Selecting the right PCB materials for harsh environments is a critical job. Multiple factors must be taken into consideration, such as weight, mechanical strength, thermal and electrical conductivity, corrosion resistance, and cost. But when a PCB operates at an altitude of a thousand miles or above, the list of decision-making factors changes dramatically. Now you have to factor in temperature, pressure, particle radiation, and more. When considering aerospace circuit board (sometimes referred to as circuit card) design, you should aim to ensure durability and reliability over a period of five to fifteen years, in addition to the following.
Aerospace Circuit Board Consideration #1: Temperature
Extreme temperatures can dramatically affect material properties and PCBA geometry. Both high and low-end temperatures can cause damage. A high temperature may weaken the chemical bond within the material, whereas an extremely low temperature may make them too brittle and develop cracks on the surface. High temperatures may also lead to thermal shock and material outgassing, ultimately leading to circuit failure.
Engineers must carefully select appropriate materials and components for use in extreme temperatures. For example, capacitors and resistors have different coefficients of thermal expansion (CTE). Thus, while designing boards for an extreme-temperature aerospace environment, thick film resistors and ceramic X7R capacitors would be your ideal bet. Apart from capacitors and resistors, you would also have to select the right material for other essential PCB elements such as connectors, semiconductors, oscillators, dielectrics, solder, and surface finish.
Aerospace Circuit Board Consideration #2: Pressure
A component designed for aerospace application is expected to experience a high differential pressure due to pressure recycling above sea level. Conditions like solder voids, trapped gas in a PCB, or exposure to permeable fluids under high or low pressure may lead to void rupture, outgassing, and increased absorption. In this regard, the choice of components and PCB material for both high pressure and low-pressure applications may differ.
As absorption is the main concern, the component material should be naturally resistant to intrusion. Designers prefer using COTS (Commercially off-the-shelf) components that ensure a void-free design to become inherently pressure tolerant and to avoid PCB potting
Aerospace Circuit Board Consideration #3: Environment Corrosion
Corrosion is another important aerospace circuit board consideration. The change in relative humidity due to temperature and pressure changes can cause water to condense on the PCB. The condensation will then mix with the board contamination and develop corrosion. Rain, oceanic sprays, or equipment washing can also exacerbate corrosion.
Depending on their source of formation, there are several types of aerospace corrosion:
- Galvanic corrosion: an oxidation reaction between two metals separated by an electrolyte or condensed water
- Electrolytic corrosion: contamination of neighboring traces by ionic liquid leading to short circuits
- Gaseous corrosion: corrosion due to atmospheric gasses
- Moisture absorption: penetration of atmospheric fluid into the PCB through micro-cracks on the surface
- Electrostatic discharge (ESD): electrostatic charge development on the PCB surface under harsh environments, creating a large voltage drop that damages the board
Designing boards with hermetically sealed components is the best way to offer protection against humidity and outgassing. Generally, LTCC and FR-4 have better moisture resistance than polyimide. Even thoroughly washing off any no-clean flux post assembly would significantly lower the risk of corrosion.
Aerospace Circuit Board Consideration #4: Vibration and Shock
PCBs are prone to experience mechanical vibrations from various sources such as in-house motors, system resonance, and air turbulence. Vibrations can slowly build up stress at solder joints and component leads, resulting in material fatigue.
Aerospace PCBs may also experience shock during sudden transitions when taking off, landing, or docking. Shocks can cause immediate cracking or material deformation.
There are a number of ways you can mitigate these effects:
- Small SMD components and through-hole parts can significantly mitigate vibrations.
- You can use higher pin count packages to effectively distribute the stress.
- Creating a mismatch between the resonant peaks of the system and the PCB would preclude any vibrations triggered by resonant behavior.
- Using high thickness boards will ensure less flexing during vibration.
- Designers can also use adhesive underfill or stacking to offer polymeric stress relief for the solder joints.
PCBA Manufacturing for Extreme Environments - Part 1
Aerospace Circuit Board Consideration #5: Particle Radiation
Particle radiation in the form of solar winds or cosmic rays can cause charge build-up and/or physical atom dislocation in PCBs. The particles can penetrate through the surface and permanently expose the components to charged and uncharged particles. They can also permanently damage a component after a single event depending on their energy level, known as a Single Event Effect (SEE).
To mitigate issues related to particle radiation, you can ask your CM to opt for ICs of the radiation-hardened version to withstand more doses of ionizing radiation.
To further ensure reliable PCB performance, source aerospace qualified electronic components and run your boards through highly accelerated life testing (HALT) alongside other test methods post assembly.
|Tempo's Custom Avionics for PCB Manufacturing Service
There are still many other design elements to consider that are beyond the scope of this blog. It is imperative to work with a contract manufacturer that is highly experienced with aerospace electronics. Tempo Automation is an industry leader in fast, high-quality prototyping and low-volume board circuits.
Tempo employs a white-box turnkey PCBA manufacturing process that promotes collaboration and transparency between engineers and CMs. This allows us to quickly deliver high-quality boards for both standard and non-standard designs that meet rigorous defense and aerospace industry criteria for prototyping and on-demand production. We also provide downloadable DRC files in Altium Designer, Cadence Allegro, Mentor Pads, other CAD formats, and Excel.