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Device construction

Devices in the Film Capacitors category are electrostatic in nature, and made using dielectric materials such as paper or various polymers that are formed into thin sheets or “films” and interleaved with electrode materials to form a capacitor.  The term “film capacitor” generically refers to any device made using this sort of process, and the term “film” is in reference to the nature of the dielectric material used.  When the term “metal” is used as a qualifier for “film” as in “metal film” or “metallized film,” it’s a more specific reference to a film capacitor sub-type in which the electrodes are built up on a supporting substrate in a very thin (10’s of nanometers) layer, usually through a vacuum deposition process.   The substrate used frequently also serves as the dielectric material for the capacitor, though this is not always the case.  In contrast, a “foil” electrode capacitor uses an electrode material more akin to household aluminum foil, which is thick enough (on the order of micrometers) to be mechanically self-supporting.

 

Film capacitors based on metal-film electrodes have the advantage of being able to self-heal; the electrode material near a localized fault in the dielectric is thin enough to be vaporized by the leakage current caused by the fault, thus eliminating (or “clearing”)  it at a cost of some lost capacitance.  This ability to self-heal permits the use of thinner dielectrics than would otherwise be feasible due to reliability or production yield concerns, and results in a high capacitance per volume.  The advantage of foil-electrode capacitors is that the thicker electrodes result in lower ESR, allowing better RMS and pulsed current handling abilities, at the expense of self-healing capability and a reduction in achievable capacitance per volume. 

 

Numerous clever combinations of and tweaks to the basic film and foil electrode types are in common use.  For example, foil and film electrodes are often combined in a single device, using a “floating electrode” configuration, which (like similarly-designated ceramic capacitors) is effectively two or more capacitors connected in series.  By making the “outer” electrodes a foil type and the “floating” electrodes a film type, one can realize a capacitor with good current handling capabilities, self-healing ability, and improved capacitance per volume.  Another technique frequently employed is the use of patterned film electrodes.  By partitioning an electrode into a number of interconnected segments, the interconnects can be made to act as fuses that limit the amount of current available to a fault site during a self-healing event, allowing the risk of cascading or short-circuit failures to be reduced.

 

Common Usages & Applications:

Film capacitors of some form are the dominant capacitor technology in power applications involving reversal of voltage applied to the device.  Metallized film types are well-suited to safety-rated applications, due to their self-healing characteristics and ability to fail open under many fault conditions.  Metal foil types are often used in applications where higher ripple current amplitudes are expected, such as in starting/running AC motors or providing capacitive reactance for bulk power distribution.  Additionally, film capacitors are often used in low-voltage signal applications where relatively high capacitance values as well as linearity and stability over temperature are required, such as in analog audio processing equipment.

 

In applications such as DC bus filtering where the polarity across the device is not reversed, film capacitors may be an alternative to aluminum electrolytic types (or vice-versa).   When comparing film capacitors with aluminum electrolytic types of similar voltage and capacitance ratings, film capacitors tend to be larger and more costly by roughly a factor of 10, but have ESR values that are lower by a factor of roughly 100.  Film capacitors’ lack of a liquid electrolyte eliminates the problem of dry-out and increase in ESR at low temperatures encountered with aluminum electrolytic devices, and they do not suffer dielectric degradation during extended periods of disuse in the way that aluminum electrolytic devices do.  Additionally, the lower ESR characteristic of film capacitors may permit use of a smaller capacitance value than would be required with an electrolytic device in some applications, offsetting the cost disadvantage of film technology relative to electrolytic types.

 

 

Common failure mechanisms/critical design considerations:

Though film capacitors are generally quite durable, they are susceptible to a few long-term wear mechanisms. Over time, the dielectric materials used weaken, become brittle, and experience degradation in their voltage withstanding capability, which eventually leads to a dielectric breakdown failure. The process is accelerated by temperature and voltage stress, and reducing either can extend service life.  Depending on the severity of the dielectric breakdown event, the failure modes exhibited can range from relatively benign to quite spectacular.  A mild breakdown event that is arrested either by a film capacitor’s self-healing properties will manifest as an incremental reduction in capacitance.  As more such events occur over time, the cumulative effect causes a reduction in capacitance and increased ESR, until the point where the device’s performance is no longer within specification and it is considered to have failed parametrically.

