Designing Thermal Vias

Enhancing thermal transfer for FR-4 PCBs can be achieved cost-effectively by incorporating thermal vias and plated through-holes (PTH) between conductive layers. These methods efficiently dissipate heat and improve the overall performance of the PCB.

Key design considerations for vias include:

- Adhering to the manufacturer's recommended number of holes (thermal vias) in the thermal pad area to ensure optimal thermal transfer.

- Connecting all holes within and outside the thermal pad area to the internal ground plane or other internal copper plane, thus maximizing the thermal dissipation efficiency.

- Employing a direct or solid connection to achieve lower thermal resistance for the internal ground plane, contributing to improved overall thermal performance.

Furthermore, for the bottom copper plane, it is recommended to encircle the vias with a ring of exposed copper (0.05 mm wide) to prevent excessive voiding. This practice, as detailed in the TI Application Report SLOA120–May 2006, ensures optimal thermal transfer and reliability.

QFN Package Stencil Design for Thermal Pad

 To effectively remove heat from the package and maximize electrical performance, it is crucial to solder the die paddle to the PCB thermal pad with minimal voids. Despite potential challenges due to thermal vias and the large size of the thermal pad for larger packages, striving for minimal voids is essential. Furthermore, careful consideration must be given to the reflow process, as out-gassing can lead to defects if the solder paste coverage is too extensive. Therefore, using smaller multiple openings in the stencil for solder paste printing is recommended, as this can result in 50% to 80% coverage, as illustrated in the figure below. While improved coverage generally enhances thermal contact, it's important to be mindful of potential voiding and floating issues in QFN packages.

Thermal Pad Stencil Design for 7mm x 7mm (Left side) and 10 x 10 QFN Packages with 4.9mm and 7.4sq Pads