These are laminates generally composed of a copper foil applied, by adhesives of different nature and features, on one or both sides of a polymer film with standard thickness that can range from 25 µm to 125 µm.
The copper foil can have a thickness from 17 to 105 µm and is available in both ED and RA type, which is recommended if dynamic flexions are expected during the use of the PCB.
In order to treat this type of laminates the production process of printed circuit boards requires proper modifications and specific technical devices in some phases. It is necessary a specific know-how and sometimes, especially if the process method is by roll-to-roll, dedicated equipments are needed.
PET (Poli Etilene Terephtalate) also known as Mylar ® (this is the name of DuPont Polyester), PET is the most common type of polyester and can be used for the production of laminates for flexible printed circuits for which no significant thermal stress are expected.
From a functional point of view, PET represents a good choice because of its economy while ensuring excellent technical performance in terms of mechanical and electrical properties. It is normally not able to withstand the soldering operations carried out with the traditional processes (in example wave and reflow), even though light soldering operation of components can be performed with appropriate precaution.
PEN (Poli Etilene Naphtalate) has molecular weight, higher than PET one, and its consequent higher resistance to heat makes it suitable for flexible printed circuits on which not very high thermal stress are expected. The technical characteristics, at the level of mechanical and electrical properties, are generally similar to those of PET. Polyimide can be an economical alternative to PI in the manufacturing of printed circuit boards on which soldering operations at not too high temperatures are planned.
PI (PolyImide) commonly known as Kapton ® (this is the name of Du Pont Polymide), the polyimide is a polymer with exceptional thermal characteristics, especially as resistance to low and high temperatures. Thanks to these properties, and together with excellent mechanical and electrical properties, the polyimide based laminates are used in the manufacturing of flexible printed circuit boards or rigid-flexible which can be used in severe environmental conditions and which can be subjected to the different soldering processes, including the lead-free ones.
For extreme applications polyimide laminates without adhesive (adhesiveless) are also available.
SEMIFLEX these materials consist of a thin layer of woven glass fiber impregnated with a modified flexible polymer resin and copper on one or both sides. The printed circuit boards manufactured with these materials combine some of the features of flexible laminates, as they can be curved or bent during assembly, with others typical of rigid materials, such as a greater consistency that allows to obtain stable geometry of the mounted particular, a higher dimensional stability and a better thermal resistance compared to polyesters.