 

In a more extreme case, which can follow a parametric failure if parametrically-failed devices are not removed from service, a cascading failure can occur when the thermal energy released during self-healing prompts additional dielectric breakdowns nearby. Because self-healing events remove portions of the capacitor from the circuit, application stresses are re-distributed across an ever-shrinking portion of the device as self-healing progresses, causing an increase in stresses placed on the portions of the device that remain effectively in-circuit.  The next weakest portion of the capacitor then fails, dumping its burden on what’s left, prompting more breakdown events, more stress concentration, more breakdown events, etc. in an exponential fashion.  If this process occurs rapidly enough, the gaseous byproducts from the self-healing process can build sufficient pressure to violently rupture the device’s case.  Larger devices often include a venting mechanism to limit/prevent collateral damage from flying debris when this happens, and may also include a fusing mechanism to remove the device from the circuit in the event of an internal overpressure condition.  Note that parametric failures due to repeated self-healing can simply be a waypoint on the route to a more catastrophic, explosive failure, if devices that have failed parametrically are left in operation.

 

Another overstress failure mode found in film capacitors occurs when peak current limits are exceeded, due to a fuse-like action at the region where the “plates” of the capacitor join to the external leads.  This is particularly common with the metallized film types, due to their very small electrode thickness and the resulting delicacy of their connection to the outside world.  Many film type capacitors will specify a maximum rate of voltage change (dV/dt) that is to be applied across the capacitor.  This is tantamount to specifying a peak current through the device since I(t)=C*dV/dt, though voltages are typically more convenient to measure than currents.

 

Environmental conditions also play a role in the longevity of film capacitors.  As with other devices, elevated temperatures reduce device lifetime considerably.  More unique to film devices is a vulnerability to moisture; prolonged exposure to high humidity environments or post-assembly wash cycles can cause the ingress of moisture into a device, through imperfections in the epoxy-to-metal seals around the device leads or by diffusion through a device’s polymer case.  Moisture ingress is bad on several fronts; it both degrades the dielectric material, and promotes corrosion of the electrode materials.  Particularly in metal-film type devices where the electrodes are only a few dozen nanometers thick to start with, it takes very little corrosion to cause problems.  Additionally, high-vibration environments can also be troublesome, by causing mechanical failure of device leads, attachment between leads and electrodes, or by exacerbating moisture ingress problems. 

 

The dominant factors in film capacitor reliability and longevity are applied voltage, followed by temperature.  Suppliers’ service life models vary, but generally are based on taking the ratio of rated and applied voltage to a large exponent (usually between 5 and 10) while the influence of temperature follows the Arrhenius relationship of a factor-of-2 change with each 10°C temperature increment.  Between the two effects, de-rating voltage by 30% and temperature by 20°C adds nearly two decimal places to service life estimates.

Dielectric types, Features, & Targeted Applications:

Acrylic:

Acrylate materials are relatively new as a dielectric material for film capacitors.  Currently available devices are often marketed as reflow-compatible film alternatives to ceramic dielectrics that avoid piezoelectric effects and loss of capacitance with DC bias, or as lower-ESR tantalum alternatives.

 

Paper:

 Kraft paper was one of the earliest dielectric materials used for film capacitors, due to its low cost and availability prior to the development of modern polymers.  Commonly impregnated with wax, various oils, or epoxy to fill voids and inhibit moisture absorption, its low dielectric strength and high moisture absorbency has caused paper to largely fall out of favor as a dielectric material, though it still finds limited use in applications that are extremely cost sensitive or where alterations to legacy specifications are extremely difficult to realize.  Due to the relative ease with which metal films can be applied to paper versus polymer materials, paper is occasionally used not as a dielectric material per se, but as a mechanical carrier of metallized electrode material, with a non-metallized polymer such as polypropylene serving as the actual dielectric.

 

Polyester/Polyethylene Terephthalate (PET):

Polyester, also known as Polyethylene Terephthalate or PET, is one of the most commonly used dielectric materials in film capacitors, alongside polypropylene.  Relative to polypropylene, polyester generally has a higher dielectric constant, lower dielectric strength, higher temperature tolerance, and higher dielectric losses.  In a nutshell, polyester dielectrics are good for film cap applications that value quantity of capacitance over quality, and which do not call for a surface mountable form factor.  Certain formulations of polyester designed for high-temperature tolerance exist that facilitate the use of polyester film capacitors in surface mount packaging, though these devices are relatively few in number.

