Selecting the right thermal materials are critical!Comments Off on Selecting the right thermal materials are critical!

Bridgelux Vero

Choosing the right thermal management solution requires electronics, mechanical engineering and materials knowledge, explains managing director of Universal Science, James Stratford.

Thermal management technology has evolved quickly to keep up with the relentless advance of electronic component and product design. The range of materials that ‘did the job’ a decade ago would now fail to make the grade as the size and format of components has continued to reduce, packaging materials have changed and device integration and power density have increased. The challenge has been compounded by sectors such as LED lighting and smart phones that create a new set of thermal challenges for design engineers.

Today everyone wants portable products which necessitates miniature power devices in high density circuits, often housed in airtight, or even watertight, enclosures to allow use anywhere. Bearing in mind these challenges, heat management is obviously a priority.

Material gains

When thermal management first started to become an issue, the choice of materials on offer was basic. Where no electrical isolation was required, a smearing of thermal grease was used to fill air gaps between power devices and heatsinks. If the tab of the device was electrically live, then the grease was supplemented with a thin piece of isolating material such as mica. It was a messy solution, but one that did the job.

Soon, thermal pads utilising a silicone elastomer loaded with electrically conductive ingredients and formed around a carrier began to emerge. These were cleaner and easier to use and enabled repeatable production. Pads of this kind are still a valuable option today.

Now, for applications where electrical isolation is not required, a range of graphite based materials are available and provide a clean alternative to thermal greases. They can be die-cut to custom shapes and even supplied with an adhesive coating to aid assembly. Materials such as UniGraph have almost consigned thermal greases to history.

Electrically isolating thermal pads are available in two main formats: cold-flow materials, which feature a loaded elastomeric binder coated on a carrier or substrate. These pads fill air voids between mating parts when subjected to assembly pressures from clips or screws and exhibit continually improving thermal
performance over time as further flowing of the material occurs.

Phase change

More recently, phase change materials have been developed. Coated to both sides of a carrier material that provides electrical isolation, these materials change state and flow to fill air voids, promoting good heat transfer when subjected to increased temperatures.

Phase change non-isolating materials also exist and can provide a highly effective grease replacement for high volume applications. The material has the appearance and texture of candlewax and can be screen printed onto heatsinks. Once assembled and heated to the phase change temperature, the material flows to provide a void-free high thermal conductivity interface. Universal Science prints its UniPhase 3500 material onto the target surface in a hexagonal pattern designed to ensure an even spread of material after phase change.

Get it on tape

The vast majority of circuits nowadays are laid out using surface mount, rather than through-hole, technology. Where high thermal performance is required, Universal Science supplies insulated metal substrate (IMS) materials. These feature an etched copper top surface onto which the components are soldered, and a rigid, usually aluminium baseplate, that acts as a heat spreader. Sandwiched between the copper circuit and the baseplate is a thin dielectric that provides several kilovolts isolation and a low thermal resistance path to the baseplate.

It is also common to attach the circuit board to a larger heatsink or to the metal chassis of the product itself. The key to achieving successful heat transfer is to have a thin interface able to fill all air voids, such as the Bondline series of thermally conductive tapes from Universal Science. These doublesided adhesive materials combine high structural bond strength and good heat transfer characteristics. Different thicknesses can be specified depending on the copper track weight or thickness on the underside of the board and the overall surface area of the mating parts.

Filling the gap

Silicone based gap fillers emerged over a decade ago. They were especially useful for bridging gaps between the top side of a large integrated circuit and the inside wall of an enclosure. Although convenient, pre-cured pads can have some drawbacks. In order to fill air voids, the pad must be subjected to an assembly pressure. Even though the materials are soft, they still exert a back pressure that can potentially damage delicate ceramic components or strain solder joints that may later cause field failures. Gap fillers must have room to move laterally as deflection occurs to keep back pressure to a minimum.

Gap filling putties such as UniPutty offer an alternative. These materials have a consistency similar to toothpaste and can be applied semi-automatically with a syringe or, for high volume applications, using automatic dispensing equipment. UniPutty can also be screen printed, which allows an accurately metered amount to be placed. The consistency of the material means there is little to no back pressure. The only major drawback is that rework can be messy.

The variety and breadth of thermal materials available today is huge. This can be perplexing, so considering all the options and tackling thermal management as early as possible remains the best approach.