The use of polymers in electronic packaging is continuously increasing, due to their relative ease of manufacturability and low cost. Since polymers exhibit time and temperature dependent behavior, their visco-elastic properties must be characterized in order to predict the behavior of package assemblies during manufacturing and operation. The testing methods for visco-elastic properties have been developed for many decades and some of them are routinely practiced using commercially available equipment. However, some of the methods are too time-consuming or complex to be implemented routinely by non-experts; the specimen preparation and the testing conditions are very critical to reliable and repeatable measurements.

A novel method is proposed to characterize the visco-elastic behavior rapidly but accurately. The method utilizes a polymer cured around a fiber Bragg grating (FBG) to form a complete specimen. An instantaneous mechanical load is applied to the specimen while equilibrated at a temperature within an environmental chamber, and the Bragg wavelength (BW) shift is documented as a function of time. The load applies an instantaneous, constant stress to the polymer substrate, which in turn applies a strain to the fiber. The relationship between the BW shift and the creep compliance can be derived directly from the theoretical behavior of the FBG. The creep compliance can then be obtained from the BW shift data at each temperature. By undergoing a de-convolution process the creep compliance can be converted into the time dependent relaxation modulus. This process can then be repeated for a range of temperatures, which results in relaxation modulus data as well as the initial modulus for each temperature.