Today, TIM is available in many variants. TIM pastes often differ in their material composition. They are highly filled with a wide variety of metals and precious metals. However, these materials, which are then abrasive, place high demands on the dispensing process: Since they are expensive, they must be dispensed precisely and without losses if possible. After application, the fillers must be evenly distributed in the heat-conducting layer so that the heat is safely dissipated and
the heat conduction pastes must be applied without bubbles, as air dissipates virtually no heat.
In one project, a heat-conducting paste filled with silver was to be applied to an electronic component. For this purpose, the material was initially dispensed by contact with an eccentric screw pump. During the test phase, various classic problems were encountered:
- Limited thermal conductivity due to unclean dispensing results caused by broken lines, air bubbles and the TIM flowing after dosing.
- Long cycle times due to slow application through feed movement in the z-axis. To form a bead on the component, the dispensing speed of the pump had to be slowed down.
- Unnecessary material consumption due to deviating volumes during reflow and dead spaces of the metering system.
- Material preparation and mixing of the micro quantities were difficult. The material had to be prepared more elaborately under vacuum to achieve the required freedom from bubbles. Static mixing tubes were not suitable for mixing the silver-containing TIM in this case. The volume flow in the static mixing tube was so low that the two components did not mix optimally.
- Costly cleaning and maintenance - The mixed TIM "infected" freshly mixed and re-feeding material in the dead spaces and changed its metering properties and function.
The solution - dispensing with jet valves
With the PDos X1 from perfecdos, the TIM is now applied contact-free in individual drops at a very high frequency and the desired application pattern is created. The optimal distribution of the fillers is ensured. With this system, the droplets can be between 500 µm and 1.5 mm in size. The materials are dispensed with a repeat accuracy of 99% in relation to the dosing volume. With regard to the above-mentioned five problems in contact dosing, jetting with this system offers the following advantages:
- With lines, metered by dot application, interruptions are reliably avoided.
- The non-contact application eliminates the need for a feed motion. The system's 300Hz dispensing frequency allows the process to be accelerated even further. Dispensing is no longer the bottleneck of the overall process.
- The system has no dead spaces. This prevents infection with "old medium" and air pockets in the system. The small internal volume of the dispensing system is also advantageous. The actuator system is completely separated from the part of the valve that comes into contact with the fluid. The internal volume of the system is only approx. 0.258 ml. Due to the principle, this volume is considerably lower than in other dosing systems. An eccentric screw pump has an internal volume of approx. 4 ml.
- The TIM are dynamically mixed and degassed after mixing.
- Quick and easy cleaning of the valves (no dead spaces) allows effective jetting processes.
The precise contactless dispensing of TIM with the PDos X1 jet valve is a concept that pays off, because high-precision contactless dispensing applies TIM only where heat is to be dissipated. "In combination with easy cleaning and the avoidance of - in several respects - cost-intensive dead spaces, this quickly saves more than €1,000 per shift (depending on the material price)," classifies Benjamin Kratz, the savings potential that can be achieved with these materials. Under total cost of ownership considerations, 300 cycles/s, which enable faster production with minimised waste, a simpler machine design and process monitoring add to this calculation.
Truss:
perfecdos GmbH specialises in solving individual and demanding dispensing tasks. The company develops, manufactures and sells microdispensing systems that can be used in industry for the application, filling and dosing of liquids of all kinds in line with requirements and processes. The basis of the solutions is the "PDos X" product series - an electro-pneumatically driven jet dispensing valve that has very harmonious, precise and fast switching characteristics thanks to a special aeration and deaeration process. This allows the dispensing of low to highly viscous liquids in the low nanolitre range. More than 17 years of experience with electropneumatically as well as piezoelectrically driven jet dispensing systems went into the development.
The dispensing system in the video: ⇒ https://www.youtube.com/channel/UC8PbNTYpSu_vYNI840qc7SA
