How To Choose And Use Thermal Gap Fillers

How To Choose And Use Thermal Gap Fillers

Most designers will position thermal materials in electronic assemblies by dispensing cure-in-place gap fillers as automotive electronics, telecom equipment, low-energy lighting, and other sectors demand higher production throughput, thinner bondlines, and reduced mechanical pressure on components and solder joints. This trend will drive changes in the characteristics and handling of thermal materials, as well as in the assembly processes and equipment used on the factory floor.

Cure-In-Place Gap Fillers

Thermally efficient pads and fillers have become vital in meeting demands to increase the performance and reduce the size of electronic assemblies, such as power supplies and control units, without compromising reliability. Today’s thermal materials are typically ceramic-filled silicone elastomers, which are easy to handle. They also conform well to the shape and surface texture of heatsinks or electronic components. And, they have high thermal conductivity relative to the still air they’re designed to eliminate.

In addition, silicone-free thermal gap fillers have been developed for applications that are extremely sensitive to the presence of silicones. These fillers are used primarily where there is a risk of electrical arcing or flashover, which can convert silicones into insulating silica and prevent the correct operation of the system.

Pre-formed gap pads in standard or custom shapes, sizes, and thickness have at least one major attraction—they’re extremely easy to use compared to older material types such as thermal greases. Greases are difficult to use and imprecise, so they aren’t desirable in the modern production environment. However, a growing need is emerging for thermal materials (Fig. 1) that can be formed in place.

Cure-in-place thermal materials, deposited as a liquid, can achieve an extremely thin bondline, which enhances thermal conductivity and is preferred in highly miniaturised assemblies such as automotive electronic control units (ECUs).

Careful calculation of the deposit shape and volume can minimize the pressure exerted on sensitive components such as small surface-mount device (SMD) passives and ball-grid array (BGA) chips when the unit is fully assembled. Manufacturers of BGA devices often specify a maximum pressure per pin, which can be exceeded when the enclosure lid is tightened against a thermal pad placed on the component.

A cure-in-place material, when deposited, also can be relied upon to “wet out” and hence conform extremely well even to rough surfaces. This enhances the elimination of tiny air pockets, improving thermal performance. The optimum volume and shape of the deposit can be assessed accurately by experimenting with glass test components, which provide a clear view of surface coverage and bondline thickness.

In practice, the characteristics of cure-in-place gap fillers can translate into better thermal performance than what can be seen simply by comparing conventional parameters quoted in material datasheets. Even if a cure-in-place material has a lower apparent thermal conductivity (W/m-K) than mature types of thermal materials such as pads, the thinner bondline and excellent wetting properties of the cure-in-place material can deliver improved thermal properties in practice.

As a further advantage of cure-in-place materials, any design changes that alter the position or types of components used can be accommodated quickly by changing the shape and volume of thermal material deposited. If automatic dispensing equipment (Fig. 2) is used, it can be reprogrammed, avoiding any need to reorder gap-filler pads in different sizes or shapes.

Bergquist has successfully developed multiple types of non-corrosive, temperature-stable, thermally conductive dispensable gap filler solutions. These include two-part materials that are designed to be mixed together when dispensed and to wet out to the adjacent surfaces filling even the smallest air gaps and voids.

Curing begins as the two parts are mixed and is completed after the material is in place. Once fully cured, the material remains a flexible and soft elastomer, which helps relieve stresses due to coefficient of thermal expansion (CTE) mismatch during thermal cycling.

Material Characteristics
Dispensable gap fillers are thixotropic to varying degrees, so they will retain their shape after dispensing. An outside force must be applied to wet-out the material to the adjacent surfaces. These materials have a relatively high at-rest viscosity. However, when a shear force is applied, such as during the dispensing process, the viscosity decreases, allowing for ease of dispensing.

In fact, the apparent viscosity depends on the shear rate. Users should bear this in mind when testing and comparing materials. After dispensing, the material will regain its viscosity. It can remain in place on the assembly, then, holding its shape without running or dripping.

The behaviour of the material after dispensing and before curing is described in terms of its slump resistance. This index provides a measure of its internal cohesive characteristics (material consistency) combined with its adhesive characteristics (ability to adhere to the target surface).

Gap fillers have a range of rheological characteristics and can be tailored to meet specific flow requirements, from self-levelling to highly thixotropic materials that maintain their form as dispensed.

Two-part gap filler systems begin curing once the two components are mixed together. The pot life, or working life, is defined as the time for the viscosity to double after this mixing. Pot life heavily depends on temperature. It will decrease at temperatures above 25°C and increase at temperatures below 25°C.

The cure time of a two-part material is defined as the time to reach 90% cure after mixing. Two-part gap fillers will cure at room temperature (25°C), or cure time can be accelerated with exposure to elevated temperatures.

Although gap fillers aren’t designed as structural adhesives, they have an appreciable natural tack when cured that permits mild adhesion to adjacent components. This helps retain the material and eliminates pump-out throughout repeated temperature cycling.

Factors affecting adhesion include surface cleanliness, geometry, and texture. Recommended best practice when using gap fillers is to clean and degrease all surfaces thoroughly, using a solvent, and to allow the surfaces to dry completely before depositing gap filler.

Usage Recommendations
Silicone-based gap fillers typically can withstand continuous use at temperatures from –60°C to 200°C for extended periods of time. In specific applications, however, it can be wise to study the performance and behaviour of the materials at both the low and high end of the temperature spectrum to ensure suitability for the conditions.

Two-part materials must be mixed in a 1-to-1 ratio by volume. As an aid to mixing, without requiring complicated measuring equipment, disposable plastic static mixing nozzles are available. These nozzles can be attached to the ends of cartridges or mounted on automated dispensing equipment and automatically mix the two parts together at the desired ratio.

Bergquist recommends purging newly tapped containers through the static mixer until a uniform colour is achieved. This will ensure a proper 1-to-1 mix ratio. Unless otherwise indicated, mixing nozzles with a minimum of 21 mixing elements are recommended to achieve proper mixing. To ensure consistent material characteristics and performance, Bergquist two-part systems always should be used with matching lot numbers for both parts.

For Best Results, Dispense
Applicator guns and static mixing equipment provide an inexpensive means of dispensing for sampling and low-volume production. Bergquist offers manual hand-triggered guns and pneumatically operated guns in various sizes. Screening and stencilling are also viable for some materials, although material will begin to cure while it’s on the screen or stencil.

Automated dispensing is by far the most suitable approach for high-speed in-line manufacturing. It permits greater repeatability as well as higher throughput.

Integrating Automated Dispensing
Formable cure-in-place gap fillers offer solutions to the challenges facing designers of miniaturised or high-volume assemblies or where the mechanical pressure exerted on components by conventional gap pads must be alleviated.

As the use of such materials increases in applications such as automotive ECUs, telecom infrastructure equipment, and low-energy lamp ballasts, new formulas will emerge offering greater thermal conductivity in addition to outstanding wet-out characteristics and bondline thickness.

A key learning task facing electronic manufacturers today is the integration of precision automated dispensing of gap fillers in high-speed inline production. This challenge is best addressed early, as this emerging class of thermal materials should soon become the norm throughout the electronic industry’s most important sectors.

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