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Dosing of thermal pastes
At the side of the material manufacturer, special thermally conductive materials that have been supplemented with fillers and additives such as, for example, thermally conductive adhesives, casting compounds or gap fillers are offered for the controlled removal of heat in electronic modules The transfer of heat is often done in these materials by the use of solid particles (metallic, ceramic or organic) that specifically increase the thermal conductivity for the fluid as a whole through their material properties. However, these particles also result in excessively abrasive properties in the heat transfer pastes. The solid particles must be transported as well as constituent parts of the paste when dosing, but also place disproportionately higher requirements on the material feed and dosing systems with regard to mechanical stress and wear when compared to dosing with purely paste-type unfilled materials. In addition, heat transfer pastes have a very high density of up to 4 g/cm³ so that when it comes to transport the material a continuously operating dosing technology has considerable advantages due to the required high acceleration forces with regard to the mechanical stress, and, last but not least, the accuracy of dosing as compared to simpler systems.
Furthermore, when dosing the heat transfer paste it is necessary to prevent any sedimentation of the fillers and any separating of the binders and the functional materials that are used in the pastes to ensure consistent product properties for a high-quality process. The effect of sedimentation is shown in the following illustration: (Fig. 2)
The sedimentation effect in a dosing system can likewise be prevented in the first instance by consistent material transport while avoiding pressure fluctuations and by keeping the pressure level low in general.
There are additional requirements for the process technology when applying the paste to joined components with an upper and lower part to produce a transition area all the way through to remove the heat and in particular to achieve a visually perfect connection without any protruding material, since the joins are frequently in a visible area. This application requires exact and consistent bead application during the dosing process and a qualitatively flawless dosing bead termination. (Fig. 3)
The dosing task imposes overall requirements in an automated, production-relevant environment that go beyond what is possible with conventional technologies. This results in the fact that it is necessary to ensure process stability to maintain product quality and economy of operation for these applications. For example, piston pumps are not suitable for dosing, because the overall friction effects are increased by the two-stage intake and delivery steps. Furthermore, the pressure pulsation that arises during the piston stroke stimulates sedimentation in the fluid during the dosing process. Massive friction effects occur also in geared type pumps as a result of their construction. What is required is a technology that makes possible the dosing process with little mechanical friction between the solid particles of the paste and the functional components of the dosing pump.
For that reason the endless piston principle used by ViscoTec has shown itself to be a suitable technology for the application of heat transfer pastes so as to be able to comprehensively meet the tough requirements stated above for this process step. The technology that is offered by ViscoTec involves a volumetric and highly dynamic dosing principle that also makes highly accurate dosing possible for other equally demanding processes. The special dosing geometry guarantees a pulsation-free conveying flow and minimal areas subject to friction in the components. The interaction between the rotor and the stator produces enclosed chambers with identical volumes that also do not change during the dosing process. This makes it possible to dose fluids that are laden with solids with very little shearing and the low friction that is required for resistance to wear. Here the material is conveyed from the inlet side to the discharge side of the pump and in an even and continuous moving flow in one direction on the basis of a dosing geometry without undercuts and a minimum of dead space volume. The amount to be dosed is regulated by the drive unit proportionally to the speed of rotation and thus can be set continuously and is free of pulses. The pump system forms a multi-dimensional sealing line between the rotor and the stator. For that reason the system does not require any additional valves, meaning that the number of components that are subject to potential wear is kept to a minimum. A number of different elastomers that are chemically very resistant are available for use as the stator material. The dosing pumps can therefore be designed optimally for the different kinds of materials.
The even and consistent volume flow reduces to a considerable extent the acceleration moments caused by the material and subsequently acting on the dosing components so that paste with high material densities can also be processed with a high degree of process safety with the ViscoTec systems. The systems are operated in the low pressure range and the continuous conveying flow suppresses pressure fluctuations in the system. Sedimentation effects are counteracted with this technology and at the same time an even application of the solid particles after the paste has been applied can be assured. The required precise and consistent bead application of pastes in connection with joining processes for parts is likewise assured by the technology. The dosing volume per time unit can be controlled exactly at the beginning and end of the application of the bead through the option for speed ramps in the control of the dispenser to be able to achieve a precise and reproducible bead connection. This precision is retained in particular even when using dosing needles of a small diameter, also in connection with materials with high viscosities. The dosing needle diameter directly affects the dosing pressure which at the same time also acts as a back pressure in the dosing principle. The endless piston principle as a purely volumetric dosing procedure therefore always provides the exact set dosing volume at the dosing outlet, regardless of the back pressure and viscosity fluctuations in the fluid.
The endless piston principle from ViscoTec also finds application in material extraction and treatment systems in addition to use in dispenser systems. In this way ViscoTec can guarantee a uniform and high quality processing of heat transfer pastes with the technology described here all the way through the entire process, from the emptying of the material up to the dosing process. High resistance to wear is ensured by the overall system which in turn assures a high level of availability for an overall production line.
The engineers of ViscoTec Pumpen- u. Dosiertechnik GmbH would be glad to support you on all matters concerning the dosing and preparation of the material discussed above.
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