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June 2014 • The PCB Magazine 21 end groups. The relative susceptibility of dif- ferent films to hydrolysis is often studied using a damp heat test (DHT) in a climatic chamber set at 85 ° C/85%RH. By using strategies such as those outlined above the lifetime to failure in DHT can be pushed from ca 1000 hours for a standard PET film to 2000–3000 hours for a hy- drolysis stabilised film. Similarly, a standard, unstabilized PET film will show poor stability to UV in outdoor use, and thus PET is often used misleadingly in comparison with other film types. PET film can be very effectively protected against the effects of UV irradiation by the addition of well-established UV stabilizers. Figure 7 shows results that illustrate how the addition of a UV stabilizer will have a significant effect on the retention of mechanical properties of PET as measured by elongation to break (ETB) on ac - celerated aging. (Aging at 10,000 hours in a Weather-Ometer ® equates to approximately three years/1000 hrs for Northern Europe, two years/1000 hrs for Southern Europe and one year/1000 hrs for Australia.) Ongoing devel- opment work is extending the lifetime of PET film further. Conclusion The understanding of the required property set for films used in flexible electronics appli- cations has moved considerably over the past decade, and significant progress has been made in developing films appropriate for use. Rather than awaiting further inventions or scale-up investments for new engineered polymer film substrates, the flexible electronics industry has been seeking to drive the processing conditions of their products to the point where they can utilize extensions of commercial polyester film substrate technology. Polyester film manufac- turers must continue to work with the flexible electronics industry to drive innovation that will meet the future performance and cost re- quirements of the flat panel display industry, as well as other flexible electronic, PV and OLED lighting markets. PCB scott gordon is business development manager at duPont teijin Films. SUBSTRATES: PoLyESTER FILM FoR THE FLExIBLE ELECTRoNICS INDUSTRy continues Figure 7: change in elongation to break vs. hours in a weather-ometer ® for Pet.