Polyester Film

This is a typical substrate used in the photographic process, and is the most common substrate in applications that involve general multiple exposures (such as PCBs, etching, printing etc.). Polyester is excellent due to its low-cost nature, but must be handled carefully and used in controlled environments if dimensional accuracy is required.

• Low Cost
• Easy to handle
• Damages easily
• Low stability
• Low resolving power

Films used for photo-masks have the following component layers: a plastic base, a photosensitive emulsion layer (silver halide) and a backing layer.

This plastic base is polyester material, and has a thickness of 0.18 mm. The emulsion layer is composed of gelatine and silver halide and other chemicals, which adds another 0.005mm in total.

Two concerns with emulsion film over glass are stability and robustness. Both the polyester and the gelatine absorb moisture and heat from their surroundings, and as such they undergo dimensional changes – quite considerably for some large area films. The other problem with the Gelatine/Emulsion is that it is relatively soft and must be handled carefully to avoid damage and scratches.

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Polyester Film Specifications:

• Thermal Coefficient of Expansion (length, width) - 0.001% / °F (0.0018% / 90)
• Humidity Coefficient of Expansion (length, width) - 0.0008% / % RH
• Specific Gravity - 1.39
• Young's Modulus - 6 3 x 105 psi
• Poison's Ratio & 1 year at room temperature - 0.25
• Tensile Strength Yield ASTM D882-67 - 13,500 psi
• Tensile Strength Break ASTM D882-67 - 25,600 psi
• Elongation at Yield ASTM D882-67 - 5.5%
• Elongation at Break ASTM D882-67 - 115%
• Tear Strength Initiation Graves Tear AST D1004-06 - 24 Ibs.
• Tear Strength Propagation Tongue Tear ASTM D1938-67 - 0.5 Ibs.
• Toughness AST D882-67 - 21,500 Ib. / cubic inch
• Refractive Index at wavelength 589 nm - N = 1.50 Vertical axis through thickness
  N = 1 64 Width direction in plane of sheet
  N = 1,66 Length direction in Diane of sheet