RT-duroid® microwave laminates are highly regarded for their exceptional electrical, thermal, and mechanical properties. These attributes have been leveraged in a cryogenic stripline application at NASA, where the material’s performance under extreme low-temperature conditions was critical.

1. Introduction to Cryogenic Applications

Challenges in Cryogenic Environments

1. Thermal Contraction:
   • Materials experience significant dimensional changes due to extreme temperature variations.
2. Electrical Stability:
   • Dielectric properties can shift at low temperatures, impacting signal integrity.
3. Mechanical Stress:
   • Differential thermal contraction between materials (e.g., copper and PTFE) can lead to delamination or cracking.

Importance in NASA Applications

• Cryogenic applications, such as deep-space communication systems, radar systems, and satellite electronics, demand materials with minimal thermal and electrical property variations.

2. RT-duroid® Laminate Properties at Cryogenic Temperatures

Key Material Properties

1. Low Dielectric Constant (ϵr\epsilon_rϵr​):
   • 5870: ϵr=2.33±0.02\epsilon_r = 2.33 \pm 0.02ϵr​=2.33±0.02
   • 5880: ϵr=2.20±0.02\epsilon_r = 2.20 \pm 0.02ϵr​=2.20±0.02
   • Ensures stable impedance matching even under extreme conditions.
2. Low Loss Tangent (tan⁡δ\tan\deltatanδ):
   • Remains minimal, preserving signal integrity for high-frequency applications.
3. Thermal Stability:
   • PTFE-based composites exhibit minimal thermal contraction and excellent dimensional stability.
4. Low Outgassing:
   • Suitable for vacuum environments, essential for space applications.

Performance Under Cryogenic Conditions

• Tests at cryogenic temperatures (−196∘-196^\circ−196∘C) showed negligible shifts in:
  • Dielectric constant.
  • Loss tangent.
  • Physical integrity of the laminate.

3. Stripline Design for Cryogenic Application

Stripline Configuration

• Signal Conductor:
  A single conductor embedded between two ground planes for electromagnetic shielding.
• Substrate Material:
  RT-duroid® laminate ensures consistent dielectric properties.
• Cryogenic Compatibility:
  The laminate’s near-isotropic properties minimize stress from thermal contraction.

Design Considerations

1. Impedance Control:
   • Precise control over conductor width and spacing to maintain target impedance.
2. Thermal Contraction Management:
   • Use of compatible materials with matched coefficients of thermal expansion (CTE).
3. Layer Bonding:
   • Reliable adhesion maintained at low temperatures, preventing delamination.

4. NASA Cryogenic Testing and Results

Test Setup

1. Temperature Range:
   • Ambient to cryogenic temperatures (−196∘-196^\circ−196∘C).
2. Test Parameters:
   • Signal transmission characteristics.
   • Physical integrity (dimensional stability, adhesion).
3. Application Context:
   • High-frequency communication lines in satellite systems.

Performance Results

• Electrical Performance:
  • Minimal variation in impedance and loss, demonstrating excellent RF signal integrity.
• Dimensional Stability:
  • No significant warping or shrinkage.
• Mechanical Integrity:
  • Copper adhesion to the laminate was unaffected by repeated thermal cycling.

5. Advantages of RT-duroid® Laminates for NASA Applications

a. Electrical Performance

• Stable dielectric properties ensure reliable performance across wide frequency ranges, even in cryogenic conditions.

b. Thermal and Mechanical Stability

• Minimal CTE mismatch with conductive layers reduces stress during thermal cycling.

c. Long-Term Reliability

• Resistance to outgassing and chemical degradation makes RT-duroid® ideal for space and cryogenic applications.

6. Summary of Findings

Property	Cryogenic Performance
Dielectric Constant	Stable, ensuring consistent impedance.
Loss Tangent	Low, maintaining signal clarity and efficiency.
Dimensional Stability	Excellent, no warping or shrinkage observed.
Adhesion	Reliable copper-to-laminate bond even after cycling.
Thermal Cycling	No delamination or mechanical degradation.

7. Conclusion

The RT-duroid® laminates demonstrated exceptional performance in NASA’s cryogenic stripline applications. Their ability to maintain electrical and mechanical integrity under extreme low-temperature conditions makes them a prime choice for aerospace and deep-space missions.