Investigation of the effects of moisture expansion of glass fiber reinforced epoxy resins on printed circuit boards using digital image correlation

Nowadays, the environmental conditions and constraints for electronic systems are further tightened to achieve higher reliability under harsh conditions. For the development of optical modules in the camera industry, the environmental impact of humidity plays a key role in engineering and product performance. In addition, optical sensors today have high resolution power, making them even more sensitive to motion associated with moisture expansion. For example, if the optical sensor is mounted on a printed circuit board (PCB), an increase in moisture in the base material will affect the associated motion and the final accuracy of the system. Finite element-based tools are needed to address these challenges in the early stages of product development. Therefore, moisture expansion must be well known so that reliable material models can be created to simulate these effects. To have a solid starting point for setting up material models, the moisture expansion of glass fiber reinforced epoxy resins must be characterized and the hygroscopic expansion mechanism must be understood. In addition, the industry needs a method for screening materials at an early stage of product development in a highly efficient way.Therefore, the present study focuses on the basic understanding of moisture-induced deformation effects on PCB base materials and PCBs on a purely experimental level. The optical three-dimensional measurement technique digital image correlation (DIC) is the core of the present study. In the first part of the study, glass fiber reinforced PCB base materials with different glass fabrics and resin contents were measured under humid conditions. Four different material families were investigated, each with two different woven fabric types. The so-called inplane hygroscopic coefficient of expansion is determined by applying the DIC method in a climate chamber. A specimen holder specially developed for the project was used for this purpose. During the experiment, the thermal strain is monitored and afterwards the in-plane hygroscopic coefficient of expansion is calculated using a python script. In the second part of the study, PCBs were produced to validate the results of the individual glass fiber reinforced epoxy measurements. The PCBs were made from the same base material as in the first part of the study. In-plane hygroscopic expansion was also determined using a similar test program in the climatic chamber. In addition, out-of-plane moisture-induced movement was also computed from the correlation calculation. From these results, the absolute and differential moisture warpage across the PCB diagonal was observed. In this way, the deformation characteristics in three-dimensional space during moisture loading was obtained and assessed. After completion of the two parts, all results were compared and final conclusions could be drawn.The in-plane hygroscopic expansion of glass fiber reinforced epoxy resins and PCBs can be investigated with the presented optical measurement method. The out-of-plane warpage due to humidity of PCBs can also be determined by this approach. Measurements on PCBs are particularly challenging due to the long measurement times.It can be concluded, that different PCB base materials result in different moisture expansion coefficients. A clear trend was observed with respect to the material family and resin content. Moreover, the behavior of each base material can be transferred to the PCB level and they follow the trends, with a much lower moisture expansion value. The out-of-plane movement of the PCBs is also clearly affected by the moisture. The final results should be taken into account as a basis for further developing material modelling techniques for simulations taking moisture-related deformations of PCBs into account. The method in general is highly suitable to assist material screening in early development phase, without building PCBs and test them.