November: Themal Vias

What significance does the number of copper-plated holes and the thickness of copper plating in these holes have for the thermal performance of the PCB?

The greater the mass of copper that can be placed under heat-generating components or in their direct vicinity, the better will be the dissipation of heat from their underside. In the case of heat dissipation with plated-through holes, both their number and the thickness of their copper layer are crucial in the thermal performance.

How does the thermal resistance of a four-layer stack up stack up with 2x 400 µm copper foils on the inner layers compare to a ten-layer stack up with 8x 105 µm copper? Both stack ups have a comparable overall thickness of copper in the inner layers and the same configuration of thermal vias?

The thicker the insulating layer in the overall stack up, the greater the thermal resistance will be because FR4 material (0.4 W/mK) represents a thermal barrier compared to copper (400 W/mK). If two 400 µm Heavy Copper layers are replaced by eight 105 µm copper layers, a greater thickness will be required for the insulation layers overall. Despite the same total mass of copper, the PCB becomes thicker as a result, which in turn leads to deterioration in thermal resistance. In the case of classical 400 µm Heavy Copper technology, the overall thickness in the sample PCB is 2.0 mm and hence 0.3 mm thinner than the PCB using 105 µm Heavy Copper technology. This advantage is reflected in a 15 % lower thermal resistance. The difference to the Heavy Copper T² technology currently developed by SCHWEIZER is even greater.

How does the thermal resistance change with a constant number of vias due to a reduction in the PCB thickness with the aid of Heavy Copper T² technology?

Heavy Copper T² technology is being developed by SCHWEIZER in order to achieve additional benefits in thermal behaviour compared with conventional Heavy Copper technology. The decisive lever is the reduction of the insulation layer, which is almost impervious to heat, by approx. 50 %. Despite the placement of CNC-drilled, copper-plated thermal vias for heat dissipation of the power components, a reduction in the thermal resistance of more than one third of the initial value is achieved in the application typically investigated.

What is the minimum distance between the hole walls of two CNC-drilled thermal vias?

The minimum distance depends on the overall thickness of the PCB and the base materials used. The resulting minimum distance from hole wall to hole wall (without metallisation) is typically between 0.2 mm and 0.5 mm.

Is the plugging and capping of vias an economically meaningful method of reducing the thermal resistance of a PCB?

Closing through-holes with plugging paste (thermal conductivity approx. 0.5 W/mK) produces practically no improvement in thermal resistance as the power losses take the path of least thermal resistance so that only the cross-section of the copper barrel functions as a heat conductor. However, plugging in combination with capping becomes indispensable with a high density of plated-through thermal vias so that a sufficiently large soldering area can be made available.

What has to be observed in the neighbouring layout area of the thermal vias in order to ensure good heat dissipation?

Coupling inner layers to plated-through thermal vias creates a spreading effect that additionally reduces the thermal resistance. A crucial factor here is the overall copper mass (thickness + area covered). The layout of the outer layers towards the heat sink has an especially large influence. This area should be designed as generously as possible in order to exploit the spreading effect to the best possible extent.

Should solder mask be applied to the thermal vias on the side facing the heat sink?

Normally the PCB is connected with the heat sink via a thermal interface material (TIM). The TIM not only guarantees the transport of heat to the heat sink but also represents an electrical insulation to the PCB and hence to the thermal vias on the rear of the PCB as well. Consequently, the function of the solder mask as an electrical insulator is not required in this case. From the thermal aspect, it is disadvantageous to overprint thermal vias with solder mask as the thermal resistance of the thin solder mask layer, due to its poor heat conductivity, is of a similar magnitude to that of the TIMs. Nevertheless, there are applications that demand overprinting with solder mask in order to minimise the risk of short circuits due to metal particles.