The Essence of Vacuum Dispensing for High-Voltage Substrates
Common knowledge tells us that corrosion is a major stress factor for unprotected wire coils and enamelled copper wires. Depending on the intended subsequent utilization of such parts also mechanical vibrations, humidity and temperature may have to be taken into account. This is of particular importance when dealing with high-voltage parts and substrates, because the volume resistivity of the dispensing material has significant influence on high-voltage insulation. Of course, simply pouring potting material over a coil does not solve all these problems. The dispensing material or the tiny gaps between the wire coils may contain air trapped within. This significantly reduces high-voltage insulation or ruins it altogether. Therefore, to ensure an impregnation that is completely free of air bubbles, the entire preparation, feeding and metering process must be carried out in a vacuum.

1 Material Processing on Solid Ground

The air bubbles still present in the thin film are made to burst by the vacuum and are thus removed from the material.

To render production guaranteed air bubble free the media must be prepared in a vacuum. A high-quality processing unit removes all traces of dissolved air by thin-film degassing within an evacuated material container. An agitator is used to further speed up the degassing process in the vacuum tank by stirring and constantly circulating the dispensing material around the container. This lets all the contained air rise to the surface of the material where it gets in contact with the surrounding vacuum. The degassing effect sets in at these upper material layers.

In order to prevent air from re-entering into the material during material reloading all fittings, material feed lines, pumps and valves are sealed air-tight or otherwise made vacuum-proof. There is even more to consider during preparation and reloading. Filler materials, especially in liquids of low viscosity, tend to sediment. The gravity causes the fillers contained in the liquid to settle at the bottom. This leads to variations of the amount of fillers contained in the dispensing material, depending on where material is taken out of the barrel. This separation of fillers from the liquid influences the material’s consistency. The resulting density change leads to material that no longer complies with specifications and impairs the quality of the mix ratio. This may result in the use of insufficiently suited dispensing material during production, which is inferior to specifications and produces low-quality results or even scrap. The solution: A well thought out stirring mechanism that ensures a consistent degree of homogeneity and a well-timed circulation of the material in the containers, pumps and feed lines. This combination effectively prevents sedimentation, particularly during production breaks.

When it comes to dispensing materials with a high content of fillers consideration must be made that fillers are often hard and abrasive. This increases the risk of damage to the feed pumps. In order to avoid unnecessary failures and high maintenance costs it is therefore advisable to consider only such systems, which are engineered specifically for the use with such abrasive media already in the procurement stage.

2 Precision is All in Metering and Dispensing

For the metering process precise metering of the two components A and B is essential. Inaccuracies occur mainly where the metering process might be influenced by variations of pressure, temperature or viscosity. A volumetric metering principle with a mechanically defined mix ratio and monitoring of the quantities to be metered increases process reliability and guarantees a consistently high quality. Especially in vacuum dispensing the metering system must be precisely matched to the material processing and dispensing process. A user-friendly metering system should be designed such that the two components mix after metering, for example, in a static mixing tube. This prevents a reaction between casting resin and hardener in the metering head. Besides rendering expensive cleaning work unnecessary, this makes a failure of the metering system on such grounds virtually impossible.

3 Parts in a Billion Mixing
After metering the two components separately they are joined and casting resin and hardener are homogeneously mixed. A highly cost-effective and efficient approach is using a static mixing tube made of synthetic material. The tube contains a series of screw-shaped deflection panels offset by 90° to one another. A two-component dispensing material flowing through the tube is fragmented billion-fold depending on the number of deflection surfaces. At the end of this process the materials are merged again with a phase offset until finally the layers are so thin that uniform cross-linking and hardening can be guaranteed.

4 Dispensing in a Vacuum
From an engineering point of view a perfect vacuum (which is entirely void of air) is not necessary when potting electronic components. Talking of vacuum in a dispensing process means a reduction in pressure down to one millibar (0.01 PSI) max. A further reduction of the atmospheric pressure takes longer and requires more energy, which would increase costs and time effort. Therefore, the vacuum level should always be matched to the task at hand. Also, not every component is capable of sustaining a strong pressure reduction, a fact to bear in mind when applying a vacuum. While wire coiled components are mostly insensitive to atmospheric pressure, air encapsulated within a capacitor can cause the substrate to burst when exposed to an external vacuum. Usually, the vacuum will range between 2 and 50mbar (0.03 – 0.73 PSI). At such values some traces of air may remain within the component and become trapped by the dispensing material. This remaining air must be eliminated during the dispensing process or instead be completely displaced by the rising dispensing material. A well thought out component design that considers all of the above significantly supports production cycle time and production economy.

The dispensing process in a vacuum chamber should be performed in different steps, particularly when processing wire coiled components. Initial filling with the dispensing material, intermediate venting to press material deeper into the coil, and last covering.

5 Component design
Horizontally positioned, wide-stretched components may block rising air bubbles and prevent air from escaping. When placing a wire coiled component under such a surface, air caught between the wires cannot rise to the surface.

6 The Target is the Perfect Dispensing Result
It is essential to take into account all manufacturing steps around the dispensing process. Because in the end success will depend on the geometry of the component, the dispensing medium, the cycle time and the ensuing production processes, such as assembly, plasma pre-treatment, pre-heating or curing. Only when all requirements and influencing factors are considered it is possible to design an ideal dispensing run and a complete manufacturing process.

Scheugenpflug Inc. USA
2125 Barrett Park Drive, Suite 104
Kennesaw, GA 30144 USA
email: sales.usa@scheugenpflug-usa.com
web: www.scheugenpflug-usa.com
phone: 770-218-0835
fax: 770-218-0931