Flexible PCB Fabrication Materials and Development Trends

The demand of advance electronic products like smart-phones, cameras, smart watches, laptops, LED TVs and many other similar is increasing day by day. People now want more miniature size and more compact devices with more features and functions with highest quality and charm and in reasonable price. Although this seems to be the long wish list, but today the technological development of Printed Circuit Boards (PCBs) has boosted to such extent where we can turn this wish list or dream into reality. Yes but this is not possible without Flexible PCBs. All above mentioned products do have flex PCBs inside them. The Flex PCBs are widely used in many electronic appliances and products and is one of the major factor of reduced size and compactness of product. The High Density Interconnect (HDI) PCBs are also highly recommended for electronic products mentioned above and can make the product performance very high and high speed processing is possible by these HDI PCBs. These HDI and Flex PCBs together form a Rigid-Flex PCB that makes a light weight (most of the weight is due to battery or power supply section), compact, small and high performance end product. These light weight flex PCBs can replace bulky wiring and cables of automotive electronics to move the auto-industry to next level.

Flexible PCB Fabrication Materials and Development Trends

Flex PCB Fabrication Material:

 

Flex Copper Clad Laminate FCCL:

 

The Flex Copper Clad Laminate (FCCL) were initially extensively used in military and aerospace  applications, but as the technology is getting common to public, this material is also now being used in commercial electronic products like digital camera, camcorders, automotive electronics like GPS, instrumentations, office automation devices, medical and health care devices.  It is very light, thin and flexible. FCCLs can be single sided, double sided or multilayer. FCCL can also be used in High Density Flex Circuit fabrication. Flexible CCL requires a halogen free environment.

 

Polyimide Flexible Substrate:

 

The substrate material for flex PCB is also flexible in nature. Usually-applied substrate material for flexible PCB includes PI (polyimide) film and PET (polyester) film apart from which polymer film is also available like PEN (polyethylene nphthalate), PTFE and Aramid etc. Substrate materials film should be picked up based on their performance and cost. PET resin has adequate mechanical and electrical performance but poor opposition to heat is a disadvantage which makes it unsuitable for direct solder and assembly. PEN is better than PET hence widely preferred as flex substrate.

 

Adhesive:

 

PI resin, PET resin, acrylic resin and modified epoxy resin are favorable for flex PCB adhesives. Modified epoxy resin is commonly used due to its high adhesive force to bond copper foil and above mentioned substrate material.

Polyimide Flexible Substrate

Developing Trend For Flex CCL (FCCL)

 

Because the future of electronic manufacturing industry is now greatly relying on Flex PCBs, so scientist and engineers are constantly working to improve the performance of FCCL to be used in flex circuits. So the 4 most popular types of FCCL recently introduced are

 

1- High Tg FCCL: The sole purpose of this type of 3 layer FCCL is to withstand high temperature at adhesive so that mechanical stability can be ensured. For this reason the acrylic resin and epoxy is used as adhesive to bear high glass transition temperatures (Tg) and ensure resistance to chemical and good insulation. The manufacturing cost of this type of FCCL is low.

 

2- Halogen and Phosphor Free FCCL: As the electronics industry is rapidly growing so does the use of hazardous materials. European Union has taken notice of this and has made mandatory to use Halogen and phosphor free FCCL to protect environment and reduce pollution.

 

3- High Speed FCCL: The use of Chip of Flex (CoF)  in LCD screens and many other miniature package ICs like Chip Scale Package (CSP), BGA, FC, PoP are increasingly used by designers today to make device compact, small size and high speed high performance. Because components are integrated very tightly and densely so the signal transmission has become fast from one point on circuit to other. So High Speed FCCL is very trendy nowadays

 

4- Ultra-Thin FCCL: The reason behind the light weight electronic gadgets we see nowadays is due to the use of Ultra-thin FCCL used in flex-rigid PCB.

 

Developing Trend For Substrate:Developing Trend For Substrate

 

Newly invented and developed technology in flexible PCB substrate is Liquid Crystal Polymer (LCP). This is a thermoplastic covered by copper foil. Highly suitable for high frequency digital circuits applications, with good thermal and dimensional stability, it has excellent humidity absorption properties. It can be recycled and reused also. Dielectric constant 2.85 @ 1GHz.

 

Recent Development For Environment Friendly:

 

The RoHS standards restricts the use of "bromine (Br)" in FR-4 and also restricts use of "Lead (Pb)" for surface finishing scheme like "Hot Air Solder Leveling (HASL)". So the trend in most of PCB manufacturers and assemblers is to ensure that those 6 restricted elements Lead (Pb), Mercury (Hg), cadmium (Cd), Hexavalent chromium (CrVI), Polybrominated biphenyls (PBB), Polybrominated diphenyl ethers (PBDE) must not be used in the process and to specially make sure that flex PCB ingredients like FCCL, solder-mask, pre-preg and coverlay all are halogen free and fire resistant.

 

A New Copper Foil Type is Trendy.

 

Mostly the copper foils used for highly flexible dynamic PCBs is a type of "Rolled and Annealed (RA)" copper. However the type of copper that is commonly used nowadays for High Density flexible PCBs is "Electro-Deposit (ED)" copper. This ED copper foil has good toughness and smoothness and highly suitable for fine pitch (40um) circuits.

 

New Conductive Silver Paste:

 

The solid, flexible and low resistance connections on flex circuits that we see is due to this type of conductive ink that is actually the paste of conductive silver. This ink is easily printed and cured quickly. This is used to print conductive images of circuits on thermoplastic or polymer film. Another version of this is the conductive oil which is less costly and environment friendly.

 

New  Photosensitive PI Coverlay:

 

A novel type of coverlay which is under research and implemented in some flex PCB applications is "Photo-Imageable-Coverlay (PIC)". This has high flexibility resistance and good bonding force when used in conjunction with epoxy or acrylic resin. But it has low Tg, low dimensional stability and low thermal resistance.