Primary Material for Flexible PCB

As the rapidly growing field of electronics and telecommunication engineering demands technological innovations, engineers and scientists are constantly finding novel ways to improve the quality, life cycle and reliability of end product. In order to do so, the Flexible PCBs materials are the focus of research today. The flex PCB is found in almost every electronic device we see around us like printer, scanner, high definition cameras, mobile phones, calculators etc. So the flex PCB material research and improvement in fabrication process can minimize the production cost and increase quality plus reliability of end product. In this article we will analyze the major types of materials that are used in the flex PCB fabrication process.


Properties of Flex PCB:


We know that flex PCB is something that can be bend easily and have miniature electronic components mounted upon. It is also very light weight and ultra thin so it can be fit inside any small compartment or enclosure designed for the subject electronic product or end product. The flex PCB is best suited for applications where enclosure space constraint is the issue.


Common Types of Substrate Materials for Flex PCB:



The most important material in flex PCB or in a rigid PCB is its base substrate material. It is the material upon which the whole PCB stands. In rigid PCB, the substrate material is FR-4 commonly used. However in Flex PCB the commonly used substrate materials are Polyimide (PI) film and PET (polyester) film apart from which polymer film is also available like PEN (polyethylene nphthalate), PTFE and Aramid etc.



The Polyimide (PI) "a thermosetting resin" is yet the most commonly used material for Flex PCBs. It has excellent tensile strength, very stable over wide operating temperatures from -200OC to 300OC, resistant to chemicals, amazing electrical properties, highly durable and excellent heat resistant. It can retain its elasticity even after thermal polymerization unlike other thermosetting resins. However disadvantages of  PI resin is poor tear strength and moisture absorption is high. On the other hand PET (Polyester) resin has poor heat resistant "which renders it not suitable for direct soldering" but has good electrical and mechanical characteristics. Another substrate PEN has medium level performance better than PET but not better than PI.


Liquid Crystal Polymer (LCP) Substrate:

The rapidly getting popular substrate material in Flex PCBs is LCP. This is because it overcomes the shortcomings of PI substrate while maintaining all properties of PI. The LCP has great resistance to moisture or humidity that is 0.04% and dielectric constant at 1GHz is 2.85. This makes it famous in high speed digital circuits and high frequency RF circuits. The melted form of LCP called TLCP can be injection molded and pressed to form flexible PCB substrate and can be recycled easily.



Another material is the resin that binds the copper foil and the substrate material together tightly. The resin can be PI resin, PET resin, modified epoxy resin and acrylic resin. The resin, copper foils (Top and Bottom) and substrate materials forms the sandwich called the "Laminate". This laminate is called FCCL ( Flexible Copper Clad laminate) is formed by applying high temperature and pressure on the "stack" by means of automatic press under controlled environment. Among these mentioned resin types the modified epoxy resin and acrylic resin have the strong adhesive properties


These adhesive resins are not good for electrical and thermal performance of Flex PCB and reduce dimensional stability. These adhesives may also contain halogen that is hazardous to environment and restricted as per EU (European Union) regulations. According to these environment protection regulation, 7 hazardous materials are restricted names of which are Lead (Pb), Mercury (Hg), Cadmium (Cd), Hexavalent chromium (Cr6+), PolyBrominated Biphenyls (PBB), PolyBrominated Diphenyl ether (PBDE), Bis(2-ethylhexyl) phthalate (DEHP) and Butyl benzyl phthalate (BBP).


So a solution to this problem is to use the 2 layer FCCL without adhesive. The 2L FCCL has good electrical properties, high heat resistance and good dimensional stability but its manufacturing is difficult and costly.


Copper Foil:

Another top notch material in flex PCBs is the copper. The PCB traces, tracks, pads, vias and holes are all filled with copper as a conducting material. We all aware about the electrical conducting properties of copper but how to print these copper traces on PCB is a subject of discussion. There are 2 ways of copper deposition on 2L-FCCL (2 Layer Flexible Copper Clad Laminate) substrate. 1- Electroplating 2- Lamination. The electroplating method is adhesive less while the lamination includes adhesives.



The conventional method of lamination of copper foil on the PI substrate by means of resin adhesive is not suitable where ultra thin Flex PCBs are demanded. This is because of 3 layer structure of lamination process i.e. (Cu-Adhesive-PI) makes the stack thicker so it is not recommended for double sided FCCL. Hence another method is used called "Sputtering" wherein the Copper is sputtered on PI layer through wet or dry process through "electroless" plating. This electroless plating deposits a very ultra thin layer (seed layer) of copper while another copper layer is deposited in next step called "electroplating" where a relatively thicker layer of Copper is deposited on thin layer (seed layer) of copper. This method results in strong adhesion between PI and copper without using resin adhesives. This method is cost effective and greatly enhance the reliability of Flexible PCBs (FPCBs).



In this method, the PI substrate is laminated with ultra thin copper foil by means of coverlay. Coverlay is a complex film wherein a thermosetting epoxy adhesive is coated on polyimide film. This coverlay adhesive has excellent heat resisting capability and a good electrical insulator with bending, fire retardant and gap filling properties. The special type of coverlay is called "Photo Imageable Coverlay (PIC)" has excellent binding force, good resistance towards flexibility plus environment friendly.  However the disadvantage of PIC is inferior thermal resistance and low glass transition temperature (Tg)


Rolled Annealed (RA)  Vs Electro Deposited (ED) Copper Foil:

The main difference between the two is in its manufacturing process. The ED Copper foil is made from CuSO4 solution through electrolysis method in which Cu2+ is dipped into spinning cathode rolls and stripped and then made it into ED copper. While RA Copper of different thicknesses is made from high purity copper (>99.98%), by means of pressure process.


The below diagram shows the difference of the surface of two types of copper foils.


Electrodeposited (ED) copper has good conductivity than Rolled Annealed (RA) copper while RA has much better extensibility than ED. For Flex PCB, RA is a better choice in terms of flexibility and ED is better choice for electrical conductivity.