What Is The Most Suitable Surface Mount Reflow Soldering Profile For A PCB With A Mix Of Low-temperature RoHS-compliant And High-temperature Leaded Components, Considering The Thermal Mass And Conductivity Of The Board, And How Can I Modify My Existing Reflow Oven To Accommodate This Profile While Minimizing Oxidation And Ensuring Reliable Solder Joint Formation?

To determine the optimal reflow soldering profile for a PCB with mixed component types and modify the existing oven, follow this structured approach:

Reflow Soldering Profile

1. Preheating Stage:

   • Temperature: 100-150°C
   • Duration: 1-2 minutes
   • Purpose: Avoid thermal shock by gradually increasing temperature.

2. Soaking Stage:

   • Temperature: 190-200°C
   • Duration: 1-2 minutes
   • Purpose: Activate flux and begin solder melting without rushing the process.

3. Reflow Stage:

   • Temperature: Peak at 240-245°C
   • Duration: 1-2 minutes
   • Purpose: Ensure lead-free solder melts while considering component temperature limits.

4. Cooling Stage:

   • Rate: 2-4°C per second
   • Purpose: Prevent thermal stress and oxidation.

Oven Modifications

1. Thermocouple Addition:

   • Install thermocouples on the PCB to monitor temperatures accurately.

2. Zone Control:

   • Adjust heating zones for uniform heating: preheating, soaking, reflow, and cooling.

3. Atmosphere Control:

   • Use nitrogen if possible to reduce oxidation; otherwise, select effective flux.

4. Cooling System Enhancement:

   • Implement forced air cooling to control the cooling rate.

5. Belt Speed Adjustment:

   • For large or thick boards, slow the belt to extend time in each zone.

Additional Considerations

• Component Specs: Check datasheets to ensure components can handle the peak temperature.
• Testing: Trial runs with thermocouples to refine the profile based on actual readings.

This approach ensures a balanced reflow process, accommodating both component types while minimizing thermal stress and oxidation.