RT-duroid® laminates are widely used for high-frequency applications, offering excellent dielectric properties and dimensional stability. However, after the etching process used to define circuit patterns, internal stresses can develop in the laminate, potentially affecting its performance and reliability. Stress relief processes are often employed to mitigate these effects.

1. Understanding After-Etch Stress

a. Sources of Stress

1. Copper Removal:
   • Etching removes significant areas of the copper foil, altering the stress distribution across the laminate.
   • As copper and PTFE have different coefficients of thermal expansion (CTE), the removal of copper can cause residual stresses.
2. Thermal and Mechanical Effects:
   • Heat and chemical exposure during etching introduce thermal stresses.
   • Mechanical stresses can also arise during handling and processing.

b. Effects on the Laminate

• Dimensional Instability:
  • Warping or distortion due to uneven stress distribution.
• Electrical Property Deviation:
  • Changes in circuit impedance due to physical deformation.
• Delamination Risk:
  • Weakening of the bond between copper foil and the PTFE substrate.

2. Stress Relief Methods

a. Thermal Stress Relief

1. Post-Etch Baking:
   • Laminates are subjected to a controlled heating process to relieve internal stresses.
   • Typical temperature: 100°C to 125°C.
   • Duration: 1–2 hours in a convection oven or as per material-specific recommendations.
2. Gradual Cooling:
   • Controlled cooling prevents the introduction of new stresses during the process.

b. Mechanical Conditioning

1. Pressing:
   • Apply light uniform pressure during the stress relief bake to ensure flatness.
   • Use a weighted or vacuum press system to maintain alignment.

c. Chemical Neutralization

• Neutralizing Residual Etchants:
  • Thorough cleaning with neutralizing agents to remove chemical residues that may weaken the bond or exacerbate stress.

3. Recommendations for Effective Stress Relief

a. Process Control

1. Temperature Consistency:
   • Avoid exceeding the recommended temperature range to prevent material degradation.
2. Avoid Rapid Temperature Changes:
   • Use gradual heating and cooling cycles to prevent thermal shock.

b. Material Handling

1. Prevent Warping:
   • Handle laminates carefully during and after etching to avoid bending or twisting.
   • Support the laminate evenly on flat surfaces.
2. Layer Stack Management:
   • For multilayer boards, ensure proper alignment and flatness during stress relief.

c. Design Considerations

1. Copper Balance:
   • Design circuits with balanced copper patterns to minimize differential stress.
2. Support Structures:
   • Include non-functional copper regions (ground planes) to maintain dimensional stability.

4. Testing and Quality Assurance

a. Flatness Inspection

• Use optical or mechanical methods to inspect for warping or distortion.

b. Adhesion Testing

• Conduct peel tests to confirm the integrity of the copper-to-substrate bond after stress relief.

c. Electrical Testing

• Measure impedance and other electrical properties to ensure consistency with design specifications.

5. Summary of Stress Relief Parameters

Stress Relief Method	Conditions	Benefits
Thermal Baking	100°C–125°C, 1–2 hours	Relieves internal stresses, prevents warping
Mechanical Pressing	Uniform pressure during baking	Maintains laminate flatness
Chemical Neutralization	Cleaning with neutralizing agents	Prevents chemical-induced weakening

6. Practical Applications

• Microwave Circuits:
  Ensures stable electrical performance for antennas, filters, and RF components.
• High-Frequency Multilayer Boards:
  Prevents alignment issues and warping in stacked designs.
• Aerospace and Defense:
  Guarantees reliability in mission-critical environments.