High Electrical Stability Epoxy Mold Compound

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Epoxy Molding Compounds (EMC) by their nature have very good electrical insulation properties. Epoxy Mold Compounds are often called "functional epoxies" or "high solid epoxies" because they are heavily loaded with fillers. These fillers, (typically silica, though other fillers are used for other properties such as thermal conductivity) are loaded more than 50% by weight be default. Highly filled systems have weight % filler loadings higher than 70% with "very highly filled loadings" as high as 92% filled by weight.

These highly-filled systems provide epoxy molding compounds with very good dielectric strength and a very high breakdown voltage, which themselves are good electrical insulation properties. These two values however are poor indicators of what is meant by "Good electrical stability" for EMCs

What is good electrical stability for Epoxy Mold Compounds?

Good electrical stability means that there is very little ionic movement within the epoxy mold compounds when semiconductor devices are under reverse bias at elevated temperatures. The High Temperature Reverse Bias (HTRB) reliability test is an excellent industry-developed and accepted test that tests the electrical stability of EMCs

Epoxy mold compound that are considered to have excellent electrically stability are thus those that:

• Have low ionic conductivity
• Have a low and stable permittivity at temperatures up to 200°C
• Have a high dielectric strength
• Have a stable dielectric constant over a frequency range from 1 kHz up to 1.8 GHz

What causes poor electrical stability in mold compounds?

There are two main causes of poor electrical stability in EMCs:

1. The ingredients used have high ionic content
2. The epoxy resin and hardner combination has high ionic conductivity

EMC subcomponents have high ionic content

Epoxy molding compounds are made up of many components including epoxy resins, curing agents, catalysts, fillers, pigments and additives. Each of these ingredients can contain ions in the form of chlorine (Cl-), Sodium (Na+), Fluorine (F-) and Potassium (K+). There are elements other than these that have ions, but these are the ions most frequently present and in the highest quantities - therefore are of the greatest interest. As you can see next to the elements listed, Chlorine and Fluorine both have negative (-) ions and Sodium and Potassium both have postitive (+) ions.

Of these four elements, the biggest culprits are Chlorine (Cl-) and Sodium (Na+). If the total ionic content is too high, or the ionic content of each of the Chlorine or Sodium is too high, then the risk of gate current leakage and device malfunction increases. In all semiconductor applications, it is prudent to have the total extractable ionic content to be less than 80ppm and the extractable content of each element to be below 20ppm. In high power semiconductor applications which operate at higher voltages and higher temperatures, it is better to have the total ionics to be below 20ppm, and of each element to be below 5ppm. As application temperature and power increase, the lower the ionic content the better.

Epoxy resin/hardner combination has high ionic conductivity

When exposed to the electric field caused by the DC voltage applied, these ions have a tendency to move. If they move too much, a gate current leakage occurs and gradually increases which ultimately leads to the device malfunction.

Different combinations of epoxy resin and hardener, will provide different ionic conductivities of the base epoxy. Specifically, as illustrated in Figure 1 below, a standard epoxy cresol novolac resin with a standard phenolic resin might have a low permittivity at lower temperatures, but quickly elevate at higher temperatures leading to gate current leakage failures for power semiconductor devices.

Why is good electrical stability important for epoxy molding compounds?

The roadmap for medium and high voltage power semiconductors manufacturers is to move from Silcion (Si) die to Silicon Carbide (SiC) and Gallium Nitride (GaN) semiconductors. SiC offers the possibility of operating at higher voltages compared to Silcion, whereas GaN offres the possibility of medium power with a much higher switching frtequency compared to Silicon.

As such, even silicon can applications have applications up to 600V which requires good electrical stability for epoxy mlding compounds. As these applications switch to SiC which can run up to 3300V, the need for improved electrical stability will only be greater.

How can epoxy mold compounds be formulated for good electrical stability?

Epoxy mold compound formulators can develop materials that will pass the HTRB test by developing EMCs that:

• Have low ionic content
• Use ion trappers
• Have low ionic conductivity
• Have low ionic mobility
• Have low permittivity
• Use epoxy resin & hardener combinations that have the lowest ionic content and mobility

How to select epoxy molding compounds with excellent electrical stability?

When looking for epoxy molding compounds that are very electrically stable, look for the following chanacteristics:

• Look for epoxies with a low permittivity over a wide temperature range as in Figure 1.
• Look for epoxies with a low ionic conductivity over a wide frequency range and at elevated temperatures as in Figure 2
• Look for epoxies with a stable dielectric constant up to 1.8 GHz as in Figure 3.
  Figure 1: Low Permittivity means high electrical stability
  Figure 2: Ionic conductivity at different frequencies
  Figure 3: Stable dielectric constant up to 1.8GHz