Creating repeatable consistent heat sink joints

Paul Boughton

As microchips continue to become more complex and faster the heat generated by them and how to effectively secure the heat sink so it dissipates the heat as efficiently as possible continues to be a design dilemma.

The issue remains a primary concern for engineers despite the emergence of low energy consuming devices.

The challenge is how to best mount the heat sink, allow it to be removable (if necessary) and provide optimal heat dissipating (functionality). Many different methods have evolved for assembling heat sinks into electronic devices for the purpose of extracting the heat the processing chips create during use.

Simply put, a good heat sink attachment needs to achieve all of the following criteria:

1. Provide a constant , even contact between heat sink and the heat source
2. The heat sink must be removable for servicing of the electronics being cooled
3. Maximise space by using as little board area for attachment as possible
4. Minimise the stresses in the PCB which can be induced by the fastening system itself
5. Provide compliance in all directions even in the event of shock loading, such as a drop
6. Minimise real estate needed on heat sink for the fastening device

Depending on the size of the heat sink required the attachment methods can vary vastly. Smaller heat sinks can generally be held in place with double-sided tapes or epoxy.

Double sided tape can act as an insulator and epoxies are good but they are permanent. Clean removal is not possible.

Another method utilises spring clips. These consist of a custom design of bent wire and may include a plastic bracket of various shapes to apply pressure to the heat sink in order to maintain firm contact to the heat source. This method requires the attachment of the retaining anchor to the PCB adjacent to the heat source usually by soldering. This technique is fine for smaller simple heat sinks since the force is limited.

Today’s larger complex and heavier heat sinks require greater force to hold them in place. With this in mind, the engineer has to be careful about inducing stresses into the circuit board, which can lead to costly fracturing of electrical tracks and even cause components or their connectors to faill.

To handle heavier heat sinks, fasteners with springs that control the load are frequently used. One style is simply a shoulder screw with a spring around it that passes through the heat sink and compresses the spring onto the top of the heat sink by a specified amount.  The stud is tightened into a nut on the opposite side of the board. However, this method is liable to over- or under-tightening, leading to incorrect pressure affecting the heat transfer.

Another style incorporates a snap top feature on the stud instead of a screw thread. The snap portion is pushed through a hole in the PCB.  These stud and spring type of fasteners are useful in mounting shallow heat pipes/heat sinks into laptop computers where space is a constraint but does reduce the amount of trace on the PCB. Also the clamping force is limited as it needs to be less than pull-out force.

Penn Engineering has developed a new solution to these fastening problems. It is a three piece engineered system containing a self-captivating screw and spring, mated to a broaching or surface mounted nut for the circuit board. It prevents damage by over-tightening yet provides a specific engineered clamp load and because it is based on captive technology, it replaces traditional loose components and removes situations when components can be left out, misplaced or lost.

Our ‘spinning clinch bolt’ is a screw which is captivated into a host material, when pressed into a punched or drilled hole. The screw is free to turn and has a vertical float in the neck area. A new version of this screw has been developed which has a shoulder which captivates and centres a spring, and this can provide a predefined pressing force onto the host/heat sink.

An audible 'click' serves to prevent over-tightening by signalling when the screw is fully engaged and installation is complete.  The screw will continue to rotate but will no longer be engaged in the threads or continue to actively tighten. The reliable and repeatable clamp force generated by the spring ultimately helps determine consistent and predictable clamp load on circuit board components.

This configuration creates an even loading on the heat sink with a predefined force at each attachment point being supplied by the compressed springs on the screws.

This unique three-piece fastening system reduces the hardware and overall 'installed costs' that are required for a typical stud and spring attachment. It provides a repeatable consistent fastening.  

Through Penn Engineering’s global custom design service, customers can have fastenings developed to meet their exact requirements based on thread size, length, spring rate, drive type and material.