An thorough grasp of how printed circuits must be developed and constructed to provide a very extended duration of operation, in frequently quite harsh working circumstances, is essential for space missions. Printed circuit boards used in aircraft applications are exposed to harsh environmental conditions, radiation, chemicals, pollutants, and more, in contrast to the majority of typical PCBs. As a result, these circuits must adhere to highly tight requirements, such as IPC-A-610E Class 3, which applies to high-performance electronic devices whose uninterrupted functioning is required even under the most demanding and challenging operating circumstances. The following are some of the principal uses covered by the IPC-A-610E Class 3 standard:
- applications for on-ground processing of data captured during flights or missions (ground stations);
- navigation systems;
- control systems;
- on-board avionic instruments;
- passive detection systems;
- unmanned aerial vehicles (UAVs);
- (Unmanned Aerial Vehicles).
Circuits used in aircraft applications must be able to tolerate and absorb significant shocks and vibrations as well as operate at extremely high temperatures. Considering that radio communication in the HF (or higher) band is a crucial component of these systems, they also share many aspects with PCBs for RF high frequency applications. Operating at high altitudes increases the danger of radiation exposure, thus PCBs and other electronic components must be built to tolerate high radiation levels for extended periods of time without suffering damage. The most typical principles and suggestions for helping the designer with the challenging but ultimately enjoyable work of developing a PCB for aerospace usage will be provided in the paragraphs that follow.
1 – Choose materials of superior quality
Reliability and durability are two specifications that must be met in the aircraft industry. Given that maintenance interventions are either impossible or extremely expensive in these types of applications, the circuits must perform constantly and without failure over extended periods of time (often from a minimum of 5 years to 15 years or more). Utilizing high-quality materials and components that are readily available on the market for extended periods of time is the usual guideline.
Anodized aluminium is a frequently used substitute for copper that can address a number of heat dissipation issues. In actuality, anodized aluminium has ten times less thickness and a thermal conductivity that is 5 to 10 times higher than that of conventional materials like FR-4. Additionally, it decreases the impacts of heat-induced oxidation and is considerably more effective in transferring heat than a traditional PCB.
2 – Use heavy copper technology
With copper thicknesses between 2 and 6 oz/ft2 (or more), heavy copper technology enables heat dissipation naturally without the need for extra cooling systems, even in the presence of high intensity currents. To further enhance heat dissipation, several manufacturers advise combining heavy copper solutions with the addition of multiple thermal vias. Figure 1 depicts a multilayer PCB with a thick copper layer in detail.
3 – Follow reference guidelines
PCBs used in the aircraft sector must function with little maintenance and adhere to exacting safety and quality criteria. Because of this, PCB designers and manufacturers for aircraft applications are required to adhere to a certain set of reference standards. The IPC 6012DS, an amendment to the IPC-6012D standard that specifies certification and performance criteria for rigid printed circuit boards for aerospace and defence applications, is one of the reference standards. You may think of this standard as an improved version of IPC Class 3.
The aerospace standard AS/EN 9100, which includes a collection of guidelines created by the IAQG (International Aircraft Quality Group) for quality and risk management in the aerospace industry, is also crucial. This international standard embodies the quality management system that is appropriate for the aerospace sector. The AS/EN 9100 standard adds further standards made especially for the aerospace environment in comparison to the ISO9001 standard, with which it shares many components. PCBs created for this kind of application are required to adhere to the standard and come with a certification attesting to the manufacturing process’ high quality.
4 – Offer superb thermal management
As was already established, aircraft PCBs need to provide outstanding heat dissipation without the need for external heatsinks. Special solutions based on materials like Pyralux AP, FR408, and other metal materials and components can be employed in addition to heavy copper technology and the widespread usage of thermal vias. In contrast to conventional PCBs, it is also recommended to widen the gap between the parts so that there is more room for heat dissipation.
5 – Use conformal coating
The PCB finishing materials should be selected to resist the most demanding working circumstances. Electrolytic nickel gold, ENIG (Electroless Nickel with Immersion Gold Coating), chemical silver, HASL (Hot Air Solder Leveling), and lead-free HASL are some of the common conformal coating methods. The conformal coating’s application offers defence against the heat, humidity, wetness, and vibrations that can all be present in aerospace applications. In order to shield the finished printed circuit from contamination or unintentional short circuits, conformal coating should be applied after acrylic-based spray. The detail of a PCB with HASL conformal coating is shown in Figure 2.
6 – Routing recommendations
In order to provide effective heat dissipation under all working situations, PCB traces should be selected for a size that can manage the highest current load. Angles on the traces must be smaller than 45°, as is the case with circuits with high frequency signals, to ensure that the signal is sent uniformly and regularly across the circuit. Separating low-frequency from high-frequency electronic components will help prevent interference. In fact, the latter can produce waveforms and disturbances that can affect how low frequency components function. The waveforms and noise reduce the signal’s quality, jeopardising the signal’s integrity, which is essential for these applications. When designing, aluminium or other comparable material casings must be used to produce the necessary physical shielding for the clock signals. The criteria must be followed in order to lessen or restrict the crosstalk phenomenon between neighbouring traces, just like it does in any RF printed circuit.
7 – Using flexible and flexible-rigid PCBs
Flexible and rigid-flexible printed circuit boards are quite popular in satellite and avionics systems, however unlike industrial or automotive applications, they are often made using polyamide rather than FR-4. This material has the capacity to adapt to tiny places with ease, is extremely light, resistant to heat and chemical agents, and ensures a high level of durability.
Due to its strong resistance to vibrations, shocks, temperature, and outside agents, superior mechanical and electrical connection, and low weight, flex and rigid-flex PCBs are widely utilised in the aircraft sector. A stiff-flex PCB is made up of printed circuit boards that are both rigid and flexible and are permanently attached to one another. For challenging and constrained space applications, the proper use of rigid-flex and flexible PCBs offers an ideal solution. With less connections and a tight connection between all of the circuit’s components, this technique also ensures contact stability and polarity.
All electronic circuits that must adhere to the IPC-A-610 Class 3 and 3A standard, including printed circuits for the aerospace industry, must be developed from the start with the goal of achieving high electrical reliability, particularly under the most challenging and unusual operating situations. Every electronic designer has an extremely difficult issue with PCB design, starting with choosing the best materials to survive harsh weather conditions with ongoing failure-free operation. In order to better comprehend the problems and offer a point of reference for the design of printed circuit boards for aerospace applications, some helpful suggestions have been offered in this article.