Film and Foil Organic Dielectric Capacitors

Manufacturing Process

The following example describes a typical manufacturing process flow for wound metallized plastic film capacitors.

metallized wound film capacitors manufacturing process; source: Wikipedia

1. Film stretching and metallization — To increase the capacitance value of the capacitor, the plastic film is drawn using a special extrusion process of bi-axial stretching in longitudinal and transverse directions, as thin as is technically possible and as allowed by the desired breakdown voltage. The thickness of these films can be as little as 0.6 μm. In a suitable evaporation system and under high vacuum conditions (about 10¹⁵ to 10¹⁹ molecules of air per cubic meter) the plastic film is metalized with aluminum or zinc. It is then wound onto a so-called “mother roll” with a width of about 1 meter.
2. Film slitting — Next, the mother rolls are slit into small strips of plastic film in the required width according to the size of the manufactured capacitors.
3. Winding — Two films are rolled together into a cylindrical winding. The two metalized films that make up a capacitor are wound slightly offset so that by the arrangement of the electrodes, one edge of the metallization on each end of the winding extends out laterally.
4. Flattening — The winding is usually flattened into an oval shape by applying mechanical pressure. Because the cost of a printed circuit board is calculated per square millimeter, a smaller capacitor footprint reduces the overall cost of the circuit.
5. Application of metallic contact layer (“school age”) — The projecting end electrodes are covered with a liquefied contact metal such as (tin, zinc, or aluminum), which is sprayed with compressed air on both lateral ends of the winding. This metalizing process is named school page after Swiss engineer Max Schoop invented a combustion spray application for tin and lead.
6. Healing — The windings now electrically connected by the school page must be “healed”. This is done by applying a precisely calibrated voltage across the electrodes of the winding so that any existing defects will be “burned away” (see also “self-healing” below).
7. Impregnation — For increased protection of the capacitor against environmental influences, especially moisture, the winding is impregnated with an insulating fluid, such as silicone oil.
8. Attachment of terminals — The capacitor terminals are soldered or welded on the end metal contact layers of the school page.
9. Coating — After attaching the terminals, the capacitor body is potted into an external casing or is dipped into a protective coating. For the lowest production costs, some film capacitors can be used “naked” without a further coating of the winding.
10. Electrical final test — All capacitors (100%) should be tested for the most important electrical parameters, capacitance (C), dissipation factor (tan δ), and impedance (Z).

1.Life test according to IEC 384-14, 1000 hrs at T_uc and 1.25xV_R + 1000 V_rms every hour for 0.1 s.

Figure 4. Life test of X- and Y-capacitors.

2.Surge voltage test according to 384-14. Three pulses of V_p = 2.5 to 5 kV depending on capacitor type.

Figure 5. Surge voltage test of X- and Y-capacitors.

3.Charge and discharge test according to IEC 384-14. 10 000 pulses at 100 V/s and 2xV_R.

Figure 6. Charge and discharge test of X and Y capacitors.

X- and Y-capacitors must have approval by national inspection authorities to be used in their respective countries. All European check routines are collected in one standard, EN 13 24 00. The USA standards are collected under UL, and the Canadian under CSA.

MP or MK?

We have to count on self-healing breakdowns in X- and Y-capacitor applications. The voltage drop caused by self-healing depends on the energy consumed to evaporate dielectric and metalizing. Here MPs with their zinc metalizing have been superior to plastic film capacitors, which have had an Al metallization whose evaporation process requires several times higher energy than Zn. Nowadays, however, plastic film capacitors (MK) are brought on the market with metallization alloys based on the advantageous characteristics of zinc but without its tendency towards aqueous corrosion.

Further on, unique designs of metalized plastic films exist where a segmented metallization is used, sometimes called a structure metallization. The surface is divided into mutually demarcated elements within reach of the charging current via narrow gates. At self-healing, the surge current burns them off. See examples in Figures 7. and 8. below. The surface element is isolated, the discharge current from other elements is cut off, and the beginning voltage drop. One gets approximately the same energy limitation as by a self-healing in an MP capacitor, especially if the structure metallization is combined with choices of modern metalizing alloys. Figure 7. shows typical self-healing effects on the voltage drop over a capacitor.

