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PCB Design for Military and Aerospace Applications

Source: Cadence Blog

PCB design for military applications

Key Takeaways

  • Adhere to IPC and MIL standards for documentation, manufacturing, and testing to meet military and aerospace requirements.
  • Choose high-temperature laminates and protective coatings to withstand extreme conditions and environmental factors.
  • Design for higher current capacity with shorter traces and heavy copper (3-4 ounces per square foot); maintain a low aspect ratio (1:10 or less) for stability.

Military and aerospace PCBs must adhere to intensely high standards to ensure their safety and functionality in the extreme conditions they're used in. For this reason, PCB design for military and aerospace applications requires incredibly stringent guidelines with built in redundancy, extra safety, and stringent tolerances, minimizing risk of failure.

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PCB Design for Military and Aerospace Applications

Category Tips and Considerations
Component Selection
  • Use mil-spec components with low tolerances (1-2%) rather than standard 5~10% tolerance components.
Current Capacity
  • Design with additional current capacity to manage unexpected spikes.
  • Handle maximum current load with shorter traces, heavy copper (90 to 120 grams per m²), and heat dissipation techniques.
Aspect Ratio
  • Maintain a low aspect ratio (1:10 or less) to ensure reliability and stability in harsh environments.
  • Recommended through-hole aspect ratio is 10:1 with a maximum board thickness of 100 mil.
Plane Separation
  • Separate power and ground planes; segregate low-frequency components from high-frequency ones to reduce interference.
  • Minimum dielectric thickness between planes should be 3.5 mil.
Signal Protection
  • Shield clock signals using materials like aluminum.
  • Protect other critical signals especially in RF applications. Guarding the clock signal with an aluminum enclosure can resolve EMI issues.
Annular Ring
  • The copper pad area around the drilled hole should have a defined width of 6-7 mils minimum for the external and internal layers.
  • Recommended drill to copper clearance is 7-8 mil.

Performance and Testing Standards for Military and Aerospace Applications

In PCB design for military and aerospace applications, adhering to standards is critical. Key IPC documents such as IPC-2610 Series, IPC-D-325, and IPC-J-STD-001, including the Space Addendum, provide comprehensive guidelines for PCB documentation, manufacturing, and assembly. Specific applications, including those for the Navy, FAA, and satellites, require compliance with IPC standards for materials and stack-ups.

Performance and testing standards such as MIL-PRF-50884, MIL-PRF-31032, MIL-PRF-55110, and MIL-PRF-19500 are mandatory for military PCBs, while AS9100 and AS9100D apply to aerospace PCBs. Additionally, compliance with the International Traffic in Arms Regulation (ITAR) is essential for military and aerospace PCB assembly. Regulated by the Department of State, ITAR mandates strict control over sensitive information related to the design and production of military and intelligence devices, ensuring the highest level of security and reflecting current technological, political, and security climates.

Wrap Plating

Wrap plating involves extending copper plating from the drilled hole onto the board surface. The IPC-6012 and IPC-A-600 standards define three classes for electrolytic hole plating thickness:

  • Class I: Thickness agreed upon between user and supplier (AABUS).
  • Class II: Continuous wrap plating through filled plated holes, extending onto the PCB surface, with a minimum thickness of 5 μm. Processes like planarization should not reduce this thickness.
  • Class III: Minimum wrap plating thickness of 12 μm for through-holes and vias greater than 2 layers. Buried via cores require at least 7 μm, and blind / buried microvias need a minimum of 6 μm.

Challenges in wrap plating include knee cracking from thermal cycles, butt joint failures, and maintaining minimum thickness standards during the planarization process, which can vary by ±0.3 mils.

