Moisture Concentration and Temperature Dependence of the Coefficient of Hygroscopic Swelling (CHS)

Abstract

This paper presents an experimental and computational study done on an epoxy to determine the effect of moisture level and temperature on the Coefficient of Hygroscopic Swelling (CHS). When a non-hermetic package is exposed to a humid environment, the adhesives used in the package absorb moisture and swell. This can induce stresses in the package that can lead to failure. The Coefficient of Hygroscopic Swelling is defined as the ratio of hygroscopic strain to the moisture concentration in the material. It has been found from prior literature that hygroscopic strains are significant and have to be accounted for in reliability modeling [1]. Prior research investigated the measurement of CHS experimentally using techniques such as thermo mechanical analysis (TMA) [1] [2], moire´ interferometry [3], and digital image correlation (DIC) [4]. An experimental method using the TMA technique was used to measure the CHS [1], but further analysis using improved techniques was recommended to get a more precise measurement. One of the goals of this paper was to investigate experimental and numerical techniques that would help better understand various factors that affect the measurement. This paper focuses on measurement of CHS for an epoxy used in optoelectronic packaging. The DIC technique was chosen for measurement of CHS. Moisture loss during the test leads to a change in the moisture concentration in the sample. While it may be thought that the moisture loss during the DIC scan can be assumed negligible due to the short test time compared to other methods, this assumption did not hold well for the epoxy material tested. The ramp rate chosen for the measurement will affect the amount of moisture lost from the sample. This has to be carefully chosen to minimize the moisture loss. These effects have to be accounted for in the CHS computation. The CHS value calculated will be significantly affected if these factors are high within the range of the measurement. This paper describes the investigation to minimize such effects in the measurement of CHS and attempts to account for them using computational methods. The CHS of an epoxy material was measured and its dependence on temperature and moisture concentration was determined.