Figure 7. Typical voltage drops DVC at a self-healing (SH) in an MP and an MK capacitor under tension. SHMP » SHMK-structure.

The metalized plastic films (MK) that hitherto have been used are polyester (MKT) and polypropylene (MKP). The latter need not be structure metalized due to its excellent self-healing chemistry. Combined with very thin ZnAl metalizing, the design gets the same characteristics as structure metalized MK. In addition, its high-frequency features are superior to those of other films.

The metalizing in structured surface elements makes great demands for the design. Even if cost-effective methods are developed, they involve a certain rise in prices. The simplified segmented metallization in Figure 8. consists of a grid-like pattern distributed over the whole surface, as shown in Figure 9.

Another very interesting structure, metallization, consists of metalized circular surfaces on top of a thin, high surface resistivity metalizing that covers the total surface. The weak circular joints serve together with the thin underlying metallization as fusing elements. The fusing function is favored by the metallization of zinc or low-energy alloy.

Figure 10. Schematic of segmented metallization.

Every self-healing reduces the capacitance correspondingly to the surface reduction. The author believes that the MP capacitor is superior to structure metalized MK types. But, of course, both types meet current standards and safety requirements.

Temperature and frequency dependencies

The following diagrams show typical graphs for paper capacitors’ temperature and frequency dependence.

In Figure 16., the impedance curve turns down at a sharp point around the resonance frequency. In low-loss components like film capacitors, the decreasing capacitive reactance curve reaches areas around the resonance frequency before it gets to the limiting ESR contribution. Here the reactance drops faster than the initial curve because of the counteracting inductive reactance.

The tip of the impedance curve in Figure 16. is in a larger magnification, not as sharp as the diagram indicates.

(In capacitors with rather high losses as, for example, electrolytes, the reactance curves reach the ESR contribution at frequencies far away from the resonance frequency. Here, the dipole-dependent capacitance decreases a deviation upwards from the initial reactance curve).

Failure modes

Penetrating moisture represents the greatest threat against paper capacitors because the paper absorbs humidity that affects the IR and damages the dielectric. In foil capacitors internal, freely suspended terminal wires risk vibrating to disruption.

Survey table

Metalized and foil capacitor specification general comparison is shown in Table 2. below.

Table 2. metallized paper and foil capacitor general characteristics

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.

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

Thus the risk of shrinking is reduced with the latter method, which is restricted to stacked designs only. Therefore the dimensions could 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.

The shrinking effects are best controlled by ESR measurements 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.

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.

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

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.

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

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 is 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.

Polypropylene (PP) is, from a molecular point of view, a non-polar dielectric with small losses and a relatively straight and moderate TC. Since the smallest film thickness is approx. 3.5 μm (0.14 mils) and ε_r ≈ 2.3, the capacitor can not come down to those sizes characterizing PET at low-rated voltages. But remaining good characteristics in many applications have brought up PP as a replacement for polycarbonate (PC) and polystyrene (PS), not least as a precision capacitor. PP exists in foil and metalized design and is adopted for AC, pulse, and transient suppression (X-capacitor) applications. The pulse and X-capacitor designs, however, require specific metalizing technology.

The MKP design had, from the beginning, problems with the adhesion between the metalized layer and the plastic film. This problem characterizes non-polar dielectrics consisting of molecules and has caused many problems, such as gluing components to circuit boards. The plastic surface film has to be raised and roughened, which among other things, can be done by etching, flame exposure, electron irradiation, or corona. Today the MKP design is well established, and the adhesion problems have passed.

The demand for filtering/interference suppression of thyristor-generated noise voltages has brought forth the same type of large capacitors as shown for PET, but here, the low ESR losses allow quite different r.m.s. currents, for example, at high frequencies.

The recommended absolute maximum temperature is +105°C. We recommend a max of +85°C with the remark that developments of new films are going on to offer a temperature range above +105°C as a maximum operating temperature. Current temperature limitations still make PP difficult for chip designs.

