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Polyester PET and Polyethylene Naphtalate PEN capacitors

This article is the third part of a complete installment on the construction, application, and features of film and foils organic dielectric capacitors, divided into 6 sections:

Introduction to film capacitors
Evolution and applications of paper capacitors
Polyester PET and Polyethylene Naphtalate PEN Capacitors
Polypropylene PP /KP/MKP capacitors
Polycarbonate PC /KC/MKC capacitors
Polystyrene PS, Polyphenylene sulfide PPS and other plastic film capacitors Teflon PTFE / Polysulfone PSU

Sometimes polyester capacitors are called Mylar. The abbreviation PET above comes from Poly-Ethylene Terephtalate (also abbreviated PETP). On the pattern of European standards, the common abbreviations is KT for film/foil design and MKT for the metalized film. Poly- Ethylene Naphtalate uses the commercial abbreviation PEN as’ polyester’.

Polyester PET Capacitors

Polyester film is the most reliable and, together with PP, most users of plastic films. It can be produced in thicknesses down to 0.7 μm (0.03 mils). Its tensional stability is high, and it’s ε_r ≈ 3.2. This has facilitated the manufacture of one for organic dielectrics very space-saving capacitor. A typical field of application is decoupling. Certain applications like switched mode power supplies (SMPS) require filtering and decoupling purposes for large capacitance and moderate losses, making the MKT capacitor an attractive replacement for ceramic X7R capacitors. A common design may look like the one in Figure 17.

Furthermore, the MKT capacitor can stand +125 °C. This has tempted many manufacturers to produce it in a chip design. But heat-conditioned shrinking is most pronounced in PET capacitors. This has led to many setbacks just for the SM types. Today mainly two different methods apply for preventing such heat rises during the soldering procedures that release a detrimental shrinking.

The components are supplied with such encapsulations and electrode designs that the heat penetration into the component is obstructed.
The molecular memory of the initial location before the elongation is deleted in a heat and pressure process during manufacture.

Figure 17. Example of an MKT capacitor design intended for interference suppression.

Thus, the risk of shrinking is reduced with the latter method, which is restricted to stacked designs only. Therefore, the dimensions can be held down with a minimum of encapsulation. Then, however, the heat penetration will be higher and at temperatures of 230 °C the film material is damaged. One solves, in other words, the shrinking problem but increases the risk of serious material damage. Soldering methods and temperatures have to be chosen and supervised so that the chosen capacitor type can stand these processes. An infrared (IR) reflecting film and IR soldering seem, for the moment, to be the only way out as long as we deal with traditional PET films. The latest films are more resistive, specified for 125 °C, and can endure peak temperatures reaching 235 °C at reflow soldering. The film manufacturer has altered the thermo-mechanical properties to reduce shrinkage when used as a dielectric in SMD capacitors.

ESR measurements best control the shrinking effects at the resonance frequency before and after soldering.

Advantages with PET

Thin film with high quality.

Disadvantages with PET

The temperature dependence is comparatively large and non-linear.
The dissipation factor is comparatively large, approximately 0.5% at 1 kHz.
The material suffers from properties that, for example, hi-fi applications (analog technique) may give perceptible distortion.

Poly-Ethylene Naphtalate (PEN)

is a relative to polyester but somewhat more heat resistant. Capacitors with this dielectric often are presented under the title polyester. The material attracts interest mostly for SMD designs. They are manufactured in stack technology with a film thickness of down to 1.5 μm (0.06 mils). According to some papers, such chips can stand up to both wave and IR soldering, but only IR and vapor phase soldering is recommended. However, recent developments and film improvements indicate a considerable step upward in the temperature range. With a derating of the applied operating voltage above 125 °C an upper temperature of + 150°C is quite possible.

The material is still too expensive to be used on a large scale.

Capacitance 1 nF… 10 μF.
Tolerance ±5% and ±10%.
Temperature range -55/+125°C (+150 °C).
Rated voltage 16…400 V DC.
Tanδ , 1kHz, 20°C, ≈ 0.4…0.5% (≤0,8%).
IR, 20°C, ”typical” 30 000 s; ≥1 GΩ or ≥ 400 s.
Stability ΔC/C ≤5%.
TC ≈ +220±10ppm/°C (average over the temperature range).
ε_r ≈ 3.2.
Dielectric absorption ≈ 0.15 %.
Recommended derating 0.6xVR.

Temperature and frequency dependencies

We will start with two three-dimensional diagrams that show how ε_r (and with that C) and Tanδ (DF) change with frequency and temperature. Specified measurement points have been inserted as ovals in the diagrams (Figures 18. and 19.).

The diagrams are interesting because they show how temperature and frequency rule the parameters in a complicated manner, especially Tanδ.

Figure 18. εr of PET versus temperature and frequency

Figure 19. Tanδ of PET versus temperature a

The following diagrams are more common and easy to interpret. In some of them, there are curves inserted for a PEN, i.e., Poly Ethylene Naphtalate. It is comparatively more temperature-resistant than PET and is used in SMD types.

Typical resonance frequency ranges are shown among the curves over the frequency dependencies of impedance. Because capacitance changes comparatively little and losses are rather small, impedance diagrams bend pliable downwards into a sharp tip around the resonance frequency. Examples of resonance frequencies are shown in Figure 25.

Figure 20. Area for typical capacitance versus temperature of PET capacitors. A typical PEN curve is also shown.

Figure 21. Typical curves for capacitance versus frequency of PET and PEN capacitors.

The temperature dependence of capacitance is also influenced by its magnitude, the dielectric thickness, and the winding design. This is reflected in the distribution at higher temperatures.

The frequency dependence of capacitance is ruled not only by the material but, to some extent, by its magnitude and capacitor construction (foil or metalized design). Sometimes there are shown examples of series capacitance measurements where an apparent increase occurs at higher frequencies. This increase is hinted at by the dotted line in Figure 21.

Figure 24. Typical curve area for the temperature dependence of IR for PET capacitors.

Figure 25. Example of resonance frequencies for 2 modules MKT capacitors.

Other manufacturers and other module distances give other resonance frequencies. Always check with the manufacturer’s datasheet.

The seemingly sharp points in the impedance curve look more round in another magnification. Figure 26. Results from measurements of MKT, MKC, and MKP capacitors, 100nF. The slightly lower losses for MKC and MKP capacitors are shown as a lower ESR.

Failure modes

Bump, vibration, and temperature cycling may, above all hermetic cans, cause an interruption in internal termination leads or in the connecting link terminal lead – spray metal compound – metalizing. Types that are molded into epoxy run less risk. If the casing, on the other hand, consists of plastic, the humidity will diffuse into the component. Humidity, however, impairs primarily only in the state of liquid (for example, at a condensation), which will cause electrolysis, especially in the presence of a DC load. Thus non-hermetic components of this type of SMD design can bear a high RH, at least for 1000 hrs. If we can accept a C of +3 to +5% (ε_r for water approximately 80) and a decreasing IR, no harm is done. When the moisture content outside of the component decreases, the humidity diffuses out of the winding, and the capacitance increase goes back. If the component, on the other hand, operates in a jungle climate for months, the IR will decrease until the leakage current starts electrolysis in the metallization. It will end in an open circuit.