A Guide to PCB Design for Military and Aerospace Systems
Printed Circuit Boards (PCBs) are the backbone of virtually every electronic system used across industries, including military and aerospace. These industries demand highly reliable, robust, and high-performance PCBs that endure the harshest operating conditions, like extreme temperatures, vibration, etc. The materials used in military and aerospace PCBs provide durability, thermal stability, and signal integrity. This is why it is essential to choose the right materials for PCBs. This post explores the key materials used in military and aerospace PCBs, explaining how these materials contribute to performance in challenging environments. It will also cover the PCB design tips for military and aerospace applications, and essential performance and testing standards.
Material Considerations for Military & Aerospace Applications
Below is material considerations for military and aerospace PCB applications:
- Substrates: Consider choosing substrates that exhibit excellent performance even in harsh environmental conditions to ensure superior electrical and thermal functionality. Opt for materials such as high-temperature laminates featuring substrates like copper or aluminum for enhanced performance. Some recommended materials to consider are FR408, Pyralux AP, and Nelco N7000-2HT, renowned for their high Tg (glass transition temperature) values, which make them particularly suitable for demanding applications.
- Coating and Surface Treatment: Select materials that can withstand corrosion and environmental degradation. To protect PCB components from moisture, humidity, and extreme temperatures, apply a conformal acrylic resin coating using a spray method. Regarding surface finishes, it's preferable to use electroless nickel with immersion gold coating (ENIG) or hot air solder leveling (HASL). These finishes offer enhanced protection and durability for the PCB components. In addition, applying conformal coatings is essential as they offer protection against factors like heat, humidity, water, and vibrations. Furthermore, an acrylic-based spray can prevent the PCB from contamination and short circuits
Performance and Testing Standards for Military & Aerospace Applications
In military and aerospace PCB design, strict performance and testing standards are critical to ensure reliability, safety, and durability under extreme conditions.
- IPC-2610 Series, IPC-D-325, IPC-J-STD-001 (with Space Addendum): These standards provide detailed guidelines for PCB documentation, manufacturing, and assembly in military and aerospace applications.
Specific applications such as Navy, Federal Aviation Administration (FAA), and satellite systems must meet IPC standards for materials and stack-ups to ensure high performance.
- Mandatory Military PCB Standards: The PCBs must meet the below military standards:
- MIL-PRF-50884: Standards for flexible PCBs.
- MIL-PRF-31032: Standards for high-reliability, rigid PCBs.
- MIL-PRF-55110: Standards for rigid PCBs used in military applications.
- MIL-PRF-19500: Standards for semiconductor devices used in military electronics.
- AS9100 and AS9100D: Industry standards specifically for aerospace systems, ensuring quality management in the design and manufacture of aerospace PCBs.
- International Traffic in Arms Regulation (ITAR): The U.S. government regulation controls the export and handling of sensitive military and aerospace technology. ITAR ensures that military and aerospace PCBs are designed and manufactured with strict controls over sensitive information, reflecting current security, technological, and political landscapes.
General PCB Design Tips for Military and Aerospace Applications
Here are some general PCB design tips for military and aerospace applications:
- Component selection: It is preferable to utilize military specification (mil-spec) components with low tolerances of 1-2% instead of using standard components with 5-10% tolerance. Low tolerance components help ensure that each part operates in the narrow range of acceptance values, reducing the likelihood of performance discrepancies.
- Current capacity: Ensure the design includes extra capacity to effectively handle any unexpected spikes. To manage the maximum current load, use shorter traces, heavy copper (3-4 ounces per square foot), and implement heat dissipation techniques to handle the load effectively.
- Plane separation: For better performance and reduced interference, it's essential to have separate power and ground planes. It is also better to separate low-frequency components from high-frequency ones. Additionally, there should be a minimum dielectric thickness of 3.5 mil between the planes.
- Aspect ratio: Maintaining a low aspect ratio of 1:10 or lower is recommended for reliable and stable performance in challenging conditions. A through-hole aspect ratio of 10:1 is best suited for boards with a maximum thickness of 100 mil.
- Shock and Vibration: The operating conditions may shake a PCB. The circuit board design must be devoid of geometrical deformities like twist, bows, warping, and so on to minimize the impact of shocks or vibrations. A symmetrical stack-up with uniform copper distribution is recommended in military and aerospace PCBs to reduce the effects of vibration and shocks.
- Signal protection: Consider using materials such as aluminum to shield clock signals and protect other critical signals, particularly in RF applications. Employing an aluminum enclosure to guard the clock signal can effectively address electromagnetic interference (EMI) issues.
- Component Placement. Ensure that heat-generating parts are separated from thermally sensitive components, like converters or regulators. Maintain a large clearance space between them.
As military defense and aerospace have become higher priorities, there is a greater demand for stronger and more adaptable circuit board assemblies. Circuit board designs and constructions must withstand extreme environments and be protected from contaminants like dirt, dust, and corrosion. Accelerated Assemblies, a leading manufacturer and supplier of circuit board assemblies for military and aerospace applications, can help create new customized circuit boards that meet the latest requirements. Contact their team to discuss your requirements.