At the core of all changes, as an NPI engineer, DFM manufacturability analysis involves a wide range of areas. Today’s focus is primarily on PCB heat dissipation holes, including their design, size, and important considerations to keep in mind.
When designing a PCB, in many cases we need to provide heat dissipation for certain components, such as linear regulators. In most cases, these are accommodated by generic through-hole parts, as the heatsink can effectively dissipate the heat within the aluminum area and keep the device at more excellent operating conditions. However, if we talk about any SMD devices, heatsinks are not available, most of the time. We must rely on copper coverage techniques to create sufficient heatsinks on the copper layers.
1. What are PCB Heat Dissipation holes?
PCB Heat dissipation holes (PCB thermal vias) are a technique used to dissipate heat to the backside of a PCB by using channels (through-holes) that penetrate through the PCB. They are placed directly under or as close as possible to the heat-generating component.
Heat dissipation holes are a method of improving the heat dissipation effect of surface-mounted components by using the PCB as a heat sink. Structurally, they involve setting through holes in the PCB.
- For single-layer and double-sided PCBs, the technique used to reduce thermal resistance is to connect the copper foil on both sides of the PCB to increase the area and volume available for heat dissipation.
- For multi-layer PCBs, the same objective can be achieved by connecting the planes between layers or limiting connections to specific layers.
2. How to Design PCB Heat Dissipation Holes?
The placement and sizing of PCB heat dissipation holes can vary greatly and depend on the type of component, different rules, and professional knowledge.
However, a primary rule is to use heat dissipation holes as close as possible to the heat source directly under the heating element. Then, in cases where the heat source is not ideal, regardless of how the component solder pads are placed, thermal vias can also be placed around the periphery of the component. In this case, the rule also remains the same, to place the heat dissipation holes as close as possible to the outer edge of the component.
To effectively use heat dissipation holes, it is important to place them close to the heat source, such as directly under the component. As shown in the figure below, utilizing the heat balance effect and connecting locations with large temperature differences is a good method. (The Benefits of PCB Prototyping for Circuit Board Design)
3. What is the typical size of PCB Heat Dissipation Holes?
To improve the thermal conductivity of PCB thermal vias, it is recommended to use small-diameter through-holes with an inner diameter of around 0.3mm that can be electroplated and filled. It should be noted that if the hole diameter is too large, there may be problems with solder wicking during the reflow soldering process.
The spacing of the thermal vias should be around 1.2mm and they should be placed directly beneath the backside heat sink of the package. If this is not enough for heat dissipation, thermal vias can also be placed around the IC. In this case, the key is to place them as close to the IC as possible.
4. Thermal Conductivity Coefficients of Different Materials
Thermal conductivity is a critical factor in determining how much heat a material can absorb. The following table provides information on the thermal conductivity of different materials, which can be a helpful reference.
| Material | Thermal Conductivity |
|---|---|
| Copper | 388 |
| Lead Frame | 277 |
| Aluminum | 205 |
| Silicon | 145 |
| Tin-Silver-Copper Solder | 57.3 |
| 63Sn37Pb Material | 50 |
| DA Epoxy Resin | 2.4 |
| Moulding Compound | 0.7 |
| FR4 Printed Circuit Board | 0.35 |
Therefore, as seen from the table above, aluminum has a lower thermal conductivity than copper. However, due to the larger surface area of aluminum heat sinks, they can provide more effective cooling for heated equipment. But as we can see, if copper is used effectively, it can dissipate more heat than aluminum of the same surface area.
Effective placement of thermal vias is when the vias are properly used in ICs or on solder pads of conductive heating elements as a method of heat transfer. Heat is distributed between multiple layers of copper and then transmitted through free air. Heat dissipation starts using the convection method to transfer heat in the air. It is recommended that the inner diameter of the thermal vias be smaller, such as about 0.35 millimeters. If the diameter is larger, there may be issues with improper soldering during reflow soldering, so extra care is needed. However, if a larger diameter is required, the thermal filling may help compensate for this.
5. Design considerations for PCB Thermal Vias
During the design process of thermal vias, there are a few important considerations to keep in mind. Here are six recommendations:
- The design approach for exposed pad packages involves transferring heat directly from the package exterior to the copper region. Using the solder as a heatsink is not very effective as it is thin and has poor thermal conductivity.
- For exposed pad packages, the most significant heat dissipation occurs through vias to the bottom layer of the PCB and dissipates into the air. Therefore, a large bottom layer area will also reduce the heat dissipation of the component package.
- Isolating the heated components and utilizing heat dissipation holes can help distribute heat evenly to other packages.
- Thermal vias are the only source of heat dissipation for DFN and QFN packages, as the top copper layer has limited space due to pin allocation. Therefore, to utilize the bottom copper layer and increase thermal conductivity, the only way is to use heat dissipation holes. (U5 and IC2 utilize heat dissipation holes. IC2 uses a QFN flat package, where thermal vias are the only viable option due to the distribution of component solder pads, which does not include a larger copper area on the solder layer.)
- The effective copper area that connects to the device through thermal vias will be the maximum copper length that is directly connected to the component package using thermal vias, irrespective of the solder layer.
- The thickness of the copper plane also affects thermal conductivity, with 2 oz copper having better heat resistance than 1.0 oz or 0.5 oz copper.
These are some suggestions for using heat dissipation holes to assist engineers who need to consider heat dissipation devices in their designs.
This concludes the knowledge about PCB heat dissipation holes. Your support is appreciated, and please feel free to leave comments in the section below to discuss and exchange ideas together.
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