How to Determine Substrate Materials for Flexible and Flex-Rigid Printed Circuit Board?

In modern electronic time, miniature and thinness of electronic devices lead to necessary advent of flexible and flex-rigid printed circuit board. Then what type of substrate material is suitable for them?

Increasing application fields of rigid PCBs and flex/rigid PCBs bring about new requirement in terms of number and performance. Polyimide film, for example, can be classified into multiple categories including transparent, white, black and yellow with high heat resistance and low coefficient of thermal expansion in order to be applied in different situations. Similarly, mylar substrate with high cost-effectiveness will be accepted by market as well owing to its advantages including high elasticity, size stability, film surface quality, photoelectric coupling and environmental resistance so as to meet the changeable requirement of users.

Similar with rigid HDI PCBs, flex PCB has to adapt to the demands of high-speed high-frequency signal transmission, dielectric constant and dielectric loss of flex substrate material have to be focused as well. Flex circuit can be composed by polytetrafluoroethylene and advancing polyimide substrate. Inorganic dust and carbon fiber can be added to polyimide resin to lead to the generation of three-layer flex thermal conducting substrate. Inorganic filling material can be either aluminum nitride, aluminum oxide or hexagonal boron nitride. This type of substrate material features thermal conducting property of 1.51W/mK and is capable of resisting a voltage of 2.5kV and curvature of 180 degrees.

Flex PCBs are mainly applied in intelligent mobile phones, wearables, medical equipment and robotics, which calls for new requirement on flex PCB structure. Up to now, some new products containing flex PCB have been developed such as ultrathin flex multi-layer PCBs whose thickness has reduced to 0.2mm from 0.4mm. High-speed transmission flex PCBs can reach a transmission speed of 5Gbps with the application of polyimide substrate material with low Dk and Df. Flex PCBs with large power apply conductors whose thickness is more than 100μm so as to meet the requirement of circuits with high power and large current. All these special flex PCBs naturally acquire unconventional substrate material.

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