Corrosion is the natural oxidation of metal. Corrosion in metals (except the noble group) may appear in several distinct forms but corrosion is most prevalent in a salt water environment. Metals occur in nature as oxides, carbonates and other metallic compounds. As raw ores are refined, the extracted metals are at a higher energy state and become vulnerable to atmospheric attack. As the metals deteriorate, they are in the process of reverting to their original natural state. In fact, corrosion can be called the 'antithesis of the metal refining process'.
Effects of corrosion on EMI gasket performance
Radio frequency (RF) joints lose shielding effectiveness over time due to corrosion. Conditions affecting this degradation include the corrosiveness of the environment, materials used for the joint, size and type of RF gasket, measurement frequency, and a variety of other factors. Corrosion build-up in the RF joint slowly forces the joint apart. As the joint opens, the shielding effectiveness degrades. The rate of degradation is directly related to the speed at which corrosion deposits are formed. Some environments, such as moist salt air, form corrosion deposits at very high rates, while dry environments produce very slow rates.
The materials present in the RF joint also affect the rate of corrosion build-up. In general, the greater the galvanic difference between metals making up the joint, the faster the corrosion rate. Galvanic compatibility is a simple but crude guide to corrosion rates. But many other factors also influence the corrosion rate.
The test frequency is important in determining degradation of RF shielding due to corrosion. The type of corrosive deposits formed can, in special cases, increase shielding effectiveness at different frequencies. This can be due to whisker growth, or simply that the corrosion deposits are conductive at radio frequencies. For this reason, DC measurements are generally a poor guide to RF shielding effectiveness, especially at high frequencies (>10 MHz).
Signal mixing: another effect of corrosion
Another phenomenon caused by the build-up of corrosion deposits in a RF joint is intermodulation, also known as the 'rusty bolt effect'. In this case, the corrosion deposits are 'nonlinear semiconductors' and cause signal mixing. The end result is interference from frequencies not transmitted at the joint, but created from the mixing of multiple signals received by the joint. The joint, in turn, acts like an electronic circuit which mixes signals. This problem is common whenever multiple electronic devices are used in close proximity. The shielding effectiveness of a non-linear joint cannot be measured using DC measurements or standard RF test methods.
Determining environment
One of the most difficult decisions in estimating life expectancy is in determining to which environment an RF joint will be exposed. The common mistake is to assume that the equipment will spend its entire life in an office with controlled temperature and humidity. In reality, the equipment is likely to be stored in an uncontrolled warehouse for months before shipment. During shipment from the warehouse, the electronic equipment is subject to vibration and different environments.
Once the environment is properly determined, the next step is to choose environmental test data that matches your environment. For example, if your environment is aboard a ship, the logical choice is moist salt air testing. The problem is that all salt air testing is not alike and may not produce identical results.
Salt fog testing using sodium chloride salt is often erroneously used to simulate an ocean environment. Many of the mineral salts found in ocean salt water can have a catalytic effect on the corrosion of different metals, speeding up or, in some cases, slowing down the corrosion rate. Most marine vessels still use fossil fuels which produce oxides of sulphur as byproducts. Oxides of sulphur greatly influence corrosion rates, and salt fog testing using sulphur dioxide is recommended.
Humidity testing
Humidity testing simulates tropical and temperate climates where there may be low levels of industrial pollution and salt. High humidity can cause significant corrosion even in indoor equipment. Part of the corrosion in humid environments is due to the growth of fungus. Acids given off by fungus are a major cause of corrosion. Special humidity tests are performed on military equipment to see if the equipment promotes fungus growth and to measure the damage by the fungus attack. This is a difficult environment to simulate, but it is a concern with equipment in humid environments.
Testing for vibration requirements
Vibration of mobile equipment and shipment of stationary equipment are common causes of corrosion in RF joints. The vibration of a joint causes rubbing of the joint against the RF gasket. This produces fine metal particles which corrode rapidly even in good environments. The end result is that many electronic enclosures do not make it to their benign environment before their performance is severely degraded by corrosion. Placing vibration stops and plastic sheets between the gasket and cover plate during shipment can reduce this problem.
Gaskets used on mobile equipment need special attention. The electronic enclosure should be rigid to minimise vibration, and the RF gasket should be flexible in all directions so it can bend and not rub against the enclosure. Vibration-induced corrosion is the main cause of failure in RF joints in mobile electronic enclosures. Some of this is external, as mentioned above, but it can also occur internally to the gasket.
Using environmental seals and desiccants
Environmental seals on RF joints require special attention. Environmental seals are designed to prevent moisture and corrosive agents from attacking the RF joint, but the seals can also trap these agents in the joint. Once trapped, moisture continuously attacks the joint and can cause more damage because of the environmental seal. Environmental seals should be designed to prevent intrusion of moisture, and the joint behind the seal should be vented to allow the escape of any moisture that may get past the seal.
For sealed electronic enclosures, the addition of dry nitrogen and/or activated desiccants into the sealed unit can absorb trapped moisture and, at least temporarily, protect the RF joint and the internal equipment. Desiccants should be chosen such that they do not give off moisture at any temperature likely to be encountered, or the desiccants will be a problem rather than a solution. Predicting RF joint life in mobile equipment is difficult.
In general, all of the major static corrosive environments can be encountered along with the corrosion potential due to vibration. The suggested way to predict long-term shielding performance of mobile equipment is to assume the worst case.
Galvanic table
Of particular interest to many engineers is the galvanic table. The probability that two dissimilar metals will corrode when coupled together can be predicted from their difference in the electrochemical potentials. This information is tabulated in the metals galvanic compatibility chart. This can be viewed on Laird's website at this link: www.lairdtech.com/catalog/staticdata/corrosion_metals/cm.htm, or with this article online at www.dataweek.co.za. The common metals and their anodic index are listed along the left side of the chart. This Web page also nicely encapsulates how designers can prevent corrosion of EMI metal gaskets.
Tests and service life prediction
Laird Technologies has developed modern test methods to measure the degradation of RF joints due to corrosion and is at the forefront of RF joint corrosion research. These test methods are used throughout the world and are under review by major professional organisations for acceptance as recognised international standards.
For more information contact Connecta, 011 463 2240, [email protected]
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