SNOAA61A October   2020  – February 2021 LMG3422R030 , LMG3422R050 , LMG3425R030 , LMG3425R050

 

  1.   Trademarks
  2. 1Introduction
  3. 2New QFN 12x12 Package
  4. 3Bottom-Side Cooling Configuration and Definition of RθJC/P
    1. 3.1 Definition of RθJC/P for Package Thermal Performance
    2. 3.2 Design Recommendations for Bottom-Side Cooling System
  5. 4Simulation Models and Results
    1. 4.1 Finite Element Models for Thermal Analysis
    2. 4.2 Thermal Simulation Results
  6. 5Experimental Setup and RθJC/P Testing Results
  7. 6Thermal Performance of QFN 12x12 Package on Half-Bridge Evaluation Board
  8. 7Summary
  9. 8References
  10. 9Revision History

3.2 Design Recommendations for Bottom-Side Cooling System

More details on how to optimize each cooling element in the system-level thermal design can be found in published application reports SNOAA14 and SNOA946. Some key points are summarized below:

  • Use thick Cu (2 oz. recommended) for multilayer PCB to spread out heat and remove solder mask for bottom Cu heat spreading plane
  • Design sufficient numbers of thermal vias to reduce RθPCB and fill vias by conductive epoxy or Cu if possible
  • Use 1.6 mm or thinner PCB if applicable and apply proper pressure to prevent board warpage when clamping it to heatsink or coldplate
  • Control thermal pad solder joint void percentage of less than 25% in total and 10% the largest
  • Make careful tradeoffs between adding more Cu pad/thermal via coverage on PCB and the resultant extra parasitic capacitance and inductance
  • Add cooling element on package top surface only as a supplement due to insufficient power dissipation from bottom side
  • Select proper TIM and heatsink to meet overall RθJA thermal requirement