THICKNESS-DIRECTION THERMAL-EXPANSION OF POLYIMIDE FILMS

The coefficient of thermal expansion (CTE) for thin polymer films is a property of considerable practical importance. In contrast to the abu ndance of CTE data in the in-plane or film direction, data in the thic kness or z-direction is rarely available, in part due to the high sens itivity required for measurement of the small thickness changes involv ed. In this paper, we will describe both a capacitance change and a Fa bry-Perot laser interferometric method for measuring z-direction CTE's . Results are presented for a variety of commercially available polyim ide films of thickness 50 to 130 mum. Upilex and Kapton were found to have z-direction CTE's of about 55 and 81 ppm/degrees-C, respectively. These CTE's for the thickness direction are many times higher than th e corresponding values in the in-plane direction. This anisotropy is a ttributed to the preferential in-plane molecular orientation of the po lymer chains.