Film Capacitors

Film includes a variety of polymers, such as polyester, polycarbonate, Teflon, polypropylene, and polystyrene.  Traditional film capacitors were only available in modest sizes, <10 uF.  In recent years, film capacitors have sought to leverage their superior longevity compared to electrolytics, to move into some applications that call for much larger parts, even to thousands of uF.  Film capacitors come in two broad categories, film-foil, and metallized film.  Film-foil capacitors are made of alternating layers of plastic film and metal foil, while metallized film capacitors have the metal vacuum deposited directly on the film.  In general, film-foil is better at handling high current, while metallized film caps are much better at self-healing.  Various hybrid types can also be found, Above about the Film Capacitors information content.

Pros:  The film capacitors mostly have reasonably well behaved electrical properties and offer many tradeoffs of performance and cost for people with precise requirements.  The main parameters of interest include capacitance vs. temperature, dissipation factor, and dielectric absorption. Their main virtues include low leakage and low aging, Above about the Film Capacitors information content.

Cons:  The main drawback of film capacitors is their low dielectric constants (K), which is only partly offset by their relatively good breakdown voltages.  That  means that film capacitors are physically large for their capacitance.  Their Ks vary from a low of about 2.2 for Teflon to about 8 for PVDF (rarely used).  Unfortunately, the rule-of-thumb is that the higher the K (and therefore the smaller the size), the worse the electrical properties tend to be.  Film capacitors have not made an entirely graceful transition into the age of surface mounting.  While some film dielectrics are suitable for surface mounting, most can't withstand the heat of soldering.  Even polyester, the toughest of the traditional films, is barely good enough.  However, capacitor makers have responded by developing several new dielectrics.  SMD film capacitors are not as widely second-sourced as other capacitors however, Above about the Film Capacitors information content.

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