Aerospace Circuit Design for Reliability and Safety
Why aerospace reliability standards are so stringent.
Determining how a circuit fails and safeguarding against these effects.
Different models for implementing worst-case circuit analysis.
Aerospace circuit design is not only demanding on its own merits but requires additional considerations for continuous operation and safety
The sting of malfunctioning electronics is something almost everyone has experienced, whether a product is newly out of the box or dug out from storage. Given the prevalence of electronics in the modern world, it’s impressive this isn’t a more common occurrence. Immense efforts by both designers and manufacturers are dedicated to circumventing these defects that may otherwise greatly reduce the service life of a device. However, what if the tolerance for failure of any kind – even momentary – were zero? This is precisely the case for aerospace circuit design, and with good reason: any disruption can lead to a loss of life and the destruction of multimillion or billion-dollar technologies.
Not All Electronics Are Created Equal
The relatively simple boards found in toys, tools, and other consumer products don’t need to be made to the same exacting standards as the PCBs that go into medical devices or those ticketed to operate in aerospace. The higher the requirement for reliability, the greater the rejection rate of the board produced during manufacturing, which would unnecessarily increase cost and decrease the availability of products that would usually be in a state of disuse or disposal long before the board powering them would wear out. These reliability requirements, also known as the class of the board, are provided by IPC:
Class 1 electronics are standard consumer use; the failure of these products poses no immediate harm due to cessation of service.
Class 2 electronics are specialty devices, typically medical, where ongoing operation is vital to the immediate health or safety of its user.
Class 3 electronics have reliability as a paramount design concern; any interruptions to service are considered unacceptable.
Class 3A electronics represent the highest possible standards; for aerospace, they are often found in satellites and other space-faring applications.
These standards play a crucial role during the formation of design constraints and inspection, but reliability extends much further into the design phase of 3A circuits due to the enhanced demands and general rigor of the environment. While all designs for manufacturing should anticipate reliability challenges rather than react to them, additional effort to optimize circuit constitution against external influence becomes central to mission success when dealing with aerospace circuit design.
Guiding Aerospace Circuit Design With Worst-Case Circuit Analysis
Designing using worst-case circuit analysis (WCCA) is an embodiment of the phrase “hope for the best, prepare for the worst.” Given the cost of aerospace product development and the extensive need for reliability, designers need to showcase that their build has a high probability of surviving a simultaneous event of every component reaching its worst functioning state. It’s useful to establish some working parameters that define the operational status of the circuit:
Initial - Off-the-shelf nominal characteristics of a component; manufacturers provide a baseline level of capability combined with a tolerance allowing for acceptable fluctuations in materials and production quality.
Transients - Excitations that move the response from a steady state possess the ability to upset standard circuit functions or overwhelm sensitive downstream components if unaccounted for.
Temperature - Heat serves as an aging mechanism for materials, which can lead to diminished performance over time due to general degradation, or more catastrophic failure in extreme cases. Since the effects of heat can differ between materials, components can either exhibit random variance or a biased, directional stray of value(s).
Radiation - Boards ticketed for exposure to high-radiation environments need to ensure that components are either largely unaffected (as is the case for most passives) or protected with specially designed cases to reduce the penetrability of high-frequency radiation. Products indicated with a radiation design margin will incorporate a safety factor that is rated for absorption two or three times that of a product's maximum allowable radiation.
Miscellaneous - Other factors pose deleterious effects on operation, such as humidity, vibration/shock, and more. While these values or ranges are often trivial to locate within datasheets, designers need to be aware that these parts may encounter momentary or continuous perturbation in excess of test conditions.
Once minimum or maximum (as appropriate) values are collected for the various failure modes present, WCCA is able to succinctly capture performance by summing these inputs into a singular function. However, this method is only applicable when test engineers know how to manipulate all parameters to reach worst-case conditions. When components under test fail while using an extreme value analysis (EVA) model, more rigorous means of evaluating a part’s ability to withstand its environment(s) during service life, such as root-sum-square, must be adopted to establish a necessary confidence level.
Synthesizing Solutions From WCCA Equations
The parameter inputs into the WCCA function provide an incomplete scope of the device’s reliability: a circuit’s components do not function within a vacuum (excuse the pun for space-bound boards). Functionality may be inhibited before reaching these extremes, however, and engineers will need to evaluate in-circuit to determine the effects on the greater system. There are a handful of modes to accomplish this goal:
- Network analysis - Development-wise, the earliest framework for reliability checks will use basic equations such as Ohm’s Law, KCL/KVL, and other idealized tools as a rough evaluation. Although lacking in specificity, the relationship between different component values will give designers an idea of where to enact changes to drive output to a maximum or minimum.
- Sensitivity - Where circuit analysis fails to accurately encapsulate system response, a more experimental approach can reveal the exact influence each input places on the board. There are some caveats: function values must exhibit monotonicity (the continual increase or decrease over a range of inputs) to determine behavior at extrema. The further inputs deviate from an operating point, the less predictive the model becomes. Within these limitations, testers can determine the partial differential effect of minute changes to a parameter on the total output, and when this calculation is performed on every parameter, the results can be summed using EVA or RSS equations to determine the maximum and minimum output variation, respectively. Unfortunately, the relatively in-depth nature of modifying inputs and measuring response is time-consuming, and ultimately infeasible for larger circuits.
- Simulation - The best tool to quickly and accurately analyze complex circuits. Simulation tools piggyback off schematics or netlists to minimize work duplication and errors caused therein. After specifying ranges of part values and parameters, a Monte Carlo simulation is run that samples at random intervals to produce a mean and standard deviation of the dataset. Once scaled properly, the optimal WCCA solution is obtained.
When Stakes Are High, the Cadence Toolset Delivers
Aerospace circuit design is a field that must commit entirely to defect prevention during manufacturing and the highest quality of ruggedness to ensure a device can continuously operate under some of the most stressful environmental conditions. At any stage of product development or revision, Cadence’s suite of PCB Design and Analysis Software allows for a granular level of simulation control to promote long-term reliability in even the most challenging boards. Alongside the power and simplicity of OrCAD PCB Designer, users can place and route complex netlists with an industry-leading toolset to reduce turnaround times and keep pushing designs to greater heights.
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