Preventing Corrosion Failures in Industrial Electronics Robert Veale Rockwell Automation 5/17/2002

Transcription

1 Preventing Corrosion Failures in Industrial Electronics Robert Veale Rockwell Automation 5/17/2002 Summary Corrosion failures of industrial electronic equipment has been a recurring problem especially in industry groups such as paper, steel and tire manufacture. Typical for these industries are sulfur compounds which can attack electrical components and lead to failure. These failures can be unexpected because the sulfur concentration levels which lead to electronic failure are not perceived as severe by workers in those facilities. Because of this, there is a need for increased awareness to prevent corrosion failures. This paper discusses the corrosion failure mechanisms and the strategies used to prevent corrosion failures. Types of Corrosion Electronics assemblies can fail due to several corrosion mechanisms. This corrosion can occur with or without electrical voltage being present. Corrosion-induced failures can be intermittent making diagnosis difficult. Examples of different types of corrosion will now be illustrated. Connector corrosion: Connector corrosion results in open or intermittent contact resistance increases. For low-voltage connectors, gold platings over nickel are used for the connector mating surface. Gold, being a noble metal, will not corrode. However, in all gold connector systems, there are always small pores in the platings and perhaps edges with exposed base metal - usually copper. The sulfur atmosphere will react with the copper base metal forming copper sulfide. With time and continued exposure, the copper sulfide corrosion products will migrate and creep over surfaces such as the gold plated connector surface. This explains the observed blackening of gold plated connectors. Where there are corrosion products in the contact area, failure of the connection can result. The following photo illustrates copper creep corrosion over gold.

2 Photo 1: Gold Fingers exhibiting copper sulfide creep corrosion. Note how the corrosion creeps inward from the edges. This is the result of more copper porosity at the edges of the fingers. Substrate corrosion: Corrosion products can also form on the printed circuit board surface and creep across the surface. Copper and silver are both susceptible to forming sulfide corrosion products with silver being the most reactive. Copper and silver sulfide are both resistive materials and if two circuit nodes are bridged by sulfide corrosion, electrical leakage results. Silver can also result in dendritic growth in high humidity applications even without sulfur attack. Photo 2: Silver electromigration bridging two circuit nodes.

3 Photo 3: Copper sulfide film growth creeping across the PWB surface. Component Corrosion: Electrical components also can fail due to corrosion. For example on integrated circuits, copper or silver corrosion products can form where the leads enter the package. Then copper sulfide creep can bridge two IC leads resulting in electrical leakage and circuit failure. Photo 4: Copper sulfide film growth at the leadpackage interface. If these films bridge two lead, resistive shorts will be present.

4 Photo 5: SEM photo of copper sulfide creep corrosion on the plastic surface of an IC. If the creep corrosion progresses and bridges two leads, then circuit failure will result. Substrate Leakage due to Deposited Contamination Technically, this is not a corrosion phenomena. Rather it is a surface leakage effect due to ionic contamination and moisture on the surface of an electrical circuit. The atmospheric ionic contamination can come from sources such as marine environments and mists from industrial machining operations. Photo 6. Surface voltage breakdown due to ionic contamination. In this case, a water-based cutting fluid mist deposited on the PWB. This resulted in a voltage breakdown.

5 Photo 7. Airborne mists and particulate can be attracted to high voltage areas on the circuit board. This photo shows the pattern of airborne particulates attracted preferentially to the high voltage areas. Environmental Variables Humidity, temperature, reactive gases and airborne contamination all contribute to the rate of corrosion in industrial environments. Humidity: Humidity is a significant variable in electronic corrosion. At levels above 70% relative humidity, thin moisture films can be present on the PWB surface. These films may cause high resistance failure in high impedance analog modules. But also humidity will accelerate the rate of corrosion including the sulfide corrosion described earlier. The electomigration shown in photo 2 is highly dependent on humidity. It has been observed that cyclical humidity with rates of change of more than 6%RH/hour can result in higher corrosion rates than constant humidity conditions. Proximity to wet processes or exposure to outdoor environments can produce cyclical changes in humidity. Severe condensation can cause instant failure of a powered electronic module. Reactive Gases: Reactive gases such as H2S,SO2,Cl2, and others react with copper and silver conductors causing the type of corrosion describe above. The higher the concentration of reactive gases, the higher the rate of corrosion. Also, two gases when present together, may have a synergistic effect which increases the corrosion rate even more. The level of concentration of gases necessary to cause electronic failure can be deceiving. Gas concentrations of H2S in the ppb range may lead quickly to electronic failure while the human perception is that environment is very benign. Once a person perceives that there

