Experimental results are presented for the time-dependent deformation of coated and uncoated flexible Mylar* diskass a function of temperature and humidity. The relative magnitude of shrinkage and creep is examined, and the nature of hygroscopic and thermal expansion is studied. Comparison between deformation of Mylar and one other potential substrate is made, and it is found that pre-annealing of substrates reduces anisotropic shrinkage. © 1977, IEEE. All rights reserved.