The Challenge and Solution of PCB Heatsink

PCB heat sink for snap-in transistor retaining springs

The different geometries of the PCB heatsink contain a threaded channel already integrated into the extrusion process, through which the individual heat sink can be screwed to the circuit board.

The simplification of transistor assembly is ensured for the various types of printed circuit board heat sink by means of a special groove geometry integrated into the heat sink and snap-in transistor retaining springs made of stainless steel. Once engaged, the spring stays in place and fixes the transistor with high contact pressure on the mounting surface.

PCB heatsink

The requirements for circuit board cooling have never been as high as they are today: The increasing integration density during assembly and the variety of housing designs used pose major challenges for developers. The CTX Thermal Solutions GmbH offers a wide range of PCB heat sink, geared to the different types of PCB assembly, to dissipate the heat quickly and reliably.

Regardless of whether it is an office, household or plant technology: The cooling of electronic assemblies is essential for their error-free and long-lasting function. CTX has several hundreds of models of PCB heatsink with thermal resistance values ​​between 6 and 72 ° C / W in the standard range. Optimally matched to the respective application and type of assembly, they ensure reliable cooling. If a standard solution is not the optimal choice, CTX develops the application-specific heat sink together with the customer – made to measure using CNC technology and based on thermal simulation.

The challenge of PCB heatsink

The pressing of the circuit board with the heat sink turns out to be a challenge in both processes. The areal distribution of the thermal adhesive without air pockets between the two components was previously an unsolved problem and is particularly due to the low flow properties of the thermal adhesive. A sufficient distribution was achieved by pressing on with so-called pressure pins, but this method is only suitable to a limited extent. The result was often that, in particular in the case of thin and large-area ceramic substrates, the punctual and thus excessive force action resulted in the ceramic board breaking.

The solution for PCB heatsink

Dow Corning, a global producer of effective thermal adhesive, was looking for a more sophisticated and more economical solution. The vacuum joining process specially developed by Scheugenpflug AG includes all requirements and presents a process-reliable result.

After the adhesive bead has been applied to the heat sink and the circuit board has been joined, the unassembled component is guided into a miniature vacuum chamber. Due to the small size of the vacuum box (15 x 12 x 7 cm³), the evacuation and subsequent ventilation take only a fraction of a second. When evacuating, all air, including that between the tracks of the adhesive beads or between the heat sink and the circuit board, is completely removed. When venting, the rapidly increasing air pressure means that the board is pressed evenly. The gap size is defined by adding larger solids in the adhesive. These are distributed homogeneously in the thermal adhesive and determine the gap size over the entire area of ​​the board. Consequently, the gap distance is defined based on the size of the solid.

However, not all dosing systems are suitable for this application. Piston dispensers that are designed for large grain sizes and can process even highly filled and abrasive adhesives in long-term use are ideal.

A central problem for developers is and remains the dissipation of the heat loss from the chips. However, there are very different ways to connect a heat sink to the heat source. How do they differ?

Microelectronic circuits are becoming increasingly complex and offer more and more computing power. The power losses generated and methods for effectively attaching heat sinks for the most efficient heat dissipation remain a key challenge for developers. Although there are more and more electronic components with low energy consumption, this problem remains a major concern.
The problem is the best way to mount a heat sink so that the connection can be detached if necessary and at the same time can offer optimal heat dissipation (functionality). Numerous methods for attaching heat sinks to electronic components have been developed to dissipate the heat generated by data processing chips during operation.

Use the circuit board as a heat sink

pcb heatsink

As the PCB heat dissipation per unit area increases with the miniaturization of electronic components, heat management is becoming increasingly important. How warm a component gets depends on the “bottom” and “top” resistance and the heat spread by the copper in the circuit board. The following article explains how thermal management can be optimized here.

To put it simply, a printed circuit board is a laminate made of copper-coated plastic plates that are pressed with the aid of synthetic resin and glass fabric. With the exception of copper, the thermal conductivity of the materials involved is very poor. Nevertheless, most components without a printed circuit board do not have a chance of thermal survival because they do not have the surface area required for the necessary PCB heat dissipation: it is only through the heat transfer into the printed circuit board and the heat spread there (or through attached heat sinks) that heat can be released to the environment at a low-temperature level become.

With the standard material FR4, the recommended maximum temperature at peak load is approx. 135 ° C. At higher temperatures there is bending and delamination and thus a loss of functionality. Table 1 shows the thermal conductivity of materials in a printed circuit board.

