Polyimide in flexible PCBs – why is PI such a popular material?
The picture above shows one of our flexible test printed circuit boards (FPCB, flex circuit). It has a very simple structure, as it is only used to study interconnection reliability. The yellow colour comes from polyimide (PI) film, while the darker areas are copper traces. So, what makes a flex circuit an interesting structure to study? And, why are PI so often used in them?
Use of flex circuits is increasing as more and more flexible electronics applications are developed. A flexible substrate is needed, if the electronics is used in applications, where it needs to be bend once or repeatedly. In addition to such applications, flex circuits are commonly used in applications which need to be very light or have severe size limitations. For example, a very complicated flex circuit can be found inside a new digital camera, since they typically require lots of electronics in a very small space.
In our earlier blog posts we have discussed about rigid PCBs which are commonly made of fibre reinforced epoxy laminates (for example here). Flex circuits, on the other hand, are fabricated using unreinforced polymeric (plastic) films, which makes them pliable and flexible. Flexible circuits have several advantages compared to fibre-reinforced substrates. As mentioned above, lighter and thinner products can be produced using them. They may also absorb stress, which can be important for the reliability of interconnections. In addition, the thermal transfer through a thin substrate is more effective, which can be very important for thermal management of a device. The thinness of a flex circuit may, however, cause problems in manufacturing and in use. Flex circuits are also more expensive, which may restrict their use in some applications.
Many materials can be used as the plastic film in a flex circuit. The most popular and common material is polyimide (PI) which we also used in our test circuit above. Actually, PI refers to a group of polymers and there are differences between the films from different manufacturer. However, all PI films have excellent thermal resistance which means that they can be used at high temperatures. This is very important, as it means that flex circuits can be used with common lead-free solders and they do not require special materials and processes for assembly. Additionally, PI films have good mechanical properties and excellent performance in dynamic flex applications, in which a flex circuit is repeatedly bent. The greatest disadvantages of PIs are their high cost and high moisture absorption. The high cost makes other materials such as PET (polyethylene terephtalate) popular for low cost applications. For demanding application PI is still the most common material.