Nobuyuki Shishido

Accuracy improvement of full-field displacement measurement using digital image correlation for images obtained with a laser scanning confocal microscope

A method for image correction is proposed for digital image correlation in conjunction with a laser scanning confocal microscope (LSCM). The scan lines of an LSCM have sub-pixel error in their position in the scanning direction (drift distortion). Drift distortion decreases the accuracy of the displacement measured using the DIC method. The correction method proposed here removes both drift distortion and spatial distortion. Drift distortion is removed using a pair of images, each of which has a different scanning direction. Spatial distortion removal is performed using a methodology that employs a series of in-plane rigid body motions and a generated distortion map. Numerical simulations demonstrated that these correction procedures can successfully detect drift distortion. The standard deviation (SD) of the obtained detection error for artificial noise with sub-pixel line-shift was 0.004 pixels. Experimental results involving rigid body motion indicate that, after the correction of drift and spatial distortions, (i) the standard deviation of the measured displacements was 0.03 pixels, and (ii) the measured displacement fields are unbiased and agree closely with those obtained using an optical microscope. The proposed correction method effectively removes the distortions of obtained images and improves the accuracy of the digital image correlation method using an LSCM.

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 systems accuracy.

An analysis of local deformation of SnAgCu solder joint using digital image correlation

The changeover from eutectic Sn-Pb solders to lead-free solders has been driven by environmental concerns and market accessibility in the last few years. Though a number of creep constitutive laws of lead-free solder have been reported due to the importance of creep in solder joint failure in the electronic packaging, these constitutive laws often show great variations across the applicable stress range. In-situ measurement of strain fields in micro regions can help make the creep model more realistic. In this study, shear tests have been carried out on small solder joints in order to gain insight into the factors affecting solder properties. In order to measure the strain in solder joints, the Digital Image Correlation Method (DICM) has been used. The experimental results proved that there are two mechanisms that disturb the uniform deformation in solder joint. One is the interfacial strain concentration which has a dimension of less than 30µm and the other is spreading across interfaces from the locally deformed area, which has a size from about 50µm to 100µm. The existence of these local concentrations of creep strain is thought to be at least one of the causes of the variations in the published creep parameters. The deformation of solder joint on the shear test has also been modeled using Finite Element Method (FEM), and a three-layered model has been proposed to simulate the experimental results.