A biaxially stretched film of BEK, a new engineering plastic with high
heat resistance (T-g: 181 degrees C,;T-m: 364 degrees C) and chemical
resistance, was developed as a base film for flexible printed circuit
boards, Its heat shrinkage percentage at 260 degrees C and the coeffi
cient of thermal expansion below T-g were decreased to below 0.1%, and
3.3 x 10(-5)/degrees C: respectively. The process of determining stre
tching and heat setting conditions for film formation with a role stre
tcher and a tenter is reported.