Most workbenches are a static-free, dry, radiation-proof area. But the real world is not as forgiving and you would be amazed at the environmental factors that can cause problems in your circuit. Exposure to water can lead to faults in exposed circuits. Seawater is incredibly corrosive and can eat away at connections. Petrol vapors are volatile so any sparks in your circuit could result in a surprise celebration.
While many devices on the market are not designed for harsh environments, there are many more that must operate effectively while being jostled, dampened, irradiated, and more. Designing systems that can withstand such conditions presents a series of problems that engineers must overcome, not least of which is assessing the requirements the environment presents and attaining the corresponding certifications.
This article will take a high-level look at the factors that can cause an environment to be labeled "harsh" and some examples of the certifications required.
Types of Harsh Environments
While classifying some environments as harsh can be easy (such as the presence of explosive materials), it's not always so straightforward. For example, a temperature of 150°F would be considered harsh by my standards but the industry recognizes temperatures above 257°F (125°C) to be harsh. (Oil rig manufacturers, for example, consider environments above 392°F (200°C)!).
So, what environments should designers look out for?
- Noxious (sensors can be affected by this)
- Static discharge
- Electromagnetic interference
A harsh mechanical environment can be one where the circuit is exposed to mechanical stresses. For example, vehicles experience a lot of vibration from both the engine and the road surface.
Vibrations can be potentially devastating to electronics and mechanical parts since solder connections can break under fatigue and mechanical fittings can “undo” themselves (this is why locking nuts are often used). But electronic circuits are not just at risk from damaged solder connections; even integrated circuits (especially MEMS) can experience damage under vibration and impact stresses. Airbags, for example, rely on accelerometers to detect sudden changes in velocity to trigger the airbag explosive (such as sodium azide). Since accelerometers often rely on tiny silicon structures that can move around, they can be prone to vibration damage common in automotive applications.
Chemical environments are those where chemicals are present (often in the form of a gaseous vapor). A classic example of how chemical environments can damage equipment can be seen by anyone who has ever made their own PCBs using ferric chloride. Any steel tools stored near a ferric chloride tank will very quickly rust and begin to dissolve due to fumes.
Chemcial environments can damage sensors and corrode surfaces - image courtesy pixnio
Circuits designed to be located in corrosive environments need to either have enclosures to prevent exposure to corrosive materials or use components that can withstand the corrosive elements.
The term "chemical environments" can also refer to those with explosives (such as petrol fumes, which can be ignited in the presence of electronic circuits. High-voltage switch presses, for example, are incredibly dangerous in such an environment which is why explosive-rated switches need to be used. Noxious fumes can also create a harsh environment as certain chemical fumes can damage sensors and detectors.
Electrical environments are those that can induce voltages into circuits via electric or magnetic fields. This could look like anything from medical equipment near MRI machines to power station circuitry which could potentially be affected by large transformers.
Static electricity is also a common problem for circuits as static discharge can flip states and damage the gate layer of MOSFETs. Static electricity can come from a number of sources including air purification where static electricity is used to remove dust particles and spraying car parts where static electricity helps the paint to adhere to the surface.
Interference can also come from other electrical circuits such as radio transceivers. This is why filter circuits can be crucial in a design and the control and immunity of emissions commonly fall under CE and FCC regulations.
Environments with Exposure to Radiation
Another harsh environment may be one where ions and radiation are present in sufficient quantity to cause problems. Every day, cosmic particles penetrate the Earth’s atmosphere. Computers, in particular, can be vulnerable to this since charge cosmic particles can cause bits to flip (such as a bit stored in RAM) or cause a latch to lock (i.e., unable to change state).
The Northen lights are incredible to view but could easily damage electronics
In most cases, these errors are self-fixing either through error correction or simple hardware resetting but circuits in space do not have the protection of the atmosphere (which absorbs a sizeable portion of cosmic rays). In aerospace applications, circuits are heavily exposed to ion radiation and, as a result, require strong shield techniques as well as protection circuitry to defend against induced voltages from ions.
Examples of Harsh Environment Certifications
While no single certification will be looked at in-depth (as they involve incredibly complex procedures and have 100+ page documentation associated with them), we will look at several examples of certification in the industry.
MIL-STD 810—Harsh Environments for Military Use
Products and devices made for military use have strict requirements and some of these requirements are found in MIL-STD 810. This standard tests for high-/low-temperature resilience, ruggedness, rain, humidity, sand, vibration, and shock.
Placing an off-the-shelf laptop in a rugged case does not make that laptop useable in the field; the laptop, itself, will still be susceptible to G-forces and sudden impacts which are all too common in military settings. But being able to survive one heat cycle or a single drop test does not make a device military grade; the MIL-STD tests often call for many drops and cycles to be done to ensure that the device still functions when exposed repeatedly to harsh, unforgiving environments.
AEC Q100—Automotive Vibration for ICs
Electronics in the automotive industry are exposed to intense vibration. With the increasing use of MEMS in cars, it’s imperative that ICs can handle such environments. The Automotive Electronics Council, or AEC, has several different certifications for electronics parts including AEC Q100, AEC Q101, and AEC Q200. AEC Q100 is concerned with how ICs fail under stresses and any part that is AEC Q100 certified can be fitted into vehicles.
Testing procedure for AEC Q100
Some of the tests involved in the AEC Q100 include ESD testing, latch-up protection, temperature cycling, wire bond pulling, dielectric breakdown, short circuits, vibration, mechanical shock, and even die shear. Even the silicon process type (CMOS, NMOS, bipolar, etc.), needs to be qualified and any process that is slightly different (despite being similar) needs to be qualified separately.
ATEX EU Directive—Explosive Atmospheres
The European Union has unified directives that describe what safety practices should be followed and how products should be capable of handling environmental factors such as electrostatic discharge and interference.
The ATEX directive, for example, is concerned with devices that operate in explosive atmospheres. It describes different category types for electronic parts, identifying potentially explosive atmospheres, and includes what environments are exempt from the directive. The directive also identifies potential ignition sources which include lightning strikes, open flames, mechanically generated sparks, electrical sparks, and radiation.
Do you have experience in designing circuits for harsh environments? Share your expertise with the community in the comments below.
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