Power requirements continue to escalate for the next generation electronic systems aboard naval ships.

New Wide Band Gap (WBG) semiconductor materials, principally SiC, offer the necessary material properties to address the higher power performance challenges. Continuous power switches, power diodes, and pulsed power switches fabricated from SiC, offer reductions in on-state resistance and switching loss over conventional silicon power devices. For a given power rating, these components can operate at a higher duty cycle. SiC power electronics also extends solid state technology by offering higher breakdown voltage levels (than current silicon technology) to address
voltage levels presently managed by electromechanical switch technology.

Wide Band Gap semiconductors represent a possible paradigm shift in semiconductor power density. WBG devices operate at higher temperatures and consequently require less cooling. The higher blocking voltages and lower switching loss at high frequency of SiC devices, as compared to silicon devices, make these devices ideal for use in smaller transformers and inductors. Improved thermal management of semiconductors and passive components through upgraded packaging would allow more current to be handled by a given device and lead to improved power density designs. Present electronics packaging technology does not allow the designer to take full advantage of the high temperature operation capabilities of WBG semiconductors. This is due to limitations on the maximum operating temperature of common organic materials used for packaging power components. The next step is to develop high temperature packaging technology that is applicable to current power devices/modules as well as readily adaptable to future power device technology.

Silicone gels have unique properties that provide mechanical protection to circuitry, high temperature stability to 200°C, electrical insulation to 20kV, chemical resistance, and sealing capability. Although silicone has a high CTE (coefficient of thermal expansion), the very low modulus of the gel allows encapsulation of delicate wire bonds without exerting excessive strain.

Figure 1-1 shows that silicones are distinctly different from organic polymers due to their silicon-oxygen (Si—O) polymer backbones. The silicone polymer dimethyl polysiloxane on the left has a Si–O backbone instead of the C–C backbone seen in the organic polymer natural rubber on the right. This silicon-oxygen linkage is responsible for its outstanding thermal and chemical resistance and is the same chemical bond found in highly stable materials such as sand, quartz, and glass. The lack of any unsaturated double bonds in the silicone backbone, unlike the organic primary backbone, make silicones highly resistant to oxidation and ozone attack.

Curing is simply the conversion of many individual polymer molecules into one highly connected molecule. Silicone polymer systems typically consist of two parts that are mixed together. By including vinyl side groups along the polymer chain (< 1 mole%), a crosslinking reaction occurs called vinyl hydrosilation (shown in Figure 1-2). This cure mechanism is called “addition” or “platinum” cure and has minimal shrinkage and produces no by-products. The two liquid parts contain a vinyl functional group (CH=CH2 on the bottom) and a hydride functional crosslinker (on the top). Mixing the two parts initiates the curing reaction and produces the solid silicone polymer (on the right).

Encapsulation of high voltage components prevents the initiation of corona and other destructive phenomena that can occur in the presence of air at high voltage levels. It significantly improves performance and reliability and provides a clean, stable, long life environment for all critical high voltage components.

In order to keep the encapsulation process void-free, special mixing and dispensing equipment and vacuum chambers are used. A planetary centrifugal mixer (Figure 1-3) is used to combine the two silicone parts by revolving the mixing container at a 400g acceleration while simultaneously rotating. If the mixing cup only revolved during processing, the air bubbles would be eliminated, but the mixture itself would separate. The rotation of the cup brings the contents back into the center to accomplish powerful mixing and de-aerating simultaneously.

To eliminate trapping any air during the encapsulation of the power module, introduction of the silicone to the module is done under vacuum using a specialized application process (Figure 1-4). The beaker of mixed and de-aired silicone is placed under the vacuum tubing which has been sealed with a pinch clamp. After placing the module to be potted (or in our test case, a simple aluminum pan with simulated wire bonds) into the encapsulator, the chamber is evacuated. By slowly releasing the pinch clamp, the silicone is pulled through the tubing into the evacuated chamber and fills the aluminum pan. Since no air is present, the silicone can evenly fill the “module” without having to displace air pockets. The thick, clear cover on the encapsulator seals the chamber while allowing observation of the filling process. After completion, the vacuum allows any residual de-aeration to occur. This results in a void-free potted module.

The chamber is then vented to atmosphere and the sample moved to a vacuum oven (Figure 1-5). Temperature drives the hydrosilation reaction. Increasing the temperature causes the silicone to cure more quickly while lowering the temperature slows down the reaction. Depending on the polymer system, some silicones can cure at room temperature requiring an extended time (usually a week) while others can be heated to cure in minutes. The curing time and temperature profile is unique to each polymer system and is provided by the silicone manufacturer. The vacuum oven provides the high temperature for curing and removes any additional air. While the oven has an internal thermocouple and a microprocessor to control the temperature, the sample temperature is more accurately determined by a closely placed additional thermocouple (shown inside figure 1-5). Since there is no air for convection to occur inside a vacuum, the thermocouple should be touching the sample side for accurate readings. The oven temperature should be adjusted so that the sample sees the precise temperature for the full curing time.

Full cure is achieved when all of the sites available for crosslinking have reacted. In a gel, the silicone does not become very hard, but does form a network of polymer chains that are immobile and protective of the circuitry that is encapsulated. The fully cured silicone gel prepared in Figure 1-6 is optically clear to allow visualization of the wire bonds and any bubble formation. Other silicones have been formulated with a wide range of physical properties, in any color and clarity from transparent to opaque depending on the properties desired. This provides the engineer great freedom in choosing the appropriate encapsulation for a particular electrical component.

The equipment described above allows the EMPF to produce void-free and crack-free encapsulations that are essential for reliable high voltage components.