6 is an environmental problem, it is usually past the damage threshold for corrosion of electronic modules. Airborne Contaminants: Many kinds of airborne contaminants such as dust particles are generally not harmful to electronic circuits. Ionic contamination is harmful if combined with moderate to high levels of humidity. Sources of ionic contamination include salt in marine environments and aqueous-based coolants in machining operations. Environmental Assessment: The environment should be evaluated before process control equipment is installed. Factors to consider are humidity, changes in humidity, type of pollutant, gas concentrations and airborne ionic contamination. For environments with reactive gases such as H2S, evaluate the environment bases on the ISA G1,G2,G3 and GX classifications. ISA Classifications: The Instrument Society of America (ISA) standard Environmental Conditions for Process Measurement and Control Systems: Airborne Contaminants (ISA-S71.04) describes many of the types of contamination and categorizes the severity of environments. The ISA defines four severity levels as described in the following table. Table 1: ISA Environmental Severity Levels It is important to note the these environmental classifications applied to the environment surrounding the electronic equipment. If the equipment is within a sealed enclosure, then the severity within the enclosure will be less than outside the enclosure. For reliable operation of electronic equipment, G1 and G2 levels should be maintained around the electronics. The following chart from the ISA standard further describes the severity levels.

7 Table 2: Classification of Reactive Environments Note that the copper reactivity level quantifies the severity level. Using this method of assessment, a clean copper coupon is exposed to the environment for one month. Then the coupon is sent to an analytical lab for analysis of the copper tarnish films on the surface. The thicker the tarnish film, the more severe is the environment. For example from table 2, tarnish films less than 1000 Angstroms will categorize a G2 environment. For more details on this analytical method, refer to the ISA standard. There are environmental control companies who can perform this type of analysis. Prevention Strategies Shielding by Enclosures By far the most effective control for reducing the rate of corrosion is shielding the electronic modules by appropriate enclosures. If the environment is not too severe then simple enclosures, such as NEMA type 12, may be adequate. To offer protection from the process environment, the enclosure must have a low air-exchange rate. Of course to achieve this, there must be no openings of the enclosure walls and doors must be maintained closed. A well-sealed enclosure will protect against fibers, dust, splashing and dripping. If temperature rise within the enclosure is a problem, heat exchangers or air conditioners can be used. Ensure that air conditioners to not direct cold air directly to

8 the electronic modules so that condensation does not occur. Some temperature rise within the enclosure actually reduces the rate of corrosion by depressing the relative humidity. This reduction is humidity can be significant even with modest temperature rises. If the environment has high humidity or cyclic high humidity, then the electronic equipment should be powered on at all times to keep the inside of the enclosure dry. One somewhat surprising effect of sealed enclosures is the reduction in reactive gas concentrations within the enclosure. It might be logical to conclude that within an enclosure the gas concentration would come to equilibrium to the outside gas concentration. However, this is not the case. The gas concentrations under discussion are in the ppb range. Within a well-sealed enclosure, the reactive gas molecules get absorbed and react with materials in the enclosure thereby depressing the gas concentration. With a low air exchange rate, these gas molecules will not be replenished quickly enough and the gas concentration will decrease. This decrease will continue indefinitely within environments of low to moderate severity. For harsh environments additional controls will be needed. Source Control In some applications, the pollutants have a localized source. Then the contamination can be isolated from the electronic equipment using ventilation or other methods. Ventilation Control and Removal Control If the environment is severe, more aggressive control may be necessary. Ventilation of the control room with clean source of air may solve the problem. Typically, a slight positive pressure avoids backfilling the room with pollutants. For ventilation alone to be successful, a clean source of air must be available. Also in severe environments, active removal control may be required. This may involve control rooms with active air filtration systems. A variety of filter media is available including activated carbon and permanganate impregnated alumina. Environmental control companies can help design the optimum system to prevent corrosion. Protecting Modules during Installation The installation period can be time where electronics are susceptible to corrosion damage. This is because the electronic modules are not yet installed within the sealed enclosure or control rooms. These modules can have direct exposure to the process environment. After this exposure, failures can occur even after the electronic modules are sealed within enclosures with most of damage occurring during installation. To minimize the potential for failure, consider the following precautions. ΠComplete the construction and wiring (as much as possible) of the control rooms before the modules are unpacked. ΠMinimize the time modules remain in an unprotected area. ΠAlways use protective bags for modules storage. ΠKeep doors to control rooms and enclosures closed unless opening the door is a necessity.