Plastic Electrical Properties

The Big Three

If you are like me, you’re often asked about certain plastic materials’ “electrical properties” and immediately you get that sinking feeling in your stomach. “Can we change the subject?” Well, let’s see if we can simplify what I call the BIG THREE ELECTRICAL PROPERTIES: Dielectric Strength, Dielectric Constant, and Dissipation Factor to have a general understanding of what these refer to in terms of plastics selection for the Semiconductor market.

DIELECTRIC STRENGTH (DS)

Let’s start with Dielectric Strength since I think it’s the most straightforward and thus easiest to understand. Simply put, a material’s DS is a measure of the material’s capability as an insulator. DS is the measure of the maximum voltage required to produce a dielectric breakdown through the material and is measured in volts/material thickness (mil).

Testing for DS is simple, a sample between 0.8mm to 3.2mm thick is placed between two electrode plates (thicker samples are often tested in oil, thinner in air). The voltage is then systematically raised to achieve dielectric breakdown which is characterized as the decomposition or burn-through of the plastic. The DS is then calculated by dividing the breakdown voltage by the thickness.

Dielectric Constant (Dk)

OK, time to move from the baby pool to the deep end. In simple terms, Dk characterizes the ability of a plastic to store electrical energy. More specifically, the Dk of plastic is the ratio of the charge stored in an insulating plastic between two metallic plates to that of the insulative material being replaced by air or vacuum. A material that has a Dk of 3 will absorb three times the electrical charge than a vacuum.

Dk is a measure of the ability to absorb and hold an electrical charge. Since capacitors are reservoirs for energy storage, the size of the reservoir is in direct relation to the Dk of the plastic material. Essentially, the Dk measures how well an insulator stores electrical energy to isolate electrical elements from each other. The more conductive a material, the greater the Dk.

DISSIPATION FACTOR (DF)

In simple terms, DF measures the ability of the insulator to not let the charge held dissipate easily as the electric field shifts. DF is a ratio of the loss factor to the dielectric constant. It measures the electrical energy stored and lost when an electrical current is applied. Most of the absorbed energy is released as heat. The dissipation factor indicates the inefficiency of material to hold energy or behave as an insulating material under current. The lower the dissipation factor, the more efficient the insulator system. DF is measured at different frequency levels for practicality.

The practical application of these electrical properties depends on many factors including the desired function of the plastic application and the frequency of the electrical system. Electrical systems use a current that alternates between positive and negative voltage. Efficiency is measured in the number of oscillations known as electrical frequency. The unit of measurement is Hertz (Hz) of which 1 Hz equals one cycle per second.

Today’s Semiconductor Tools run at extremely high frequencies. While a standard power line generally runs at 50-60 Hz, it is not uncommon to have Semiconductor systems that operate in multiples of the MHz range or 1+ million Hz. Thus, understanding the electrical properties of plastics as insulators at the desired frequency range is critical to designing a system that performs and is safe.