Ji-Gao Zhang

Preliminary investigation on error code rate of high speed digital circuitry caused by electrical contact failure

It is found that one of the most important reasons causing high error code rates in digital communication systems is the contact failure of coaxial connectors installed in the systems. Preliminary investigation into the failed connectors collected in-situ has revealed that the contact failure of coaxial connectors may be caused by many factors, such as corroded products or dust particles on the contact surfaces, and the change of normal contact force caused by relaxation of the spring elements, etc. It is interesting to learn how the contact failure causes high and erratic contact impedance and results in error codes in digital communication systems which are still unknown. The effects of tarnish materials on the contact impedance and also the influence of the contact failure on digital signal transmission are studied theoretically and experimentally. Some of the results and tentative conclusions are presented in this paper.

The application and mechanism of lubricants on electrical contacts

Lubricants are widely used in electronic connector applications. Mainly it is for increasing reliability and reducing the cost of electric contacts. However, from the users point of view, proper application and the mechanism of the lubrication effect of the lubricants are still not fully understood. By means of micro-observation techniques, the effect of corrosion inhibition, wear resistance and reducing the failure caused by insulated particulates are studied and described in this paper. The mechanism of static coverage, dynamic coverage as well as floating and moving ability of the particles are discussed. The effect of important parameters such as contact angle, surface tension, contact angle ratio and adhesion are also introduced. This paper is expected to be helpful to both lubricant users and manufacturers.

Electric contact performance: effects of contact surface morphology and the size of dust particles

Dust deposition is influenced by many factors such as orientation of the coupon, electric static field, and permittivity of the lubricant which covers the coupon surface. Morphology of the surface is found to be an important factor which effects the dust size and density distribution curve: the rougher the surface, the larger the particles that are deposited. This phenomena is explained theoretically by means of the balance among various forces, including electric attracting force and mass gravity force of the dust particle. Experimental results showed that electric contact failure is closely related to the particle size and the morphology of the contact surface. Theoretical analysis indicates that there is a hazardous size range of the dust particles, within which dust particles are difficult to be moved away from the contact and thus cause the contact failure.<<ETX>>

Island growth of corroded products on various plated surfaces after long-term indoor air exposure in China

The environmental effects on several contact platings including gold, silver, nickel and tin were studied after investigating the morphology, composition and fretting behavior of natural corroded products, which were formed in Shanghai, China after long-term indoor air exposure. Corrosion formed on all of the testing coupons are found as discrete islands rather than continuous layers as predicted. Not only the protrusive core product but also one or more dark rings centrically surrounding the core are seen on gold plated coupons. Micro-observation on the ring shows that it is formed by accumulated and discrete islands similar to that of the core. The height of corroded products on the ring is much lower than that of the core. The composition of corroded product on the ring is similar to that of the core. Fretting across the dark ring also causes high contact resistance. Preliminary investigation shows that this may cause sudden and rapid high impedance and error codes in telecommunication systems.

Failure analysis on bolt-type power connector's application

Bolt-type power connectors are widely used in China due to their simple structure and low cost. However, investigation shows that this kind of power connectors needs continuous maintenance and frequent replacement. By analyzing the failed connectors that were collected from different cities and sites, high connection resistance of most samples was found. Clearly high temperature has been occurred at each connector. Bolts of nearly 45% of the testing connectors appeared loosening, which caused low contact pressure, evidently the bolts were stress relaxed. Even the bolts were rusted and locked, the strand wires could still rock in the grooves of the connectors. Sliding and fretting tracks were clearly observed and appeared different kinds of morphologies. Serious contaminants and corrosive film covered on the groove surfaces, which caused high temperature rise and further enhanced stress relaxation. Surfaces of almost 58% samples preserved obvious melting or welding appearance, some of them were even burnt out, evidently there were gaps between wires and connector. Therefore, the structure of the bolt type power connectors can hardly stand for the harsh environment in China. It is also found that the material used for making connectors was not fitted to the standard.

Characteristics and electric contact behavior of tarnished film covered on silver plated surface formed by indoor air exposure

Previous experimental results showed that seriously contaminated films were found on silver contact surfaces which caused electric contact failure. To investigate the tarnished film formed by natural air exposure, silver plated coupons were exposed indoors for 18 months. Tarnished ring patterns with various sizes were observed. The thickness of the film on ring area was as high as 1500 A and a polarity effect was also found. The composition mainly seems to be copper sulfide. The contact resistance on outside ring areas appeared lower than that on the ring areas but they were still beyond the normal value. The inside ring areas looked to be least corroded. Sliding contact behavior showed high resistance and only part of the film was peeled away. The existence of copper on the surface is believed to result from the corroded copper products which have crept through the tiny holes and micro-cracks in the silver plating and nickel underplates. A circular ring stain pattern was formed probably due to water droplets. The tarnished film was mainly composed of oxides, sulfides, and chlorides of silver, copper and nickel. Even it was formed at moderate indoor conditions, experimental results showed that serious failure can occur in static contacts, sliding and fretting contacts as well.

