How to Specify Electrical Insulating Material
Choosing the right material combinations for a specific application can mean the difference between success and failure. Richard Orstad of Fralock presents a primer on how to specify electrical insulating material.
There are hundreds of decisions required when designing mission-critical electromechanical components. In each of these decisions, choosing the right material combinations for a specific application can mean the difference between success and failure. Most of these design decisions are made utilizing the same material choices design after design, which can compromise the true intent of the project simply because the designer is unaware of the many other material choices available. With this in mind, a primer in common and not-so-common electrical insulating materials may be helpful.
For simple electrical insulating applications where a thin film is required, polyester films are probably the most common type of electrical insulator material used. Polyester films are commonly characterized as either PET (polyethylene terephthalate) or PEN (polyethylene naphthalate), the differences found in their chemical composition and their resulting physical and electrical insulating properties.
PET films, such as Mylar or Melinex, offer good dielectric withstanding resistance within a relative temperature range (Tg of +78°C), whereas PEN films, such as Teonex, typically provide similar electrical performance at a much higher temperature range (Tg of +120°C). In applications involving higher temperatures, PEN films are often a better choice than PET films given they are three to four times stiffer at temperatures above +125°C , as well as their higher operating temperature as compared to PET (+180°C vs. +160°C). PEN films are more expensive than PET films, but for mission critical applications this cost difference may be worthwhile. These thin films are found in thousands of applications including flexible electronics, battery and motor insulations, and electronic component manufacturing.
Where higher temperature environments prevent the use of PEN films, often times the design engineer will consider the use of poly ether ether ketone (PEEK) or polyetherimide (PEI) materials such as Ultem. PEI materials have a significantly higher glass transition temperature than PEN films, at +216°C. A common PEI material, Ultem 1000 film has a continuous operating temperature rating of +171°C with exceptional flame and heat resistance. Another benefit of Ultem 1000 material is its availability in film, sheet, or extruded rod forms, which gives the engineer greater flexibility to design critical mechanical components based on the specifics of the application.
Again, just as PEN films are typically more expensive than PET films, the greater performance attributes of the PEI materials are typically more expensive than PEN or PET films. The specific requirements of the electromechanical component, as well as the requirements of the specific application, must be weighed when choosing a material. Because of the different material forms available, PEI materials are often used in aircraft components, microwave applications, and electric/electrical components.
When even greater temperature resistance is required while still maintaining electrical insulating performance, polyimide materials are often used. Polyimide films such as KAPTON® have been used for years as electrical insulating material, and thicker polyimide materials such as CIRLEX®, Vespel, and Torlon continue to be utilized as alternatives to PEI materials where critical applications require highly engineered materials. CIRLEX, made from 100% KAPTON polyimide film, has a glass transition temperature of +351°C, far exceeding that of PEI materials, so it provides exceptional stability at demanding temperatures. Its very low coefficient of thermal expansion (20ppm/°C) along with its high tensile strength (32,000 psi at +200°C at 9mil) makes CIRLEX an exceptionally strong and stable material across its operating temperature range of -269°C to +351°C. (A chart comparing these materials is shown below.)
Another unique characteristic of CIRLEX is its availability in increments of 0.001″, from 0.004″ up to 0.125″ or thicker. This allows a tremendous amount of design flexibility, as the engineer can enter the design phase without thinking of material thickness limitations but rather design specifically for the application, confident that the right thickness material is available. The availability of CIRLEX in 0.001″ increments also reduces the amount of machining time required to get to final thickness tolerances, which must be considered when designing for manufacturability. CIRLEX’s physical and mechanical stability are also evident during and after the machining process, as there are no residual stresses built up in the material, so the final design is left intact in its dimensionally stable form. In applications where extreme temperature or environmental requirements apply, the benefits of a polyimide material such as CIRLEX outweigh other material choices.
Finally, there are a variety of instances where a mechanical component requires a metal-to-polymer lamination process, such as bonding copper to polyimide. In many cases, an appropriate PSA or B-stage adhesive is chosen to bond the two materials. However, the addition of an adhesive creates potential failure modes such as outgassing or delamination, especially under elevated temperatures or other physical or environment conditions. Eliminate the adhesive layer and the engineer eliminates a weak link in the design; this may be readily accomplished by using adhesiveless laminate technology (ALT).
Thermal stress is a design consideration inherent when differing materials are physically connected such as in laminations. The CTE of polyimide is closely matched to that of copper, and with use of an adhesiveless laminate, a seamless marriage of conductor and insulator can be achieved, allowing greater flexibility. Whereas the use of copper-clad laminates was once a constraint in flexible circuits, the advent of ALT with polyimide allows for near limitless possibilities in advanced, mission-critical electromechanical component design.
Choosing the right material and processes for a high-performance electromechanical components is a delicate balance between form, fit, function, and cost. It is important for the engineer to consider all aspects of the part’s functional design as it relates to the physical, electrical, and thermal properties of the environment in which it will perform. The selection of the optimal electrical insulating material, whether it is a PET, PEN, PEI, or polyimide, is a function of the anticipated operational and environmental requirements. For mission-critical applications, the correct choice of electrical insulation materials and processes starts when the engineer addresses these requirements and balances the various design and application risks against expected cost constraints.
Richard Orstad, P.E., M.B.A., is a professional engineer with more than 25 years of experience. He is a strategic account manager for Fralock.
Mylar®, Melinex®, Teonex®, and Vespel® are registered tradenames of E.I. du Pont de Nemours and Company.
Ultem 1000® is a registered tradename of Sabic Innovative Plastics.
Torlon® is a registered tradename of Solvay Advanced Polymers.
CIRLEX® is a registered trademark of DuPont used under license by Fralock.
KAPTON® is a registered trademark of DuPont.