Flexible PCBs are an integral part of today’s modern electronics. Unlike rigid PCBs, flexible PCBs can bend, are lightweight and can be used in very tight enclosures and therefore are very useful for various applications. While there are many options for flexible PCB materials like PET ( Polyethylene Terephthalate), polyimides and PEN (Polyethylene naphthalate) in the market there is one name synonymous with high-quality flexible PCBs, and that is Kapton PCBs.
What are Kapton PCBs?
Kapton PCBs are boards made from Kapton matreials. Kapton is a polyimide material series offered by DuPont, an American material science company established in 1802. Kapton polyimide films were introduced in the 1960s as a part of DuPont’s high-performance materials capable of withstanding harsh and extreme environmental conditions. This material series was initially intended to serve aerospace and defence applications that had high budgets and had to withstand hostile terrains. Kapton films were able to maintain their physical and electrical properties in temperatures as low as -269°C to as high as 400°C.
The material gained popularity when it was used in the NASA Apollo Lunar Module in the 1970s where it was used for thermal insulation. It was also used in electrical insulation for military applications. In the 1980s Kapton became popular and was being extensively used as an insulation material for electronics. Not only could it withstand harsh conditions, it had good electrical properties and it could bend. Since then Kapton PCB has been a renowned name for flexible PCBs with high reliability. Today, Kapton is one of the preferred materials for flexible PCBs and is a popular choice among commercial and industrial applications.
Properties of Kapton PCBs
Since Kapton is a polyimide-based PCB material, it has excellent physical, electrical and chemical properties. A few of its properties are –
- Dielectric Constant – The dielectric constant of a Kapton PCB material is the measure of the material’s ability to store electrical energy. Ideally, the dielectric constant should be either very high or very low depending on the application. The Dk of Kapton PCB is around 3.8.
- Dissipation Factor – It is the measure of energy that will be lost as heat when in the presence of an applied electrical field. Ideally, a material should have a low value of Df as the generated heat is formed by the attenuation of the material to the signal which would deteriorate the signal integrity of the signal. The Dissipation factor of Kapton PCB is 0.002.
- Glass Transition Temperature – Also denoted by Tg, is the temperature at which a material becomes like a rubbery soft state from a glass hard state. Ideally, Tg must be high to maintain physical properties under high temperatures. Kapton PCB material has a Tg of 400°C.
- Chemical and Solvent Resistance – Kapton PCBs are resistant to most of the hydrocarbons, organic solvents, oils and acids. However, since it is a polyimide-based material it is known to have limited resistance to alkalis and alkaline environments.
- Outgassing Loss – Outgassing loss is the physical property of a material implying the amount or mass of the gas released under a vacuum or vacuum-like condition. A low-outgassing material is preferable for any material used in outer space or low-pressure applications. The outgassing mass of Kapton is 0.58% under vacuum.
- Moisture Absorption – Moisture absorption is the percentage of water the material may absorb under humid or wet conditions which may affect the electrical performance of a material. The moisture absorption rate of any material should be preferably close to zero. Polyimide has around 1.5% to 2% moisture absorption rate.
- Young’s Modulus – Young’s Modulus is a metric for the stiffness or rigidity of a material. For flexible PCBs, Young’s Modulus must be as low as possible which means the material should not resist bending or flexural force. Kapton PCBs have a Young’s Modulus of 2.5 GPa.
- Flexural Strength – It is the amount of stress a material can endure before cracking at the surface or breaking under a constant and normal (perpendicular) load. Ideally, the flexural strength must be high for flexible PCBs to maintain the integrity of the circuitry on the PCB. The Flexural Strength of Polyimide PCB is around 200MPa.
- Bending Cycles – This is the measure of the number of times a PCB can be bent without failure or damage. A high bending cycle is a significant property of a good flexible material. The bending cycle for Kapton PCBs is 100,000 to 1,000,000 bending cycles under ideal conditions.