Properties of corrosion stains on thin gold plating

Surface analysis of gold platings with different thicknesses after 3, 8, 15 and 22 months indoor air exposure was carried out. Some attractive phenomena are described in this paper. Corrosion products on gold plating are observed as the core products surrounded by different sizes of corrosion stains under an optical microscope. The stain appears as numerous tiny islands under SEM observation, which is spread and accumulated as rings or part of rings. The pore corrosion on thin gold plating is much more serious than that on the thick gold plating. It is found that contact resistance changes from point to point along a straight line through the stain area, and the values are much higher than normal in the whole stain area no matter where the testing probe is located inside the stain. It is reasonable to count the whole stain area at the contact region as failure surface area. Ratio of corrosion stain area to the sampling area versus exposure time matches Weibull probability distribution curve. Calculation leads to prediction of the reliability of contacts. Gold plating thickness below 0.5 /spl mu/m may appear high risk in this environment. Dust particles are also deposited on gold plating during indoor air exposure. In some corrosion stain areas, elements of dust are found. Dust may enhance the corrosion and also cause high contact resistance.

Comparison of sulfide films formed on copper by surface segregation and wet H 2 S exposure

It is shown that the sulfide films formed in a technical vacuum on copper are composed mainly of Cu/sub 2/O and Cu/sub 2/S, and have a composition slightly different from films formed by an H/sub 2/S exposure. AES (Auger electron spectroscopy) depth profiles and ESCA (electron spectroscopy for chemical analysis) experiments showed that surface segregated sulfide films formed in ultrahigh vacuum as well as those formed in a technical vacuum disappeared on air exposure of approximately 5 to 7 days. At the same time the characteristic oxygen photoemission lines from Cu/sub 2/O and CuO increased significantly. It is concluded that the sulfur was entirely replaced by oxygen and that the resulting free sulfur, which has a high vapor pressure at room temperature, simply evaporated. A similar tendency was observed for the sulfide film formed on wet H/sub 2/S exposure (1.5-2 p.p.m.) after a 7 day air exposure. An AES depth profile showed that part of the sulfide film was buried by oxide and part of it vanished. The polarity effect on thick films formed by wet H/sub 2/S was significant; it decreased with decreasing film thickness. A polarity effect was also found on a sulfur segregated surface formed in technical vacuum; however, it degraded rapidly with time.<<ETX>>

Dust effects on various lubricated sliding contacts

Sliding electric contact experiments show that, in dusty environments, liquid lubricants appear to perform better than wax lubricants. Experimental and theoretical analysis indicates that high permittivity of the lubricants plays an important role in attracting dust. The mixture of dust particles and wax could be very harmful to the contact, especially under low normal force. Dust particles floated on the surface of liquid droplets appear to have high mobility, which reduces the dust problem. Electric current flowing through the contacts possibly melts the wax and decreases contact resistance. On the contrary, electric current weakens the mobility and floating ability of dust on the liquid surface, which leads to an increase in resistance. Simply reducing the contact size can cause the embedding of dust into wear tracks in low normal force applications.<<ETX>>

Investigation of the thickness of lubricant film on gold plated surface

Lubricants for electrical contacts are usually diluted to low concentrations in order to keep boundary lubrication between contacts after solvent is volatilized. Evidence shows that the thickness and coverage of lubricants play an important role in reducing wear, preventing corrosion and blocking pores on the gold plating surface of electrical contacts. The distribution of lubricant film usually is observed by putting colorful dyes into lubricant. Average film thickness can be estimated by the weight of lubricant on the sample divided by surface area. Two new methods are introduced in this paper to measure the coverage and film thickness on a surface with much higher accuracy: (1) backscattering electron (BSE) method-by changing acceleration voltage of the scanning electron microscope (SEM) at low ratings, the film distribution can be shown visually by the BSE image; (2) three dimensional profiler (3D profiler) method-by insertion of the optical parameters of the lubricants, film thickness can be measured with accuracy. This paper illustrates and compares the advantages and disadvantages of the above four methods.