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June 2014 • The PCB Magazine 15 SUBSTRATES: PoLyESTER FILM FoR THE FLExIBLE ELECTRoNICS INDUSTRy continues The Flexible Substrate Property Challenge Over a decade ago, the film's dimensional reproducibility at processing temperatures, sur- face smoothness, high barrier, and inertness to the chemicals used in processing were high- lighted as the key property requirements for film substrates. Cost and commercial availabil- ity have emerged as further film requirements as device manufacturers have moved from dem- onstrators to prototypes to commercial manu- facture with the realization that only film man- ufactured at a commercial scale is likely to meet the consistency and quality required for volume manufacturing. Figure 1 shows the main plastic film types currently under consideration by the flexible electronics industries. Films can be categorised into semicrystal- line and amorphous. The combination of di- mensional reproducibility, surface quality , chemical inertness, cost, and commercial avail- ability have given biaxially oriented, semicrys- talline, heat-stabilized polyester films a leading position in these emerging markets. The films that are circled have emerged as substrates of interest over the past decade. Oriented polypro- pylene film (O-PP) is finding use in "simple" printed electronic applications where process- ing and performance at higher temperatures is not required. Polyether ether ketone (PEEK) is the premier high performance semicrystalline engineering polymer and offers an excellent high temperature, chemical inertness property set. The amorphous high T g resin developed by Akron Polymer Systems offers high temperature processing. However new polymer and or film developments have the issue of the prohibitive costs of scaling up to a commercial quality film line for what remains at present a low-volume market. Other materials such as stainless steel and flexible glass have also been worked on. Stainless steel finds use in PV devices, but is not being actively used in flexible electronics. Flex- ible glass offers transparency and excellent bar- rier properties, but faces the challenges of scale up to a commercial scale, plus other technical and processing issues including edge cracking. Experience over the past decade has shown that "off the shelf" material is unlikely to have the complete desired property set required, but good progress may be achieved when the device manufacturer and the substrate supplier work together to match process capability to an op- timized substrate property set. Taking the prop- erty set highlighted earlier and using the biaxi- ally oriented polyester films polyethylene tere- phthalate (PET) and polyethylene naphthalate (PEN) as examples, it is possible to review the progress that has been achieved. 1. Flexible Transparent High Barrier Considerable progress has been made in developing high barrier films, and commercial products are now available. However, achiev- ing a cost effect barrier of >10 -5 g/m2/day for moisture and >10 -5 mL/m 2 /day remains a key challenge and a rate-limiting step in develop- ing commercial quality flexible OLED displays. A pristine film surface is one of the key enablers to achieving this and progress towards this will be discussed later. 2. Dimensionally Reproducibility The Melinex ® (PET) and Teonex ® (PEN) fam- ily of polyester films exhibit dimensional sta- bility up to approximately 150 ° C and 200 ° C re- spectively, and PEN films have been shown to survive solder reflow temperatures >230 ° C. The dimensional reproducibility of the films can be enhanced further by the use of a rigid carrier, the use of process technology that allows for predictable dimensional change, and the con- trol of moisture in the processing atmosphere. Once a certain level of shrinkage is achieved, the consistent dimensional reproducibility is more important than further reductions in shrink- age. Companies such as Plastic Logic, Polymer Vision and the Flexible Electronics and Display Center at Arizona State University have demon- strated the successful manufacture of thin film transistor (TFT) backplanes on polyester film, proving that the required dimensional repro- ducibility can be achieved. 3. Surface Quality Defects on the surface of the film are clearly undesirable and can lead to pinhole defects in subsequent barrier coatings, or line outs and spot defects in displays. It is probably impos- sible to achieve glass-like perfection in terms of surface smoothness with a plastic film; howev-