Start at the device documentation
Thermal vias provide conductive paths between copper regions in the board. They can help move heat from a device land to internal planes or the opposite surface. The recommended land and via arrangement must start with the exact component package and its application guidance.
An exposed package pad may also have an electrical function. Confirm its required net connection before adding planes or vias. Do not infer the connection from the fact that the pad is described as thermal, and do not connect it to an arbitrary heatsink or chassis potential.
Follow heat beyond the via array
The useful path includes the die and package, solder interface, board copper, spreading region and the eventual transfer to ambient or a heatsink. Improving one segment may have little effect if another dominates the thermal resistance. Review the complete assembly, including enclosure and airflow.
Via diameter, barrel copper, length, count and distribution affect the local path. So do plane connections and the available spreading area. A fixed number of vias is not a universal solution for every power level or board construction.
Coordinate vias with solder deposition
Open vias in a solder land can change solder flow and the resulting joint. Tenting, plugging, filling and capping are distinct treatments with different manufacturing and assembly implications. Select the treatment using the package recommendation and the qualified assembly process.
Review the paste pattern, mask definition and acceptable joint condition together. Increasing paste to compensate for an undefined via treatment can introduce other defects. First articles should be evaluated with the actual land, stencil and reflow process intended for production.
Validate representative operating conditions
Measure or model the assembly at relevant loads, ambient conditions and cooling arrangements. Identify the temperature being evaluated and the method used to relate it to the device limits. A bare-board material conductivity value cannot establish the operating temperature of the finished product.
For quotation and DFM review, include the exact component, land pattern, via treatment, stackup and intended cooling interface. Define the inspection and test evidence required. Any change to the copper connection or via construction should be checked against both the electrical and thermal design.