 

Polyethylene Naphthalate (PEN):

Polyethylene Naphthalate (PEN) is a polymer dielectric material designed to tolerate higher temperatures, in order to permit the use of film capacitor technology in surface mountable, reflow-compatible packaging.  In application concept, it can be thought of as a reflow-compatible version of polyethylene (PET), delivering quantity of capacitance over quality.  In exchange for gaining reflow soldering compatibility, PEN gives up a bit of specific capacitance (capacitance per volume) has higher dielectric absorption, and is more prone to moisture absorption issues, although dissipation factor at low frequency may be slightly improved relative to polyethylene.

 

Polypropylene (PP):

Polypropylene exhibits the lowest dielectric losses, lowest dielectric constant, and lowest maximum working temperatures of the commonly used film capacitor dielectrics.  It also exhibits one of the highest dielectric strengths among these polymers, as well as good parameter stability over temperature.  Overall, polypropylene is a dielectric of choice for film cap applications calling for quality of capacitance over quantity thereof.

 

Due to its low temperature tolerance, polypropylene dielectrics aren’t compatible with reflow soldering processes, and are therefore found almost exclusively in through-hole or chassis-mount packaging of some form.  Due to its superior loss characteristics, polypropylene film capacitors are a device of choice in high-current, high-frequency applications such as induction heating and thyristor commutation, as well as applications where a stable, linear capacitance is desired and other capacitor types are unavailable or unfeasible for some reason.

 

Polyphenylene Sulphide (PPS):

Polyphenylene Sulphide (PPS) dielectrics can be regarded as a reflow-compatible alternative to polypropylene, for applications where quality of capacitance is more important than quantity.  Relative to polypropylene, PPS capacitors exhibit a higher specific capacitance and dissipation factor over the range of applicable frequencies by a factor or roughly 2 to 3, though stability of capacitance over the temperature range is slightly improved.

 

Other Dielectrics

A number of film capacitor dielectric materials have either come & gone with time, or lingered in obscurity.  While not readily available or advisable for use in new applications, mention is made here for reference and comparison.

 

Polycarbonate

Polycarbonate is a rigid, transparent thermoplastic often used to make lenses for safety glasses, helmet visors, or other impact-resistant optics.  Its manufacture for use as a dielectric film was discontinued around the year 2000, and remaining material stocks for capacitor applications have largely been consumed.  As a dielectric material it was quite good, with electrical properties similar though slightly inferior to polypropylene in most cases, although with superior temperature characteristics that permitted use over the military (-55°C to +125°C) temperature range with relatively stable parameters and frequently without de-rating at elevated temperatures.  Polyphenylene sulphide (PPS) is commonly cited as an available alternative that is likely to be suitable for applications previously using polycarbonate-based devices.

Polyimide

Polyimide is a high-temperature polymer often sold under the trade name Kapton, and which finds use in many electronics applications as a substrate for flexible circuits.  As a dielectric for capacitor applications it offers moderate performance comparable with polyester/PET, though its high temperature stability enables operation at elevated temperatures in excess of 200°C. While its high dielectric strength suggests potential for devices with good volumetric density, difficulties in producing the stuff as a very thin film have tended to limit appeal/availability of capacitors based on this dielectric material.

Polystyrene

Polystyrene film capacitors are largely an extinct species at this point, having fallen out of favor primarily because of the assembly and manufacturing difficulties associated with a very low temperature tolerance of only 85°C.  At modest operating temperatures the electrical performance of polystyrene capacitors is quite good, and for a time such devices were a go-to choice when stability and electrical performance characteristics were the driving selection criteria.  For the most part, these devices have been supplanted by polypropylene film capacitors.

Polysulfone

Polysulfone is a rigid, transparent thermoplastic similar to polycarbonates both electrically and in terms of being high-cost and relatively unavailable.

Teflon/PTFE

“Teflon” is a DuPont Trade name which encompasses a number of fluoropolymers, principally polytetrafluoroethylene (PTFE) though fluorinated ethylene propylene (FEP) and others can be found with the “Teflon” moniker.  These polymers tend to be very stable and possess many admirable qualities as precision dielectrics, including high temperature tolerance and excellent stability over time, temperature, voltage, frequency etc.  The mechanical properties of PTFE films and difficulties in metallization thereof make production of PTFE-based film capacitors a difficult and costly affair, so few such devices are available in the market.  

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