The evolution of thermal management. The dispensing of in-situ curing gap fillers is expected to become the dominant technique for placing thermal materials in electronic assemblies, as sectors such as automotive electronics, telecommunications equipment, and energy-efficient lighting demand higher production capacity, thinner glue lines, and reduced mechanical stress on components and solder joints. This trend will drive changes in the characteristics and handling of thermal materials, as well as in assembly processes and the equipment used in factories. In-situ curing void fillers
Thermally efficient pads and fillers have become essential to meet the demand for increased performance and reduced size in electronic assemblies, such as power supplies and control units, without compromising reliability. Today, thermal materials are typically ceramic-filled silicone elastomers, which are easy to handle, conform well to the shape and texture of the surface of heat sinks or other electronic components, and have high thermal conductivity relative to the still air for which they are designed to be evacuated.
Furthermore, silicone-free thermal gap fillers have been developed for applications highly sensitive to the presence of silicone. They are primarily used when there is a risk of arcing or convolution, which can convert silicones into isolated silica, preventing proper system operation.
Pre-molded filling pads in standard or custom shapes, sizes, and thicknesses have at least one major advantage: they are extremely easy to use compared to older materials such as thermal greases. Greases are generally considered difficult to use and imprecise, and therefore undesirable in modern production environments. However, there is a growing need for thermal materials that can be formed in situ (Figure 1).
Liquid-deposited, heat-curing thermal materials can achieve an extremely thin glue line, which improves thermal conductivity and is preferred in highly miniaturized assemblies such as automotive electronic control units (ECUs). Careful calculation of the deposit size and volume can minimize the pressure exerted on sensitive components such as small SMD passives and BGA chips when the unit is fully assembled; BGA device manufacturers often specify a maximum pressure per pin, which can be exceeded when the locking cap is pressed against the thermal pad on the component.
Furthermore, once deposited, the in-situ curing material impregnates itself completely and therefore conforms extremely well even to rough surfaces. This improves the elimination of small air pockets, thus increasing thermal performance. The optimal volume and shape of the deposit can be precisely assessed by experimenting with glass test pieces, which allow for clear visualization of surface coverage and adhesive line thickness.
In practice, the characteristics of in-situ curing gap fillers can translate into improved thermal performance, which is readily apparent when comparing the conventional parameters listed in material data sheets. Even if an in-situ curing material has a seemingly lower thermal conductivity (W/mK) compared to established thermal materials like pads, the thinner glue lines and excellent impregnation properties of the in-situ curing material can contribute to better thermal characteristics in practice.
As an added advantage of in-situ curing materials, any design changes that alter the position or types of components used can be quickly implemented by changing the shape and volume of the deposited thermal material. If an automatic dispenser is used, it can be reprogrammed. This eliminates the need to order new filler pads in different shapes or sizes (Figure 2).
Bergquist has successfully developed multiple types of non-corrosive, temperature-stable, and thermally conductive dispenseable filler solutions. These include materials composed of two different elements, designed to mix upon dispensing and impregnate adjacent surfaces to fill even the smallest gaps and air voids. Curing begins when the two components mix and is completed after the material is placed in position. Once fully cured, the material remains a soft, flexible elastomer, which helps alleviate stresses caused by unequal coefficients of thermal expansion (CTE) during thermal cycling.
Material Characteristics:
Bergquist dispensable gap fillers are thixotropic to varying degrees and will therefore retain their shape after dispensing. An external force must be applied for the material to penetrate all adjacent surfaces. These materials have a relatively high viscosity at rest. However, when a shear stress is applied, such as during dispensing, the viscosity decreases, facilitating dispensing. In fact, the apparent viscosity depends on the shear rate; users should take this into account when testing and comparing materials. After dispensing, the material will regain its viscosity and thus be able to stay in place within the assembly, retaining its shape without moving or dripping.
