RT-duroid® 5870 laminates, consisting of a PTFE-based substrate reinforced with glass microfiber and bonded to copper foil, are widely used in high-frequency applications. The adhesion between the copper foil and the PTFE substrate is a critical factor for the reliability and performance of these laminates, particularly during soldering processes.

This study outlines the effects of soldering temperatures on the copper foil adhesion to RT-duroid® 5870 material and provides guidelines for ensuring long-term reliability.

1. Key Factors in Copper Foil Adhesion

1. Material Properties:
   • PTFE Substrate:
     PTFE is chemically inert and thermally stable, but it has low surface energy, making adhesion to copper challenging.
   • Glass Microfiber Reinforcement:
     Provides mechanical strength and dimensional stability, improving adhesion performance.
2. Copper Surface Treatment:
   • The copper foil is typically treated (e.g., roughened or coated) to enhance its bond to the PTFE substrate.
3. Adhesive Layer or Bonding Process:
   • Adhesion relies on the quality of the bonding process, which may include mechanical or chemical methods.

2. Effect of Soldering Temperatures

a. Thermal Stress and Expansion Mismatch

• Differential Expansion:
  PTFE has a higher coefficient of thermal expansion (CTE) than copper. During soldering, temperature-induced stress can weaken the bond due to CTE mismatch.

b. Decomposition or Degradation

• PTFE Stability:
  PTFE is stable up to ~260°C, but excessive soldering temperatures can cause thermal degradation, reducing adhesion strength.
• Copper Oxidation:
  Prolonged high temperatures may oxidize the copper surface, weakening the adhesion.

c. Solder Process Parameters

• Time-Temperature Profile:
  Excessive dwell time at high soldering temperatures exacerbates thermal stress and bond weakening.
• Reflow Cycles:
  Multiple soldering cycles can lead to cumulative stress, impacting adhesion.

3. Experimental Analysis

a. Test Conditions

1. Soldering Temperatures:
   • Typical soldering temperatures: 240°C–260°C.
   • High-temperature scenarios: Up to 300°C for lead-free soldering.
2. Adhesion Measurement:
   • Pull tests or peel strength tests are used to evaluate copper foil adhesion before and after thermal exposure.
3. Cycles and Duration:
   • Simulated soldering cycles are applied to assess the cumulative effect of thermal stress.

b. Observed Effects

1. Adhesion Degradation:
   • Adhesion strength decreases significantly above 260°C due to PTFE softening and thermal expansion stress.
2. Copper Delamination:
   • Higher soldering temperatures (e.g., >280°C) increase the risk of copper delamination, especially after multiple reflow cycles.
3. Thermal Degradation of PTFE:
   • Decomposition of the PTFE matrix around 300°C leads to reduced dielectric and mechanical performance.

4. Recommendations for Mitigating Adhesion Loss

a. Optimize Soldering Process

1. Temperature Control:
   • Maintain soldering temperatures within PTFE’s safe range (≤260°C).
   • Use rapid heating and cooling profiles to minimize thermal stress.
2. Time Optimization:
   • Limit the dwell time at peak soldering temperatures to reduce stress on the adhesive bond.

b. Material Selection

1. Enhanced Adhesion Copper Foil:
   • Use copper foils with enhanced surface treatments to improve adhesion.
2. Higher Thermal Stability Laminates:
   • Consider materials with higher thermal stability for lead-free soldering processes.

c. Post-Soldering Stress Relief

• Allow the laminate to cool gradually after soldering to relieve residual thermal stress.

d. Multi-Layer PCB Considerations

• For multilayer designs, use thermal vias and balanced layer stack-ups to mitigate warpage and stress.

5. Summary of Results

Soldering Temperature	Effect on Adhesion	Recommendation
< 240°C	Minimal impact on adhesion strength	Safe operating range
240°C–260°C	Moderate stress; adhesion slightly reduced	Optimize dwell time
> 260°C	Significant degradation; copper delamination likely	Avoid prolonged exposure above 260°C
> 300°C	Severe damage to PTFE; unacceptable adhesion loss	Not recommended for RT-duroid® 5870

6. Applications and Practical Implications

• High-Frequency Circuits:
  Adherence to soldering guidelines ensures stable electrical and mechanical performance for antennas, filters, and RF components.
• Aerospace and Defense:
  Controlled soldering profiles prevent failures in mission-critical systems.
• Medical Electronics:
  Ensures long-term reliability in sensitive devices such as imaging and diagnostic tools.