What materials are used in PCB in space?

Common PCB materials Used in Space

The most commonly used PCB materials for space applications include:

Polyimide

Polyimide (PI) is a high-performance polymer that offers excellent thermal stability, chemical resistance, and mechanical properties. It can maintain its properties over a wide temperature range from -269°C to +400°C. PI is often used as the base material for flex and rigid-flex PCBs in space.

Some commonly used PI films for space PCBs include:

• Dupont Kapton®: Kapton is a polyimide film developed by Dupont. It offers excellent thermal stability, low outgassing, and resistance to radiation. Kapton is widely used in space applications.

• UBE UPILEX®: UPILEX is another polyimide film that provides high heat resistance, low thermal expansion, and good electrical properties. It is suitable for high-reliability space PCBs.

PTFE (Teflon)

Polytetrafluoroethylene (PTFE), also known as Teflon, is a fluoropolymer with excellent dielectric properties and low loss at high frequencies. It offers high thermal stability, low moisture absorption and low outgassing. PTFE PCBs are often used in RF/microwave circuits for space communications.

Some PTFE-based laminates used for space PCBs include:

• Rogers RT/duroid®: RT/duroid laminates use PTFE composite as the dielectric material. They offer low dielectric loss, uniform electrical properties and good thermal stability. RT/duroid is commonly used for high-frequency space electronics.

• Gore PHASEFLEX®: PHASEFLEX is a PTFE-based material reinforced with ceramic fillers. It provides low loss, low skew, and phase stability for high-speed digital and RF applications in space.

Ceramic

Ceramic PCBs use inorganic ceramic materials as the substrate, such as alumina (Al2O3) or aluminum nitride (AlN). Ceramics offer high thermal conductivity, low CTE, and hermetic properties. They can operate in extreme temperatures and are resistant to radiation. Ceramic PCBs are often used for high-power, high-reliability space electronics.

Some ceramic PCB materials include:

• Alumina (Al2O3): Alumina is the most widely used ceramic substrate. It has high hardness, high thermal conductivity, and good electrical insulation. Alumina PCBs are suitable for high-temperature and high-voltage applications in space.

• Aluminum Nitride (AlN): AlN offers even higher thermal conductivity than alumina, making it ideal for heat dissipation. It also has low CTE, high dielectric strength, and low loss. AlN PCBs are used for high-power space electronics.

Factors to Consider When Choosing PCB Materials for Space

When selecting PCB materials for space applications, several key factors must be considered:

Outgassing

Materials used in space must have low outgassing to avoid contamination of sensitive equipment and degradation of the vacuum environment. Outgassing refers to the release of gas from a material when exposed to vacuum. The total mass loss (TML) and collected volatile condensable material (CVCM) are measured to assess the outgassing properties of materials. NASA has established outgassing requirements for space materials:

• TML < 1.0%
• CVCM < 0.1%

PCB materials that meet these requirements, such as polyimide, PTFE, and ceramics, are preferred for space use.

Thermal Stability

Space PCBs must withstand extreme temperature fluctuations, from the cryogenic temperatures of deep space to the high temperatures during spacecraft operation. The glass transition temperature (Tg) and thermal decomposition temperature (Td) are important indicators of a material’s thermal stability. Higher Tg and Td values signify better thermal performance.

Polyimide and PTFE have high Tg and Td, making them suitable for space applications. Ceramics also offer excellent high-temperature stability.

Coefficient of Thermal Expansion (CTE)

CTE mismatch between the PCB substrate and electronic components can cause stress and fatigue during thermal cycling, leading to reliability issues. Space PCBs require materials with low CTE to minimize this mismatch.

Polyimide has a relatively low CTE compared to other polymer substrates. PTFE-based laminates can be engineered to have controlled CTE by adding ceramic fillers. Ceramics, such as alumina and AlN, have very low CTE, making them compatible with bare die components.

Radiation Resistance

Space electronics are exposed to various types of radiation, including cosmic rays, solar wind, and trapped radiation belts. Radiation can cause damage to PCB materials, such as degradation of electrical and mechanical properties, and single-event effects in electronic components.

