Published papers

The delamination of copper lead frames from epoxy molding compounds (EMC) is a severe problem for microelectronic devices, as it leads to reduced heat dissipation or circuit breakage. The micro/nanoscale surface structuring of copper is a promising method to improve the copper–EMC interfacial adhesion. In this study, the generation of micro/nano hybrid structures on copper surfaces through femtosecond pulsed laser irradiation is proposed to improve interfacial adhesion. The micro/nano hybrid structures were realized by generating nanoscale laser-induced periodic surface structures (LIPSS) on microscale parallel grooves. Several types of hybrid surface structures were generated by changing the laser polarization direction, fluence, and scanning speed. At a specific aspect ratio of microgrooves, a latticed structure was generated on the sides of microgrooves by combining LIPSS formation and direct laser interference patterning. This study provides an efficient method for the micro/nanoscale hybrid surface structure formation for interfacial adhesion improvement between copper and EMC.

In the present study, the effects of rubber modification on the joint strength and fracture toughness of epoxy adhesives under mode I and II loading were investigated using Arcan-type adhesively bonded joints. The results of the fracture tests under mode I loading revealed the occurrence of stress whitening owing to triaxial tensile stresses near the crack tip. The joint strength and fracture toughness were calculated using the J-integral value, where the adhesive layer was considered as an elastic–plastic material. The crack-growth energy was also calculated using the J-integral value, where the adhesive layer was considered as an elastic material. The results revealed that rubber modification under mode I loading induced an increase in the joint strength, fracture toughness, and crack-growth energy. Rubber modification under mode II loading did not induce stress whitening near the crack tip owing to low triaxial tensile stresses. However, macroscopic void growth and significant plastic deformation were observed in the adhesive layer. Rubber modification induced a slight decrease in the joint strength; however, it induced a significant increase in the fracture toughness owing to the growth of the plastic zone in the adhesive layer. The calculated crack-growth energy decreased slightly, in accordance with the joint strength. It was concluded that rubber modification contributed to an increase in the crack-growth resistance under mode I loading and not under mode II loading. Furthermore, a correlation was observed between the joint strength and crack-growth energy.

Advanced high-strength steels (AHSSs) are widely adopted in car body structures to achieve weight reduction for energy efficiency and crash safety. This study investigates the effects of material strength and notch shapes on crack initiation and propagation behaviors in AHSS sheets used in car body structures. Tensile tests were conducted for three types of Japanese grade AHSS (JSC590, JSC980, and JSC1180) specimens and two notch root radii (R0.5 and R5). The crack initiation and propagation behaviors were found to strongly depend on the material strength and notch root radius. The crack generated from the notch root for the R0.5 specimens, whereas it generated from 1 to 2 mm inside the notch root for the R5 specimens. Unstable crack propagation occurred beyond the maximum load for the R5 and JSC1180 specimens, whereas only stable crack propagation occurred for the R0.5 specimens. From the digital image correlation analyses, we found that the specimen notch radius affects the stress triaxiality and equivalent plastic strain around the notch root. The JSC980 material had the highest crack initiation energy, which correlates with the plastic dissipation energy utilized for thickness reduction. The JSC590 material exhibited the highest crack propagation energy, which correlates with the true fracture strain.

This study investigated the effects of micro- and nanostructures on the adhesion strength between copper and an encapsulating mold compound (EMC). The shape of the microgrooves was designed based on fracture mechanics theory, and the effect of laser-induced periodic structure surface (LIPSS) formation on the microgroove was investigated using numerical simulations. The designed surface profiles were fabricated using femtosecond laser treatments, and the adhesion strength between copper and the EMC was evaluated. The results show that deep microgrooves improved the adhesion strength owing to the anchor effect. However, the aspect ratio between the pitch and depth of the microgroove had a limitation in enhancing the adhesion strength. The formation of LIPSSs on the mountaintop of the microgroove was very effective; however, an LIPSS on the valley of the microgroove was ineffective. Numerical analyses revealed that LIPSSs suppressed the shear deformation of the EMC and worked as resistance to interface delamination. Based on the findings obtained in this study, combining micro- and nano-scale surface structure formation with femtosecond laser treatments is an effective and eco-friendly method for improving adhesion strength.

This study presents a gradient damage model for ductile fracture, in which the damage hardening modulus is degraded by the accumulation of plastic deformation and the volume expansion caused by negative hydrostatic pressure. The proposed model fulfills the thermodynamic requirements, and the governing equations are derived from energy minimization principles. Two parameter studies are carried out to confirm the basic performance of the proposed model, in which some typical ductile fracture responses are demonstrated by changing parameters for degrading the damage hardening modulus. Also, a series of numerical experiments are presented to reveal the ability of the proposed model to successfully simulate the fracture tests of advanced high strength steel sheets with different tensile strengths. It is indeed confirmed by the close agreement with experimental results that the proposed model is capable of realizing the breaking elongation, the transitional behavior from unstable to stable crack propagations, and the corresponding load–displacement curves. Also, the model successfully reproduces and predicts the crack initiation positions in notched specimens with different notch radii.