1. Introduction

RT/duroid and RO4000 series high-frequency laminates, produced by Rogers Corporation, are widely used in advanced RF and microwave applications. Proper device attachment and wirebonding techniques are crucial for ensuring optimal performance and reliability in circuits using these materials. This comprehensive guide explores various device attachment methods and provides detailed wirebonding notes specific to RT/duroid and RO4000 series laminates.

2. Overview of RT/duroid and RO4000 Series Laminates

2.1 RT/duroid Laminates

RT/duroid laminates are PTFE-based composites designed for high-frequency, low-loss applications. Key features include:

• Low dielectric constant (Dk) ranging from 2.2 to 10.2
• Low dissipation factor
• Excellent dimensional stability
• High thermal conductivity options available

Common RT/duroid variants:

• RT/duroid 5870/5880
• RT/duroid 6002/6010LM
• RT/duroid 6202

2.2 RO4000 Series Laminates

RO4000 series laminates are hydrocarbon ceramic-based materials offering a balance between performance and processability. Key features include:

• Dk values ranging from 3.38 to 3.55
• Low loss tangent
• Stable electrical properties over frequency
• FR-4-like processing capabilities

Common RO4000 variants:

• RO4003C
• RO4350B
• RO4360G2

3. Device Attachment Methods

3.1 Soldering

Soldering is a common method for attaching devices to RT/duroid and RO4000 series laminates. However, special considerations are necessary due to the materials’ unique properties.

3.1.1 Solder Selection

• Use high-temperature solders (e.g., Au80/Sn20, Au88/Ge12) for RT/duroid laminates to withstand subsequent assembly steps.
• For RO4000 series, standard Sn63/Pb37 or lead-free alternatives like SAC305 can be used.

3.1.2 Surface Preparation

• Ensure the laminate surface is clean and free from oxidation.
• For RT/duroid, plasma cleaning or chemical etching may be necessary to improve adhesion.
• RO4000 series typically requires standard FR-4 preparation techniques.

3.1.3 Temperature Control

• For RT/duroid, use a temperature-controlled soldering iron set below the material’s maximum operating temperature (typically 280°C).
• RO4000 series can withstand standard soldering temperatures up to 280°C for short durations.

3.1.4 Flux Selection

• Use no-clean or easily removable fluxes to prevent residue-related issues.
• For RT/duroid, choose fluxes compatible with PTFE surfaces.

3.2 Epoxy Attachment

Epoxy attachment is often preferred for sensitive devices or when precise positioning is required.

3.2.1 Epoxy Selection

• For RT/duroid, use PTFE-compatible epoxies with good thermal and electrical properties.
• For RO4000 series, standard conductive or non-conductive epoxies suitable for FR-4 can be used.

3.2.2 Application Techniques

• Use automated dispensing equipment for precise epoxy placement.
• Control epoxy thickness to ensure consistent electrical and thermal performance.
• Consider using stencils for repeatable epoxy deposition.

3.2.3 Curing Process

• Follow the epoxy manufacturer’s recommended curing profile.
• For RT/duroid, ensure the curing temperature doesn’t exceed the material’s maximum operating temperature.
• RO4000 series can typically withstand standard epoxy curing temperatures.

3.3 Eutectic Die Attach

Eutectic die attach is often used for high-reliability applications and provides excellent thermal and electrical conductivity.

3.3.1 Material Selection

• Gold-tin (Au80/Sn20) and gold-germanium (Au88/Ge12) are common eutectic alloys used with RT/duroid and RO4000 series laminates.

3.3.2 Surface Metallization

• Ensure proper metallization of both the die and the laminate surface.
• For RT/duroid, specialized metallization techniques may be required to achieve good adhesion to the PTFE surface.

3.3.3 Process Control

• Use precise temperature control to achieve proper eutectic bonding.
• Implement inert atmosphere or forming gas to prevent oxidation during the bonding process.

Read more about:

• RF PCB
• Taconic PCB
• Rogers 3003
• Rogers 5880

4. Wirebonding Notes

Wirebonding is a critical process for connecting devices to the circuit traces on RT/duroid and RO4000 series laminates. Proper techniques are essential for achieving reliable connections.

4.1 Surface Preparation for Wirebonding

4.1.1 RT/duroid Laminates

• PTFE surfaces require special treatment to achieve good bondability.
• Plasma etching with oxygen or CF4/O2 mixture can improve surface energy.
• Chemical treatment using sodium naphthalene solution can also enhance bondability.

