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 boardheat 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.
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
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?
Use the circuit board as a heat sink
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.
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
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.