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Nowadays, flexible PCB (printed circuit board) develops so fast that its market share goes up and much progress has been made in terms of technology. The advent of new flexible PCB fabrication technologies drives flexible PCB to feature its advantages of light weight, thinness and flexibility, leading to its wide range of applications.
The fundamental properties of PCB depend on performance of substrate material, so it's foremost to improve substrate material performance to truly improve technical performance of PCB, which also works on flexible PCB.
The functions of film substrate material lie in its capability to provide conductor carrier and insulating medium between circuits. Moreover, it has to be able to be bent and curled.
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.
The leading substrate material for flexible copper clad laminate (FCCL) covers PI, a type of thermosetting resin that won't reach a temperature to become softened or be able to flow. However, it can still maintain flexibility and elasticity after thermal polymerization, which is different from majority of thermosetting resin. PI features high thermal resistance and excellent electrical characteristics. However, PI leads to high humidity absorption and bad tear strength, which should be improved. Upgraded PI film features humidity absorption of 0.7% which is much lower than ordinary rate that is 1.6% and features higher dimensional stability, converting from ±0.04% to ±0.02%.
Both flexible CCL and rigid CCL call for environmental requirement that is halogen free, which is an inevitable and stringent trend of electronics industry. Based on regulations released by EU (Europe Union) and lots of countries, 6 types of hazardous substances are forbidden to be available in electronic devices since July, 2006 and PCBs including flexible PCBs mustn't contain brominated flame retardant.
PET resin features agreeable mechanical and electrical performance and its biggest disadvantage is bad heat resistance, leading it to be incapable of direct soldering and assembly. Performance of PEN is better than that of PET and worse than that of PI so that the applications of PEN keep going up.
In the world, categories of applicable plastic film exceed 2000 among which there must be some types suitable for flexible PCB fabrication. Thus, as applications of flexible PCBs become expanded, new flexible PCB substrate material will be applied.
Adhesive plays a role in bonding copper foil and substrate material film and its ordinary classifications cover PI resin, PET resin, modified epoxy resin and acrylic resin among which modified epoxy resin and acrylic resin are more used owing to their high adhesive force.
Flexible CCL usually contains three layers: polyimide, adhesive and copper foil. Since adhesive tends to reduce flexible PCB performance, especially electrical performance and dimensional stability, two-layer flexible CCL (2L-FCCL) without adhesive is developed. In addition, because 2L-FCCL doesn't contain adhesive that might contain halogen, it's good for environmental protection and is capable of meeting the demand of lead-free soldering by improving temperature from 220°C to 260°C to 300°C. Performance comparison between 2L-FCCL with no adhesive and 3L-FCCL with adhesive can be summarized in Table 1 below.
FCCL with Adhesive
FCCL without AdhesiveSubstrate material thickness Film + Adhesive (12μm-25μm) Film (12.5μm-125μm) Heat resistance Low High Dimensional stability Bad Good Flexibility resistance Good Based on types Compatibility with cover film Good Based on types Manufacturing applicability Long-term and easy Short-term and difficult Cost Low High
Three types of manufacturing methods of 2L-FCCL are available:
• Film coating;
By comparing three methods, it can be concluded that deposited electroplating metal layer on polyimide film is easy to be produced in rolling and thinner substrate material and copper foil can be picked up with low cost. Film coating is applicable for mass-volume production with a low cost. Lamination works better on double-side board fabrication.
To essentially modify disadvantage of polyimide substrate material, liquid crystal polymer (LCP) is newly developed. As thermoplastic LCP film is covered with copper foil, which then receives constant hot pressing, single-side or double-side CCL will be obtained. This type of CCL features a water absorption rate that is only 0.04% and dielectric constant that is 2.85 (1GHz), compatible with demands of high-frequency digital circuits.
Polymer features a liquid state and it will be melted into hot melting liquid crystal polymer (TLCP). When it comes to merits of TLCP, it can be molded through injection and fabricated by pressing into thin film that will be substrate material for PCB and flexible PCB. Moreover, it is capable of going through secondary processing, recycled and reused. Owing to low humidity absorption, high-frequency suitability and thermal dimensional stability of TLCP, it starts being applied in flexible PCB.
As early as 2003, EU published RoHS and WEEE to forbid applications of 6 types of hazardous substances and waste electronic and electric equipment processing. RoHS is concerned with bromine used in flame retardant in PCB and lead in surface coating.
Halogen-free substrate has been developed and applied in both rigid PCB and flexible PCB. Flexible substrate material, such as FCCL, coverlay, prepreg and solder mask and reinforcement board have to feature fire resistance and be halogen free.
Leading conducting material of flexible PCB is copper or copper foil and sometimes alloy is also used including aluminum, nickel, gold and silver etc. Apart from conduction, conductor layer should be flexibility resistant. In accordance with different manufacturing methods, copper foil is classified into electro deposit (ED) copper foil and rolled and annealed (RA) copper foil. Difference between the two types of copper foil lies in different crystal shapes: RA copper foil features a column array shape, leading to even and flat structure, accessible to roughening and etching processing. ED copper foil features fish scale, leading to smooth copper foil with good toughness, but inaccessible to roughening or etching processing. As far as dynamically flexible PCB is concerned calling for high flexibility, RA copper foil is usually applied.
Currently, high-density flexible PCB mainly depends on ED copper foil. To be able to compatible with requirement of mass-volume production of PCB whose pitch is within the range from 40μm to 50μm, new demands are laid. One is that copper foil surface should receive low roughness and the other is that copper foil should be ultra thin.
During the process of flexible PCB fabrication, conductive ink is printed on insulating film with wire or shielding layer generated and this type of conductive ink is mainly conductive silver paste. Printed conductive layer is required to be low resistance, solid connection and flexibility. Moreover, printing should be easily implemented and curing should be quick.
New conductive silver paste meets the demand of low resistance and flexibility and is capable of making conductive images formed on thermosetting or thermoplastic polymer film, fabric and paper. It is also capable of making graphics that are used on RFID products. Final products with the application of conductive silver paste are qualified in terms of high-temperature storage, humidity test and high and low temperature cycle performance. Conductive oil is also a type of technology compatible with environmental protection and low cost requirement.
Traditional PI/adhesive coverlay fails to achieve flexible PCB requirement such as high density, high dimensional stability and environmental protection, so PIC (photo-imageable coverlay) has been developed with high flexibility resistance, which is similar to solder mask oil.
Up to now, liquid or film-type PIC depending on modified epoxy resin or acrylic resin receives a wide range of research and application due to their high resolution, excellent binding force and flexibility. The disadvantage of PIC depending on modified epoxy resin or acrylic resin lies in its low dimensional stability applied on high-density PCB, low Tg and low thermal resistance.
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