative humidity and temperature. Contact resistance increases very slowly in dry air and quickly in humid air with little rate difference between 15 and 88% relative humidity. The increase of contact resistance when tin‐nickel is in contact with water has the same kinetics initially as when it is aged in humid air, but resistance continues to grow with time well beyond the air‐limiting value. The initial kinetics of contact resistance increase at 100°C are the same as at room temperature, although much higher levels are attained on prolonged aging at 100°C. The contact resistance of tin‐nickel is also inversely related to its surface roughness.