4.1.2 RO4000 Series Laminates

• Standard cleaning procedures used for FR-4 materials are typically sufficient.
• Plasma cleaning can be used to remove organic contaminants and improve bond strength.

4.2 Metallization Considerations

4.2.1 RT/duroid Laminates

• Direct bonding to copper may be challenging due to the soft PTFE substrate.
• Recommend using additional metallization layers:
  • Nickel (1-5 µm) as a barrier layer
  • Gold (0.5-1 µm) as a bondable surface

4.2.2 RO4000 Series Laminates

• Standard metallization schemes used for FR-4 are generally suitable.
• Typical stack-up: Copper > Nickel (2-5 µm) > Gold (0.05-0.1 µm)

4.3 Wirebonding Parameters

4.3.1 Gold Wire Bonding

• Wire diameter: Typically 18-33 µm (0.7-1.3 mil)
• Bonding force: 15-30 gf for RT/duroid, 20-40 gf for RO4000 series
• Ultrasonic power: Adjust based on substrate hardness (lower for RT/duroid)
• Bond time: 10-20 ms

4.3.2 Aluminum Wire Bonding

• Wire diameter: Typically 25-33 µm (1-1.3 mil)
• Bonding force: 20-40 gf for RT/duroid, 30-50 gf for RO4000 series
• Ultrasonic power: Higher than gold wire, adjust based on substrate
• Bond time: 15-30 ms

4.4 Temperature Considerations

• For RT/duroid, maintain substrate temperature below 200°C during bonding.
• RO4000 series can typically withstand temperatures up to 280°C for short durations.

4.5 Loop Profile Optimization

• Optimize loop height and shape to minimize wire length and inductance.
• Consider the coefficient of thermal expansion (CTE) mismatch between wire and substrate when designing loop profiles.

4.6 Bond Pad Design

• Minimum pad size: 3-4 times the wire diameter
• Recommended pad thickness: 3-5 µm for gold, 5-10 µm for aluminum
• Include fiducials for automated bonding systems

5. Reliability Considerations

5.1 Environmental Factors

• Both RT/duroid and RO4000 series laminates offer good resistance to humidity and chemicals.
• For high-reliability applications, consider hermetic packaging to protect wirebonds and devices.

5.2 Thermal Management

• RT/duroid laminates with high thermal conductivity (e.g., RT/duroid 6010.2LM) can aid in heat dissipation.
• For RO4000 series, consider additional thermal management strategies for high-power applications.

5.3 Stress Mitigation

• Use stress-relief loops in wirebonds to accommodate CTE mismatches.
• Consider underfill materials for large devices to distribute stress.

5.4 Testing and Qualification

• Perform pull and shear tests to verify bond strength.
• Conduct environmental stress tests (temperature cycling, humidity, etc.) to ensure long-term reliability.

6. Advanced Techniques

6.1 Ribbon Bonding

• Useful for high-current applications or where lower inductance is required.
• Typically easier to implement on RO4000 series due to its higher rigidity compared to RT/duroid.

6.2 Automated Bonding

• Both RT/duroid and RO4000 series are compatible with automated bonding equipment.
• Proper fixturing and temperature control are critical, especially for RT/duroid laminates.

6.3 Flip-Chip Bonding

• Can be used with both laminate series for reduced parasitics and improved thermal performance.
• Requires careful consideration of underfill materials, particularly for RT/duroid laminates.

7. Troubleshooting Common Issues

7.1 Poor Bond Adhesion

• For RT/duroid: Verify surface treatment effectiveness, consider alternative metallization.
• For RO4000 series: Check for surface contamination, adjust bonding parameters.

7.2 Wire Breakage

• Examine loop profile for stress concentrations.
• Verify wire quality and bonding parameters.

7.3 Pad Lifting

• Check metallization adhesion to the laminate.
• Adjust bonding force and ultrasonic power.

7.4 Inconsistent Bond Quality

• Ensure consistent surface preparation across the substrate.
• Verify equipment calibration and maintenance.

Conclusion

Successful device attachment and wirebonding on RT/duroid and RO4000 series high-frequency laminates require a thorough understanding of the materials’ properties and careful process control. By following the guidelines outlined in this document and considering the unique characteristics of each laminate series, engineers and technicians can achieve reliable and high-performance RF and microwave assemblies.

It’s important to note that specific applications may require further optimization of these general guidelines. Always consult with Rogers Corporation’s technical support and conduct thorough testing for your particular use case to ensure optimal results.