PCB design considerations for aerospace and military applications

By Linda Liu

SHENZHEN, China - Printed circuit board (PCB)designs for aerospace and military applications should factor in expectations such as longer product lifespans, high-performance, and temperature swings. Aerospace and military PCBs should be more reliable, stable, and rugged compared to other electronics. This calls for stringent design considerations.

There are multiple design considerations for building aerospace and military circuit boards. This article takes a deep dive into three main factors for designing aerospace and military circuits: design and layout, thermal management, and radiofrequency.

Design and Layout

PCB manufacturers handling aerospace and military products must populate their designs with mil-spec parts and not the standard ones. Mil-spec elements provide low tolerances of 1-2%, whereas the standard ones display high tolerances of between 5-10%.

Manufacturers should consider additional current cushions to strengthen the current in aerospace and military circuit boards. If the requirements indicate a maximum load of 2 amperes, the designers should build with 3 amperes in mind. This additional margin is essential in controlling undue spikes or any other unexpected anomaly.

The aspect ratio (AR) also affects the stability of the board. Basically, AR is how the PCB thickness compares with via thinness. The recommended AR ratio is 1:10 or anything less. Higher ARs threaten PCB reliability. In addition, higher AR boards are challenging and expensive to build. They need more accuracy yet present high failure chances, particularly in harsh environmental conditions. 

In such designs, you must separate power and ground planes. Additionally, you should separate low-frequency parts from their high-frequency counterparts. Rotating parts at higher frequencies produces waveforms that hinder lower frequency parts. Furthermore, these effects lower the signal integrity by making noise.

You should clean and protect your clock signals. During the design stage, you can do this using aluminum or a similar substrate to protect and complement your clock design.

Besides, you should prove your impedance calculations and carry out pre-layout simulations since it is pretty challenging to test aerospace and military PCBs in natural environments. There is a simulation program that will help you conduct this process. The program confirms the various loads at particular points and offers recommendations for layout modifications, guaranteeing optimum performance. It creates a similar working environment; hence, the results are closely related.

You should keep your board routings maximally at 450C and curve them. This maintains the conducted current effectively throughout the board. Trace angles of 90⁰ C are forbidden since the current transmission reaches the upper point of the 90⁰. They then turn and echo back, generating a ripple effect and consequently creating a poor signal.

Mechanical apertures and other measurements should contain an additional consideration to ensure higher efficiency and stability when necessary. For instance, a cable cut-out can utilize an extra twist space. In case it is damaged, the board incurs negligible impacts.

You should also shield all primary circuit board modules. You can achieve this by designing the modules to be separately tested out of the system. Besides, you must consider the PCB manufacturing process as part and parcel of your design because PCB materials affect the design and layout process.

Finally, you should use FR-4, G10, polyimide, or Cynanate Ester PCB materials for low-frequency use cases. On the other hand, use Rogers Series RO4003, Duroids, Polyimide, or other Teflon-based PCB materials for high-frequency applications. 

Thermal Management

The component application should be the first aspect you should consider to manage heat. For instance, LEDs emit a lot of heat and power. If your board requires LEDs, you should look for other options, like low-power LEDs. If your application requires ball-grid arrays (BGAs), ensure their top layer is metallic to absorb the heat and scatter it in the device.

Remember that the circuitry also generates a lot of heat. Therefore, it is recommendable to distinguish analog from digital circuitry. This is because waveforms produced by high-power analogs significantly hinder the digital circuitry waveforms. To prevent this issue, you should effectively sandwich the two circuitry between ground planes, subduing any damaging signal.  

Thermal management also entails the proper selection of PCB materials and parts that can tolerate high temperatures. Metal-core boards, for example, are applied in aerospace and military devices that produce low amounts of heat – one part is epoxy-based, and the other one is aluminum-based.

You can also use laser drills to create more space on your PCB. If you require more space, laser drills will reduce your vias from 15 to 5 mils. You can use the extra space to lengthen the ground plane to the surface, acting as a heat transmission tool.

Handling Radio Frequency (RF)

RF refers to the signals transmitted at high-frequency (500 MHZ to 2 GHz) along a regulated impedance line, using the ground to maintain signal integrity. For instance, in a coaxial cable, the RF conducts current through wires, creating electromagnetic fields. These fields create minor impacts at low frequencies, but the effects become more pronounced at high frequencies. In such cases, you need connectors to create successful conduction lines since they mate without affecting the signal quality. 

When matching PCB impedances, it is important to ensure that when the signal enters the power source outside the board, less signal loss is sustained by applying a non-functional RF cable or shield.

You also need a transmission signal length to prevent any mismatch between the conduction and reception. A mismatch generates an imbalanced return signal, which creates an incoming wave, lowering the signal integrity.

When the mismatch is high, the inbound signal can be lost since the return signal is quite strong, revoking it completely. For radio frequency applications, you should ensure that the transmission and return signals match. They should also be short and move from one point to the other with minimal curves.   

Remember to shield your RF signals since they are susceptible to frequencies. Otherwise, they will create a ripple or skin effect. To reduce these effects, ensure you protect your signals well. In some situations, you can develop antennas in a shielding pattern, away from the noise-producing parts of the PCB.

Conclusion

PCBs for aerospace and military applications should be reliable, robust, and rugged than other boards. To achieve this, you must factor in various considerations, like design and layout, thermal management, and radiofrequency, from the beginning of your project. This article has offered a detailed analysis of those factors, and we believe you can now design a quality PCB for use in aerospace and military projects.

Author bio

Linda Liu is the overseas marketing manager for MKTPCB, a leading PCB manufacturer that offers high-quality PCB products and services. Since 2012, she has established “first-of-its-kind” industry-changing and transformational businesses initiatives that increased revenue growth, brand exposure and market expansion for MKTPCB. Linda graduated from Western University with a bachelors degree in marketing.