The thermal power loss of components

How warm the component gets in the assembly depends on its thermal power loss, the “bottom” thermal resistance between the component and the circuit board, the “top” resistance between the top of the component and the air and not least on the heat spread by the copper in the circuit board. Depending on the component type, the bottom or top resistance (and thus the temperature increase) can be reduced by using suitable cooling hardware (underfill or heat sink).

However, the hotspot (thus the target area for cooling air and radiation) can also be enlarged by using the appropriate copper in the layout; thus reducing the temperature and also reducing the thermal stresses in the circuit board caused by the temperature differences. For small components such as LEDs, this is the only way of cooling (possibly in combination with heat vias).

Circuit board on the heat sink

Circuit board on the heat sink

MOKO Technology has developed a special process for vacuum joining for heat dissipation in power semiconductors.

High component performance and packaging density on circuit boards require higher heat dissipation via adjacent heat sinks. If this is not guaranteed, the components will overheat and be destroyed or fail to function. So-called heat-conducting adhesives are used to dissipate the heat to the adjacent cooling surface, usually made of metal (aluminum). These fill the gap between the two contact surfaces that arises due to unevenness or other influences.

The deformation properties make adhesives the ideal material. However, their most highly viscous consistency often brings with it greater challenges when joining and pressing. A new process technology offers a quick and economical solution for this.

Thermally conductive adhesives are highly filled with one- or two-component adhesives. Depending on the layer thickness, they can be applied to the PCB heatsink using the stamping or dosing method. The higher the filler content, the higher the thermal conductivity. Thermally conductive adhesives with up to 70% fillers are currently used.

In order to meet the requirements of the development engineers with regard to heat dissipation, the filler content of the adhesives was constantly increased. The limit is not the enrichment with fillers, but the machine processing. Thermal adhesive with a high filler content (density higher than 3g / cm³) can no longer be applied to the heat sink surface due to the paste-like consistency. Alternatively, there is the possibility of applying highly viscous substances using the casting technique. The somewhat longer processing time can be compensated for by applying adhesive beads with relatively large line spacings.

A good PCB heatsink attachment must meet all of the following criteria:

• It must ensure permanent, even contact between the PCB heatsink and the heat source.
• The circuit board heat sink must be removable in order to be able to service the electronics to be cooled.
• The fastener must allow maximum space by taking up so little space occupied as possible for mounting on the circuit board.
• The mechanical load on the circuit board introduced by the fastener itself must be as low as possible.
• The attachment must be able to yield in all directions in order to absorb shock loads, for example when falling.
• The fastening element may only occupy a minimum of space for fastening on the heat sink.

Different types of fastening PCB heatsink

Different types of fastening PCB heatsink

Depending on the size of the required heat sink, there is a wide selection of fastening methods. Smaller PCB heatsink can usually be fixed with double-sided adhesive tape or epoxy resin. Double-sided adhesive tape can have an insulating effect; Epoxy resin adhesives are good, but they are a permanent bond. But neither method enables clean removal.
Spring clips are another option. They consist of specially-shaped wires and may contain plastic clips in different shapes. These exert pressure on the heat sink to ensure close contact with the heat source. For this purpose, a holding element is attached directly to the heat source on the circuit board, usually in a soldering process. This technique works well for smaller, simple heat sinks because the holding forces are limited.

However, fixing larger, more complex and heavier heat sinks require greater forces. The developer must ensure that the lowest possible forces are applied to the circuit board, as this can lead to expensive damage, such as the breakage of electrical conductor tracks or failure of components or connectors.

When handling heavier PCB heatsink, fasteners with springs are often used to limit the load. One possibility is the use of a simple fitting screw surrounded by a spring, which is passed through the heat sink and presses the spring onto the top of the heat sink with a defined force. The fitting screw is fastened with a nut on the other side of the circuit board. However, this procedure allows the tightening to be too strong or too weak so that an incorrect contact pressure can impair the heat transfer.

Another fastening concept uses a snap mechanism instead of a thread on the fitting bolt. The snap mechanism is pressed through a hole in the circuit board. Such fastening elements with bolts and springs are suitable for fastening flat heat pipes or heat sinks in laptop computers with limited space, but they reduce the space for conductor tracks on the circuit board. In addition, the clamping force is limited because it must be less than the pull-out force.

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