A power-device pad filled with thermal vias may still fail to drop temperature as expected. The usual reason is not that there are too few holes. It is that heat leaving the vias never reaches a larger copper plane, a metal-core board, or a heatsink. Via count only creates a local path. Total thermal resistance is set by the entire thermal path.
In PCB thermal design, hole diameter and hole count are the easiest quantities to measure, so many layouts treat "add more vias" as the default answer. The first few added vias can help a lot. After the array becomes very dense, the temperature drop shrinks and can bring solder wicking, voids, and lost routing space.
Shift the judgment from via count to heat flow: can heat from junction temperature travel through the pad, via-wall copper, and inner and outer copper planes, and finally enter the ambient air or the chassis? If any segment has too little cross-section or a broken contact, extra vias only queue in front of the bottleneck.
Via-wall copper is the main conduction cross-section
An ordinary thermal through-hole is not a solid copper pillar. Vertical conduction is carried mainly by plated copper on the hole wall and by any fill that may be present. A larger diameter or thicker plating increases the conduction cross-section, but that must be judged together with fabricator capability, pitch, and pad integrity.
If the holes open inside an exposed pad, solder can also be drawn into the holes during reflow. Plug, resin fill, copper cap, or backside opening affect conduction, soldering, and cost at the same time. Do not treat them as ideal copper pillars from a thermal simulation.
Metal-core and ceramic substrates in the source material point to another direction: when power density per unit area is high, changing the base material and the spreading surface is often more direct than adding endless vias on ordinary FR-4.
The far end of the via must land on effective copper
If thermal vias go from the top pad to the bottom layer, but inner layers connect only through a very thin thermal pad, or the bottom copper is shredded by traces, vertical heat flow still finds no low-resistance exit after it arrives. The via array looks dense, yet equivalent thermal resistance is limited by the narrowest copper neck.
The check is straightforward: inspect via-to-copper connections layer by layer. Confirm that inner layers are not islands and that bottom copper is not cut by slots, splits, or a dense via field. If the path is intended to reach a chassis or heatsink, include thermal-pad thickness, clamping force, and contact area in the path.
Ceramic boards, aluminum boards, and multilayer copper planes do not help because the material name sounds more advanced. They help because they provide lower vertical or lateral thermal resistance. Before choosing a material, decide whether heat must travel to the board back, the board edge, or the chassis.
Vias that are too dense also steal lateral copper
Thermal vias need antipads, solder mask, and manufacturing clearance. When pitch is too small, the pad area is cut by via rings, and a previously continuous copper plane becomes many narrow necks. Lateral spreading falls. On multilayer boards, also check how the vias actually connect to power and ground planes.
A via array can also occupy current loops and routing channels next to the device. After a power-device electrical loop is forced to detour, parasitic inductance and voltage drop rise. The source of temperature rise can shift from the die to copper, vias, or a connector.
Via diameter, pitch, and copper area are therefore not three independent parameters. A sound approach is to keep continuous copper first, then use enough—but not overcrowded—through-holes to move heat to the next layer. Do not turn the entire available area into holes.
Use temperature-rise results to judge diminishing returns
On the same board, two comparable regions can be built: keep the device, copper area, and airflow unchanged, and change only via count or diameter. After thermal steady state, record case temperature, board-back temperature, and ambient temperature, then compare the drop from each added group of vias.
If board-back temperature rises clearly while device temperature barely falls, heat has reached the backside, but external convection or chassis contact is the new bottleneck. If the board back is also not hot, return to via-wall copper, inner-layer connection, and pad copper necks.
Temperature measurement must lock power dissipation, airflow, mounting attitude, and test points. Infrared images also require attention to surface emissivity. Otherwise brightness differences can be misread as real temperature differences.
Thermal simulation should also avoid defining through-holes as ideal solid copper. If the model does not include diameter, plating thickness, fill material, and inter-layer connection, the result overestimates vertical conduction. On a prototype, place test points near the power pad, at the matching board-back location, and at the thermal interface. Use temperature differences to judge which segment holds most of the thermal resistance.
For exposed-pad devices, also check the solder window. Holes that are too open, unplugged vias, or poor solder-mask design can pull solder away during reflow and reduce the real contact area under the package. Thermal vias increase, solder-layer thermal resistance also increases, and temperature may not improve.
If a thermal pad will be attached on the bottom side, form a sufficiently flat, continuous copper surface around the thermal vias. A dense via field can leave surface pits or resin shrinkage and lower actual contact ratio. Compression after chassis assembly must be checked against the material specification. Do not measure temperature only on a bare board.
Design review can lock four questions: where heat is generated, which materials it crosses, on which layer it spreads, and what finally removes it. If any question has no clear answer, complete the path evidence before discussing more vias. That turns "add vias by habit" into a thermal design that can be verified.
Also confirm whether the temperature limit is junction temperature, case temperature, or board-surface temperature. Package and solder sit between the three. An easy-to-measure location cannot stand in for the full result. If the device provides a junction-temperature estimate, cross-check it with board-surface measurements.
Conclusion
More thermal vias are not always better. Heat must complete a continuous low-resistance path. If via-wall copper, inter-layer copper, board-back spreading, or the external cooling interface is missing, the return from adding more vias shrinks quickly.
The next time a hot spot appears, first draw the thermal path from the die to ambient and mark the narrowest cross-section. Fixing the bottleneck is usually more effective than copying more thermal vias.
On your board, is the device hot and the board back also hot, or is only a local spot on the device heating up?