Military and Aerospace Applications Require PCB Design for Harsh Conditions

Category Tips and Considerations
Acceleration and Vibration
  • Design to withstand forces from rapid accelerations. Use through-hole assembly for strong component adherence, which also improves signal transmission.
  • Mount military-grade components to minimize vibration effects. Design a symmetrical stackup with uniform copper distribution for enhanced stability.
Lightning Protection
  • Include surge protection circuits and use materials that are resistant to high transient voltages to safeguard against lightning strikes.
Radiation Effects
  • Employ radiation-hardened designs to protect from ionizing radiation in space and high-altitude applications.
  • Use rad-hard components as per military standards (MIL-PRF-38535), and incorporate antifuse technology in aerospace PCB assembly for enhanced reliability and security.
Packaging Challenges
  • Design rugged packaging to protect the electronics from shocks, vibrations, and environmental factors. This includes creating enclosures that can withstand external pressures and impacts.
Test Coverage and Access
  • Ensure comprehensive test coverage and facilitate easy test access by incorporating test points and in-circuit testing to verify functionality and durability under test conditions.

Material Considerations for Military and Aerospace Applications

Category Tips and Considerations
Resins and Foils
  • Select resins and foils that perform well under extreme conditions for optimal electrical and thermal performance.
  • Suitable materials include high-temperature laminates with substrates like copper or aluminum.
  • Recommended materials are FR408, Pyralux AP, and Nelco N7000-2HT, all known for their high Tg (glass transition temperature) values.
Moisture, Salt, and Sand Effects
  • Opt for materials that resist corrosion and environmental degradation. To safeguard PCB components from moisture, humidity, and extreme temperatures, a conformal coating of acrylic resin applied via spray is advised.
  • Preferred surface finishes include electroless nickel with immersion gold coating (ENIG) or hot air solder leveling (HASL).
Conformal Coatings, Potting Compounds, Staking Compounds
  • Utilize these materials to protect components from environmental damage while improving insulation and thermal management.

Conformal Coating

For extreme operating conditions, choose PCB finishing materials like electrolytic nickel gold, ENIG, chemical silver, HASL, and lead-free HASL. Conformal coatings protect against heat, humidity, water, and vibrations, crucial for aerospace applications. Additionally, applying an acrylic-based spray protects the PCB from contamination and short circuits.

Allegro X System Capture is a comprehensive tool for PCB design, particularly suited for high-reliability applications such as military and aerospace. The tool integrates various analyses to ensure the robustness and reliability of PCB designs under extreme conditions.

Allegro X Aids in PCB Design for Military and Aerospace Applications

Allegro X provides Electrical Stress Analysis, which helps in identifying components that might be operating beyond their safe limits, ensuring that the design can handle worst-case scenarios. The key features include:

  • Partitioning the PCB Circuit: The design is divided into multiple subcircuits, making the analysis more manageable and accurate.
  • Parameter Calculation: It calculates parameters like power dissipation, voltage, and current for various components, including mixed-signal, analog, digital, and RF components.
  • Derating Reports: The tool generates derating reports, which predict the circuit performance under worst-case conditions, helping in ensuring the longevity and reliability of the components.

Electrical Stress Analysis Results Dashboard within Allegro X System Capture displaying the design summary and estimated stress for all the devices analyzed.

Electrical Stress Analysis Results Dashboard within Allegro X System Capture
MTBF Analysis Results Dashboard within Allegro X System Capture

Allegro X Design for Reliability

For aerospace applications, reliability is paramount. Allegro X supports various analyses to ensure the reliability of the PCB design:

  • Mean Time Between Failures (MTBF) Analysis: This estimates the durability and safety of electrical and electro-mechanical parts. The tool supports standards like MIL-HDBK 217F and FIDES for real-time reliability analysis.
  • Power Topology Analysis: This validates the power distribution network across the PCB, ensuring that all components receive adequate power without causing excessive heat or voltage drops.
  • Thermal Analysis with Celsius Thermal Solver: Integrated thermal analysis allows for early detection of potential thermal issues, helping in optimizing the placement of components and ensuring effective heat dissipation.

MTBF Analysis Results Dashboard within Allegro X System Capture displaying the design summary and estimated life expectancy of the PCB.

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