Polyimide and PTFE have good radiation resistance compared to other polymer materials. Ceramics are inherently resistant to radiation due to their inorganic nature. For high-radiation environments, such as in satellites and deep space probes, additional shielding or radiation-hardened components may be necessary.

Electrical Properties

PCB materials for space must have stable electrical properties over a wide frequency range and temperature range. The dielectric constant (Dk) and dissipation factor (Df) are key parameters that affect signal integrity and power loss.

PTFE has a low Dk and Df, making it suitable for high-frequency applications. Polyimide also has good electrical properties, with moderate Dk and low Df. Ceramics have high Dk but very low Df, which is advantageous for certain RF and microwave circuits.

Space-Grade PCB Manufacturing Considerations

In addition to material selection, the manufacturing process of space-grade PCBs must meet stringent quality and reliability standards. Some key considerations include:

Cleanliness

Space PCBs must be free of contaminants that could outgas or cause short circuits. Clean room manufacturing, ultrasonic cleaning, and special handling procedures are employed to ensure the cleanliness of the PCBs.

Controlled Impedance

For high-speed digital and RF applications, controlled impedance PCBs are essential to maintain signal integrity. The impedance of the PCB traces must be tightly controlled through proper material selection, stack-up design, and manufacturing processes.

Conformal Coating

Conformal coating is often applied to space PCBs to provide additional protection against moisture, contamination, and mechanical damage. Parylene and urethane are common conformal coating materials used in space applications.

Qualification and Testing

Space-grade PCBs undergo extensive qualification and testing to ensure their reliability and performance in the harsh space environment. This includes thermal cycling, vacuum testing, vibration and shock testing, and radiation testing. PCBs must pass these tests before they can be used in space missions.

FAQ

Q: What is the most commonly used PCB material for space applications?

A: Polyimide (PI) is one of the most widely used PCB materials for space due to its excellent thermal stability, chemical resistance, and mechanical properties. PI films like Kapton and UPILEX are commonly used as base materials for space PCBs.

Q: Why is low outgassing important for space PCB materials?

A: Low outgassing is critical for space materials to prevent contamination of sensitive equipment and degradation of the vacuum environment. Outgassing can lead to condensation of volatile materials on optical surfaces, solar panels, and thermal radiators, affecting their performance. NASA has established outgassing requirements (TML < 1.0%, CVCM < 0.1%) for space materials.

Q: What are the advantages of using ceramic PCBs in space?

A: Ceramic PCBs offer several advantages for space applications, including high thermal conductivity, low CTE, and hermetic properties. They can operate in extreme temperatures and are resistant to radiation. Ceramic materials like alumina and aluminum nitride are used for high-power, high-reliability space electronics.

Q: How does radiation affect PCB materials in space?

A: Radiation in space can cause damage to PCB materials, such as degradation of electrical and mechanical properties, and single-event effects in electronic components. Polyimide, PTFE, and ceramics have good radiation resistance compared to other materials. For high-radiation environments, additional shielding or radiation-hardened components may be necessary.

Q: What special manufacturing considerations are required for space-grade PCBs?

A: Space-grade PCBs must meet stringent quality and reliability standards. Clean room manufacturing, ultrasonic cleaning, and special handling procedures are used to ensure PCB cleanliness. Controlled impedance, conformal coating, and extensive qualification testing are also important considerations for space PCB manufacturing.

Conclusion

The selection of PCB materials for space applications is a critical factor in ensuring the reliability and performance of spacecraft electronics. Polyimide, PTFE, and ceramics are commonly used materials that offer excellent thermal stability, low outgassing, and good electrical properties. When choosing PCB materials for space, factors such as outgassing, thermal stability, CTE, radiation resistance, and electrical properties must be carefully considered.

Space-grade PCB manufacturing also requires special considerations, including cleanliness, controlled impedance, conformal coating, and extensive qualification testing. By using appropriate materials and manufacturing processes, PCBs can be designed to withstand the harsh environment of space and provide reliable operation for mission-critical electronics.

As space exploration continues to advance, the development of new PCB materials and technologies will be essential to meet the evolving demands of space applications. Researchers and engineers are continuously working on improving the performance, reliability, and cost-effectiveness of space-grade PCBs, enabling future space missions to push the boundaries of scientific discovery and technological innovation.