Temperature and frequency dependencies

The frequency dependence of capacitance for PP capacitors is moderate. In Figure 29. the broken part of the curve indicates an increase in capacitance. This increase is not physical but depends on the influence of series capacitance measurements.

Note in Figure 30. the outstanding low loss factor for PP over the whole temperature range. The curve area in Figure 31. represents capacitance up to hundreds of nF: the higher the capacitance, the more significant losses.

Designs with foil or hermetic seals have higher IR than corresponding metalized and plastic encapsulated types.

Failure modes

Any characteristic failure mode just for PP capacitors is not distinguishable. Here what was said about PET usually applies.

Table 4. POLYPROPYLEN (PP) / KP / MKP

Failure modes

Some mysterious short-circuiting failures were reported from precision capacitors in high impedance low voltage space applications. In 1982 a Voyager satellite was met with a failure in an 8 μF/ 75V polycarbonate capacitor. Halfway to Jupiter, the IR suddenly dropped from hundreds of MΩ to 2.8 kΩ. All over the world, component experts worked with this kind of problem, and by and by, the picture became clearer. The reasons are resulting effects of self-healing.

The issue is a critically high carbon deposit in former self-healing sites, which is connected with the high carbon contents in the polycarbonate film. But the failure only appears in that very narrow application sector where precision capacitors are usually used at low voltages and in high impedance circuits, e.g., integration and ramp voltages. Even that very low energy needed to burn away the formed carbon string may be missing. The cure is some kind of Burn-in treatment. From a failure rate in the magnitude of, say, one percent, we should get down to or below one per thousand. We find the most sophisticated (and expensive) Burn-in program in US MIL-C-87217. In conclusion, it is built on a collection of all those environments that release this kind of short circuit: cyclic voltage ramps with polarity changes + gradual temperature fluctuations.

Table 5. POLYCARBONATE (PC) / KC / MKC

General comments to PPS

Foil types exist, but the metalized design is a rule. If nothing else is said, metalized film applies.

ε_r ≈3.0. Max. temp.+140 °C; most manufacturers recommend a maximum of +125 °C. Dielectric absorption ≈ 0.1%.

The smallest film thickness is 1.2 μm (0.05 mils), but former quality problems with the thinnest film may cause stronger derating of the lowest rated voltage. However, the supply of film is stabilized, thus indicating an established demand…

The film is used in capacitors both for lead mount and SMD. Existing chip types are made in a stacked design.

Temperature and frequency dependencies for PPS

Other plastic, organic film capacitors used in special designs:

Teflon (PTFE)

Teflon exists both in metalized and foil designs. The greatest advantage of the film is its temperature resistance. On the other hand, the price is high, giving cause for exclusive hermetic designs.

• Capacitance 470pF….4.3μF.
• Tolerance ±0.25….±20%.
• Temperature range -65/+200°C.
• TC -200…+50ppm/°C.
• Rated voltage 50…600 V DC.
• Tanδ , @ 1kHz, 20°C, ≤0.1%.
• IR, @ 20°C, foil ≥500GΩ/ ≥100 000s,
• IR, @ 20°C, metallized film 50GΩ/ 10 000s.
• Stability  ΔC/C max -1%.
• ε_r ≈ 2.2.
• Dielectric absorption  ≤0.02%.
• Recommended derating 0.6xV_R.

Polysulfone (PSU)

Polysulfone capacitors belong to the same price group as one Teflon. They are used only in special applications where high-temperature capabilities and excellent characteristics are of vital importance. The capacitors have been manufactured only in hermetic design and with foil electrodes.

• Capacitance 1nF….22μF.
• Tolerance ±0.25….±10%.
• Temperature range -65/+150°C.
• TC -200…+50ppm/°C.
• Rated voltage 50…400 V DC.
• Tanδ , @ 1kHz, 20°C, ≤0.15%.
• IR, @ 20°C, foil ≥1000GΩ
• ε_r ≈ 3.2.
• Dielectric absorption  ≤0.2%.
• Recommended derating 0.6xV_R.