Understanding Gap Filler MaterialsComments Off on Understanding Gap Filler Materials

Understanding Gap Filler Materials

This article appeared in Electronics Sourcing magazine. Download the file here.


IN NORMAL USE most electronic components generate heat, is unavoidable and quite normal. Kept below a reasonable level, temperature will have little effect on the performance or life of a component or circuit. However, if a component operates either periodically or permanently outside its specified temperature envelope (and appropriate thermal management is not put in place) then a dramatic shortening of its useful life or immediate/latent failure can occur.

Thermal management has become a high-profile issue in recent years as components shrink in size and designers are under pressure to feed industry and consumer desire for high levels of functionality in smaller equipment. Add to this a trend towards portability, resulting in completely sealed enclosures, and you have a thermal storm brewing.

Thermal issues used to only emanate from analogue power devices. Now, as digital electronics have miniaturised and clock speeds accelerated, thermal issues have beset microprocessors and other digital devices.

In recent years gap fillers have become a knight in shining armour for engineers tasked with solving unwanted thermal issues related to innovative, high-functionality designs. However, gap fillers have themselves advanced and now purchasers are presented with a baffling choice of materials.

This article breaks down what is on offer and the key factors that need to be considered to make the right choice.

What are the main types of gap filler on offer?

Gap fillers can generally be split into two categories: sheet material and putty. Both use silicone elastomer as their primary ingredient. Silicone is used because of its softness (a key desirable of any thermally efficient gap filling material) and its ability to perform across a wide temperature range: typically –50 to 200°C). Silicone contamination, which occurs when material migrates and deposits silicone on adjacent components and pads was a major concern with early gap fillers. Current formulations control the problem effectively enough for this to no longer to be a major concern.

Die cut gap filling pads

Die-cut Gap Filling Pads

The heat transfer performance of silicone based gap fillers is achieved by blending conductive fillers, usually boron nitride or alumina, with the elastomer. In general, the more higher quality filler used the better the thermal conductivity achieved. However, too high a percentage of filler will negatively affect other material properties.

Sheet form gap fillers usually feature an embedded reinforcing material. This is required to offset the Achilles heel of silicone elastomers: poor tear resistance. The material most often used to perform this function is fibreglass matting. The matting gives exceptional strength in X and Y-planes without seriously compromising thermal performance.

Reinforced gap filler in sheet form is offered in a range of thicknesses, usually between around 0.5 and 5.0mm. From a commercial perspective, it is important to design-in the thinnest possible material for a given application to reduce costs. For small production runs it is advisable to purchase the material in sheet form and cut it by hand to the required size. Once production volumes are reached, relatively low cost tooling can be developed that allows parts to be die-cut and delivered ready for immediate assembly.

UniPutty 1800 in application

Gap Filling Putty in Application

Putty type gap fillers too are silicone based and contain conductive fillers. They differ from sheet material in that they are supplied in bulk form for application via a syringe or automated metered dispensing system. They can also be screen printed onto the target surface. These materials are easy to use as, unlike many established dispensable approaches, they do not require a post-application curing cycle and are a one-part, rather than two-part compound. The thixotropic nature of the material means that putty does not migrate or run once dispensed. Thermal putty is a relatively new arrival on the market, providing engineers a range of options when looking for the optimum thermal management solution.

Materials with different levels of thermal performance are available when purchasing either putties or sheet material. Quoted thermal conductivity figures range from less than 1W/m-K, up to values exceeding 6W/m-K. Generally, the higher the thermal conductivity the higher the cost. This is mainly because of the more refined and exotic fillers used in the top performing materials.

Both putties and sheet can provide a degree of electrical isolation between the surfaces being coupled. However, if an application needs a guaranteed amount of electrical isolation, then a thicker sheet material reinforced with fibreglass mating is the best option. This guarantees a minimum assembled thickness meaning there is no chance of ‘metal-on-metal’ contact.

Are regular thermal pads and greases still relevant?

Thermal pads comprising a fibreglass or Kapton carrier coated with a thin layer of thermally-efficient elastomer are still useful in many applications. However, materials like gap fillers and putties, plus surface mount printed circuit board materials with built-in heat spreaders, are much more adept at meeting the thermal management needs of surface mount technology.

Traditional thermal greases always offered a low cost approach where heat transfer without electrical isolation was required. However, they tended to dry out and flow from the interface over time causing a potentially troublesome decline in the thermal efficiency of an assembly. The more advanced thermal putties, with their long-term stability and thixotropic characteristics, offer a more dependable approach.

When thermal putty?

When sheet material?

With its ability to be applied by automated metered dispensing equipment, thermal putty is well-suited to high volume production. The application of sheet or die-cut material is difficult to automate so the need for manual placement makes it more appropriate for lower volumes.

In a like-for-like assembly, putty is more economical than sheet gap filler material. Although it can be used for thermally coupling devices to heatsinks or heat spreaders with large surface areas, putty works better in applications with small surface areas where good repeatability and faster cycle times can be achieved.

Many high-cost devices such as powerful microcontrollers are now housed in ceramic packages. In these applications only minimal assembly forces can be used when attaching a heatsink to the chip as large forces increase the risk of the ceramic package cracking. Putty materials can deflect beyond 70 per cent of their original thickness at light assembly pressures: better than gap filling sheet material. This makes them ideal for filling air voids and promoting good heat transfer in ceramic packages without damaging the device.

Die-cut gap fillers provide the best solution for larger areas where the area to be covered has a complex profile with details such as multiple cut-outs. The part can be designed such that the custom pad just drops into place.

Another important factor regarding thick pieces of die-cut gap filler is their ability to provide vibration dampening. As well as heat transfer, a piece of gap filler compressed between a PCB and chassis can significantly reduce the risk of failure caused by mechanical vibration. In assemblies such as modules fitted under bonnet in passenger cars, vibration can, over time, cause the failure of solder joints and thus in-service failure of the module itself.

Both putty and sheet materials suit post-design quick fixes. It is not always possible to calculate or predict how a finished assembly or piece of equipment will perform from a thermal point-of-view. Although not the ideal approach from a project management standpoint, gap fillers of all types are often ideal for squeezing between a hot component and the nearest metalwork.

What are the main parameters for comparing different gap filling materials?

Data sheets for gap filling materials contain a host of technical specifications, the key criteria that allow a basic comparison to be made are:

  •  Thermal conductivity: Quoted in units of W/m-K thermal conductivity gives a measure of the material’s ability to conduct heat. The higher the figure the more effective the material is at taking heat away from a device. It is important to note that there are other factors that affect the overall thermal performance of a material in an application. For example: assembled thickness, ambient temperature, roughness and flatness of device and heatsink surfaces, and assembly pressure all have a significant impact.
  •  Hardness: Quoted in Shore OO, the softer the material the more effectively it will fill air voids at the interface (for a given assembly pressure) and promote good heat transfer. Softer materials also allow engineers to use lower assembly pressures. This puts less strain on devices and can simplify the mechanical method employed to assemble parts.
  •  Breakdown voltage: Is a measure of the potential difference that can be applied across the material before breakdown occurs. Breakdown voltage is usually quoted in Volts AC.

Do gap filling materials comply with popular standards and new legislation?

      Most gap filling materials are UL94 flame retardant. This is a prerequisite for many types of equipment destined for industrial, consumer and automotive applications. Gap fillers are also compliant with Restriction on Hazardous Substances (RoHS) legislation and certificates of compliance are available on request.