Advantages of Kapton PCB
Kapton PCBs have various advantages which make them a very popular choice of material for flexible PCBs. A few of the advantages of them are –
- High Bending Cycles – Many flexible PCBs have low and limited bending cycles. Being a polyimide-based material, Kapton flex PCBs have a very high bending cycle. Being able to withstand up to 1 million bending cycles, is significantly higher than many other flexible and rigid-flex PCB materials available in the market. So Kapton flex PCBs became popular in the PCB markets.
- Extremely Flexible – Kapton PCBs have low Young’s modulus while having high tensile strength which is desirable for a flexible PCB. These parameters result in a material that is easy to bend but hard to break. Kapton PCBs can easily bent to a full fold or 180° fold without damaging the circuitry.
- Wide Operating Temperatures – Kapton PCB material can be used in temperatures as low as -269°C and as high as 260°C in continuous operation. They can withstand temperatures up to their Tg i.e.400°C for a short span.
- Wide Availability – Kapton Flex circuit boards have a wide availability in the market. Most of the PCB vendors who offer flexible PCBs are likely to offer Kapton PCBs too. This makes Kapton flex circuit boards much cheaper than other Polyimide-based flexible PCBs. They are so popular that they are synonymous with the term polyimide PCB.
- Good Electrical Properties – They have a rather low Df and Dk which is required for PCBs designed for high signal integrity or high-speed signals.
- Chemical Resistivity – Kapton PCBs are chemically inert. They are highly resistant to most of the organic and inorganic solvents, acids, and even gases. This makes it a preferred choice for hostile and corrosive environmental applications.
- Suitable for Aerospace Applications – Kapton PCBs have a low density, therefore, it is lightweight which is preferable for aerospace applications. Kapton circuit boards also have excellent outgassing properties which are required for operating in a vacuum or very low pressure. Very few PCB materials are suitable for aerospace applications and the list gets even more diminished when considering flexible PCB materials. So Kapton flex circuit boards are a better choice.
Applications of Kapton PCBs
Kapton PCbs are widely used in various grades of industries. Some of the applications of them are –
- Aerospace Applications – As discussed before, Kapton is one of the few materials suitable for aerospace applications. It is lightweight, has excellent outgassing properties and has a wide operating temperature. Such properties make it suitable for space and satellite applications. Also, note that PCB Kapton material has been used in NASA’s Lunar missions ( not as PCBs) therefore it is a proven material for aerospace applications. They are used as interconnects for various electrical systems for aviation and satellites which require high reliability and decent high-speed signal capabilities.
- Military Applications – Electronics used in defence applications are subjected to high temperatures, hostile and corrosive environments and high jerks and shocks. The PCBs used in military applications are expected to endure all that along with providing high reliability. Kapton PCBs, are known for their durability, thermal stability, and chemical resistance making them ideal for electronics in defence equipment and communication systems.
- Industrial Equipment – Industrial equipment, comes in all sorts of different requirements, like high-temperature resistance, chemical resistance or high mechanical endurance. Kapton PCBs are favoured because they are suitable for all of the above-mentioned requirements. They are used in sensor circuitry in highly corrosive environments like chemical plants and high-temperature environments like furnaces as they are capable of enduring high temperatures and are inert to most chemicals.
- Wearable Devices – Many wearable devices are required to be as light as possible along with being flexible. Kapton PCBs are one of the most suitable materials for such applications. Not only are they flexible and lightweight, but Kapton PCBs also have good electrical properties that enable them to support high speed and RF signals which is crucial for modern wearable devices connectivity to devices such as laptops and mobile phones.
- Automotive Grade Application- Modern vehicles are becoming more electronics-reliant than ever. Systems like engine control units, battery management systems, ABS, and other sensory systems are required to be highly reliable even under high temperatures and in the presence of various hydrocarbons. Kapton PCBs are suitable for automotive-grade applications since they are resilient to high temperatures while maintaining high reliability. PCB Kapton materials are also less prone to corrosion and are suitable for use under diverse weather conditions.
