Strain measurement in the microstructure of advanced electronic packages using digital image correlation

Recently, the sizes of electronic products have been decreasing rapidly, with many electronic devices embedded in print circuit boards (PCBs), a phenomenon known as system in package (SiP). In the near future, not only passive devices but also active devices are embedded in the PCBs. It is thought that stress and strain around embedded devices affects the functions of embedded devices. A measurement system of stress and strain in the microstructures of PCBs is needed. In this study, a system for measuring thermal strain in the micro region of PCBs using the digital image correlation method (DICM) in conjunction with an optical microscope was developed. The accuracy of the measurement of thermal strain was verified by measuring the distribution of strain on the surface of a homogeneous aluminum alloy that is heated uniformly. Then, the strain distribution in a PCB was measured using the developed system. Although the measured distribution of strain in the PCB using the DICM was very complicated, the warpage of the PCB calculated from the measured strain accurately corresponded with the macroscopic warpage measured using a laser displacement meter. The accuracy of measurement was affected by the image distortion caused by the optical system and by the nonlinearity of the image sensor of the complementary metal-oxide semiconductor (CMOS) camera. An error correction method was introduced into the present measurement system to increase the system's accuracy.