In aviation, there’s no room for error. Every component must function flawlessly – from navigation and communication systems to flight controls and instrumentation – to ensure a safe and successful flight. And at the heart of these critical systems are printed circuit boards, or PCBs, that power and control every aspect of the aircraft.
Designing and building PCBs for avionics requires a unique set of considerations that go far beyond traditional circuit board design. Engineers must carefully consider factors such as signal integrity, power distribution, electromagnetic interference, and thermal management to ensure reliable and consistent performance in extreme flight conditions.
In this article, we’ll dive into the essential design considerations for building PCBs for avionics, offering comprehensive insights to help you navigate the complexities of this demanding project.
Choose the Right Materials
To ensure optimal performance and reliability, consider designing avionic PCBs with substrate materials that can withstand high temperatures. This requires careful consideration of the substrate’s heat resistance (Tg) value, with a minimum margin of 20°C included to ensure proper functioning in the operating environment.
For example, if the required operating temperature is 160°C, a substrate with a Tg value of 180°C is recommended. High-temperature laminates with copper or aluminum substrates are well-suited for these applications, as they can withstand the thermal stresses of extreme temperatures.
Another good option is polyimide. It boasts a high Tg value, often in the range of 250°C or more, which allows it to perform well under extreme thermal stress. Additionally, this substrate material is highly resistant to chemicals, making it an ideal choice for harsh environments where chemical exposure is a concern. Its flexibility and durability also make it well-suited for applications requiring mechanical stress resistance, such as aerospace and defense systems.
Design and layout
When designing aerospace and military printed circuit boards, it’s essential to use mil-spec parts with low tolerances of 1%-2% to ensure optimal performance. Additionally, it’s crucial to add extra current cushions to handle unexpected anomalies and ensure stability. For example, if a design requires a maximum load of 2 amperes, it’s best to build with 3 amperes in mind.
The aspect ratio (AR) is also a crucial factor affecting the PCB’s stability. The recommended AR ratio is 1:10 or less to ensure reliability. Higher ARs can lead to PCB failure, especially in harsh environmental conditions, and they are challenging and expensive to build.
You may also want to consider separating power and ground planes and low-frequency parts from high-frequency ones. Rotating parts at higher frequencies can generate waveforms that interfere with lower-frequency parts, resulting in lower signal integrity.
Finally, it’s important to prove impedance calculations and carry out pre-layout simulations since it’s challenging to test aerospace and military PCBs in natural environments. The simulation program can help to confirm the various loads at specific points and offer recommendations for layout modifications, guaranteeing optimal performance.
PCB designers in the avionics industry must consider thermal management a critical factor when designing boards for use in space. Once a satellite is in orbit, it is impossible to replace any defective boards, making failure due to heat buildup unacceptable. Without air, convection cooling is not an option, so designers must rely on metal’s natural conduction properties to dissipate unwanted heat.
However, the heavy copper commonly used in multilayer PCBs with 4-oz copper and 20 layers in the aerospace industry presents a significant challenge for fabrication and assembly. While heavy copper does help to dissipate heat during active use, it also conducts heat away from SMD terminals during reflow soldering, making it difficult to achieve reliable solder joints.
Essentially, the PCB assembly process must compensate by having a longer pre-heat time, which increases the prospect of delamination as copper traces heat up to a higher temperature. Cooling time after soldering is also longer, which may allow vibrations to displace SMD components before the solder solidifies. Therefore, designers must consider thermal management challenges when choosing materials and planning the assembly process to ensure reliable avionics PCB performance.
Handling RF (Radio Frequency)
Radiofrequency operation is also crucial when designing a PCB for avionics. In military and aerospace applications, RF is used for communication, and the RF components must meet the performance specifications described in the MIL-PRF-19500 and MIL-STD-461 standards. Ideally, the PCB must be designed to prevent electromagnetic interference (EMI) and ensure that it does not interrupt other surrounding devices.
The type of PCB used in military and aerospace applications will depend on its function and design. Rigid PCBs are suitable for aircraft cockpit instrumentation, auxiliary power units, control tower instrumentation, and other similar uses.
Testing After Manufacturing
Testing is a crucial step that ensures the board meets the required specifications and functions reliably. The testing process involves several stages, including visual inspection, automated optical inspection, electrical testing, and environmental testing.
Visual inspection is the first stage of testing, where the board is checked for any defects or manufacturing errors. This is done by checking the board for any misaligned components, incorrect soldering, or physical damage that could affect the board’s performance.
Automated optical inspection (AOI) is the next stage of testing, where the board is scanned for any defects using a machine vision system. AOI checks for defects such as incorrect component placement, solder joint quality, and missing components.
The electrical testing stage involves testing the board’s electrical characteristics to ensure that it functions as intended. This is done using automated test equipment (ATE) that checks the board’s functionality, electrical parameters, and signal integrity.
Finally, you should conduct environmental testing. This involves subjecting the board to extreme temperatures, humidity, and vibrations to test its resilience under harsh conditions. This is important for aerospace and aviation applications where the circuit board must function reliably in extreme environments.
Designing printed PCBs for avionics requires careful consideration of various factors to ensure reliability and optimal performance. To ensure that these considerations are properly addressed, we recommend working with an experienced EMS PCB assembly company that can provide expertise in PCB design, manufacturing, and testing. With the right partner, avionics manufacturers can produce high-quality, reliable PCBs that meet the needs of their demanding applications.