Inexpensive, high-resolution, high-temperature characterization of thermally induced strains: Applications to microelectronics and fuel cells

The reliability of electronic systems is often determined by the thermo-mechanical properties of materials, influenced by their microstructure and by interfaces between materials. Thus, there is a need to characterize the behavior of materials and components under various loading conditions. The high-sensitivity laser moiré interferometry has proven to be invaluable in characterizing electronic systems. However, many times, simpler techniques do need to be validated by the more accurate moiré measurements. Thus, we carried out a systematic characterization of printed circuit board (PCB) samples using strain gages, further validated by moiré interferometry, to understand the impact of CTE variation on the reliability of solder joints. It was shown that the measured effective CTE varied in a wide range and could caused impact the reliability significantly for a representative plastic ball grid array package assembly.; Cross-sectional moiré is used to study the behavior of multilayered specimens such as electronic packages. Here, to evaluate the accuracy of the stress in moiré measurement, a five-layered circular specimen consisting of copper and epoxy layers (deemed representative of electronic packages) was tested under a thermal excursion. The thermal strains were measured over the top surface of the circular specimen as well as in each layer of the cross-sectioned specimen. The tensile test was also performed to obtain the mechanical properties of each layer. The results showed that there was only 0.4% difference in strain between the whole and the cross-sectioned specimens, but the corresponding difference in stress was as high as 40 percent.; The constitutive behavior of materials used in electronic packages is highly nonlinear and time-dependent. A high-resolution experimental technique is needed to accurately measure force and deformation. Thus, the coupling of a micro mechanical tester with the digital image correlation (DIC) was developed to enable a study of time-dependent behavior. The CTE of Inconel X-750 at up to 1000°C and those of PCB coupons were also measured. The strain in a layer of copper-epoxy-copper specimen was determined next using an inverse analysis. The glass transition temperature (T_g ) of RT/duroid 6002 PTFE was also determined and was validated using Dynamic Mechanical Analyzer. Finally, a stress relaxation test was carried out on RT/duroid 6002 PTFE to illustrate the experimental setup for time-dependent behavior characterization. The developed setup was thus demonstrated to be a simple to build but precise tool for high temperature, whole field characterization of thermal strains.