The material's behavior after dispensing and before curing is described in terms of slump strength. This index provides a measure of the internal cohesive characteristics (material consistency) in combination with its adhesive characteristics (ability to bond to the target surface). Bergquist's void filler incorporates a range of rheological characteristics and can be customized to meet specific customer fluid requirements, from self-stabilizing to highly thixotropic materials that maintain their shape upon dispensing.
Two-component filler systems begin curing once the components are mixed. Pot life is defined as the time it takes for the viscosity to double after mixing. Pot life is highly temperature-dependent; it decreases above 25°C and increases below 25°C.
The curing time of the two-component material is defined as the time until 90% curing is reached after mixing. Two-component void fillers will cure at room temperature (25°C), or curing can be accelerated by exposure to elevated temperatures. Bergquist materials are available in various pot life and curing time variations; these parameters are indicated on each product's data sheet.
Durability under specific storage conditions is also indicated.
Although gap fillers are not designed as structural adhesives, they have a noticeable natural tack upon curing, which allows for slight adhesion to adjacent components. This helps retain the material and eliminates leakage over repeated temperature cycles. Factors affecting adhesion include cleanliness, geometry, and surface texture. Therefore, as a best practice when using gap fillers, we recommend thoroughly cleaning and degreasing all surfaces with a solvent and then allowing them to dry completely before applying the gap filler.
Usage Recommendations:
Silicone-based gap fillers typically withstand continuous use at temperatures between -60°C and 200°C for extended periods. However, in specific applications, it is worthwhile to study the performance and behavior of the materials across the spectrum of extreme temperatures, both high and low, to ensure their suitability for these conditions.
The two-component material must be mixed at a 1:1 ratio by volume. To aid in mixing without the need for complex measuring equipment, Bergquist provides disposable static plastic mixing injectors. These can be attached to the ends of the cartridges or mounted on the automated dispenser to automatically mix the two components at the desired ratio. Bergquist recommends purging freshly capped containers through the static mixer until a uniform color is achieved. This will ensure a correct 1:1 mixing ratio. Unless otherwise specified, mixing injectors with a minimum of 21 mixing elements are recommended to achieve the proper mix. To guarantee consistent material characteristics and performance, Bergquist two-component systems must always be used with the corresponding lot numbers for both components.
For best results, the dispenser,
application guns, and static mixer are cost-effective dispensing methods for samples and small-batch production. Bergquist offers manual guns for 50cc, 200cc, and 400cc cartridge sizes, as well as pneumatic guns for 200cc and 400cc cartridge sizes. Figure 3 illustrates examples of the available two-component cartridge sizes. Screening and stenciling are also viable for some materials, although users should consider that the material will begin curing upon screening or stenciling.
Automatic dispensing is by far the most suitable method for high-speed in-line manufacturing; it allows for greater repeatability as well as higher throughput. Bergquist supplies filler materials in large-capacity container assemblies up to 10 gallons (37.8 liters), designed to ensure quick and easy loading and unloading.
Bergquist has partnered with several experienced automated dispenser suppliers to help customers create an optimized dispensing process. This type of cooperation can contribute to optimizing equipment design and performance.
Close collaboration with equipment suppliers also helps minimize equipment lead times, allowing customers to move quickly once dispensing equipment requirements are finalized. This can be an important criterion for rapidly changing industries, such as consumer electronics and automotive, to adopt in-place curing gap fillers as a higher-performance alternative to conventional thermal materials.
Conclusion:
Conformable, in-situ curing gap fillers offer solutions to the challenges faced by designers of miniaturized or high-volume assemblies, or where the mechanical stress exerted on components by conventional gap pads must be reduced. As the use of these materials increases in applications such as automotive ECUs, telecommunications infrastructure equipment, and energy-efficient lamp ballasts, new formulations will emerge offering higher thermal conductivity, excellent impregnation characteristics, and exceptional glue line thicknesses.
A key learning challenge facing electronics manufacturers today is integrating precision automated gap-filler dispensers into high-speed online production. This challenge is best addressed in the early stages, as this emerging class of thermal materials is expected to become the norm in major sectors of the electronics industry soon.
Author:
Uwe Jessen, The Bergquist Company
