Embedded systems contain electronic circuits that control products or processes. These systems are subjected to a variety of environmental conditions, including moisture. In most cases, the control electronics are implemented using technology that cannot tolerate environmental contamination without negative effects. At the same time, it might be important to know whether environmental contamination is present in order for the control module to execute a fail-safe mechanism, such as a warning or controlled shutdown of the plant under control.
As an example, many embedded control systems are implemented using printed circuit boards (PCBs) that contain multiple layers of insulating substrate. Each layer has a deposited (“printed”) set of conductors, usually made of metal representing wires that carry signals or power. The layers are glued together to form a single board that is populated on the external surfaces with integrated circuits. If moisture contacts these layers, thin whiskers of metal called dendrites can form between the conductors potentially leading to bridging faults (short circuits) between the conductors. If two shorted conductors carry signals, the resulting signal carried by either or both of the shorted conductors may be unpredictable. Thus, the function performed by the controller would be unpredictable. If the conductors carry power, the short can lead to melting or even ignition of the PCB.
Furthermore, if the embedded system is safety-relevant, that is, the safety of the product depends on the hazard-free operation of the controller, then contamination of the controller electronics must be monitored and controlled. To prevent damage to the electronics in an embedded system caused by environmental contamination, collisions or other causes, the electronics are typically enclosed and/or sealed in a case or housing. Often this single layer of protection is not sufficient to prevent environmental contamination and additional inner layers are necessary. A typical solution to the problem of inner layer protection is to cover the controller PCB with a protective coating, such as an electronics potting compound made of silicone or polyurethane.
For safety-relevant systems it is sometimes important to know if contamination has occurred, even if the protective layers have prevented the contamination from negatively affecting the embedded controller.
One solution to the problem of detecting the contamination is to add a sensor, e.g., a moisture sensor in the case where water contamination is to be detected. A similar approach, appropriate in cases where environmental conditions can cause premature aging of components, is to monitor a redundant component or circuit with slightly greater sensitivity than the primary circuit. A failure in the redundant circuit indicates environmental conditions that could adversely affect the primary circuit. Such redundant circuits are frequently called canary circuits. However, both of these solutions add cost, complexity and potentially unreliability due to the additional components.