Toru Ikeda

Effect of bond thickness on the fracture toughness of adhesive joints

The effect of bond thickness on the fracture toughness of adhesive joints was investigated from a microstructural perspective, using compact tension (CT) adhesive-joint specimens with different bond thicknesses. The adhesive material was a rubber-modified epoxy resin with 12.5 wt% carboxy-terminated butadiene acrylonitrile (CTBN) elastomer. The shapes of the rubber particles dispersed in adhesive layers of damaged and undamaged specimens were observed with an optical microscope. The damage was distributed along the interfaces between the adhesive layer and the two adherends. The results show that the primary causes of variations in the fracture toughness of at adhesive joint with the bond thickness are not only a damage zone around a crack tip hut also the combination of a damage zone around a crack tip and additional damage zones along the interfaces.

Failure of a Ductile Adhesive Layer Constrained by Hard Adherends

The evaluation of a fracture from a thin layer constrained by a hard material is important in relation to the structural integrity of adhesive joints and composite materials. It has been reported that the fracture toughness of a crack in a ductile adhesive joint depends on the bond thickness, but the mechanism has not yet been elucidated clearly. In this study, the J-integral and the near-tip stress of a crack in an adhesive joint are investigated. It is determined that a decrease of the bond thickness increases the stress ahead of a crack tip, which results in the decrease of fracture toughness.

Evaluation of the Delamination in a Flip Chip Using Anisotropic Conductive Adhesive Films Under Moisture/Reflow Sensitivity Test

Anisotropic conductive adhesive films (ACFs) have been used for electronic assemblies such as the connection between a liquid crystal display panel and a flexible printed circuit board. ACF interconnection is expected to be a key technology for flip chip packaging, system-in-packaging, and chip size packaging. This paper presents a methodology for quantitative evaluation of the delamination in a flip chip interconnected by an ACF under moisture/reflow sensitivity tests. Moisture concentration after moisture absorption was obtained by the finite element method. Then, the vapor pressure in the flip chip during solder reflow process was estimated. Finally the delamination was predicted by comparing the stress intensity factor of an interface crack due to vapor pressure with the delamination toughness. It is found that the delamination is well predicted by the present methodology

Residual stress evaluation in resin-molded IC chips using finite element analysis and piezoresistive gauges

The high residual stress in a resin-molded electronic package sometimes makes the electronic functions unstable. Therefore the residual stress in electronic packages, especially on the top surfaces of semiconductor chips, should be evaluated. The objective of this study is to present a simple method for evaluating residual stress in resin-molded semiconductor chips using a combination of experimental and numerical methods. The actual residual stress of the packaging process was measured by using test chips that included piezoresistive gauges. A linear thermoelastic finite element analysis was then carried out using a three-dimensional model. The finite element analysis was performed under a stress-free temperature determined by the temperature dependence of the residual stress, which was experimentally measured by using the piezoresistive test chips. The measured residual stress using the test chips agreed well with the results of the finite element analysis. It was therefore confirmed that the present evaluation method, combining experimental and numerical methods, is reliable and reasonable.

Constraint effects of clad on underclad crack

The finite element method is applied to two-dimensional elastic-plastic analyses for underclad crack problems. The analyses are performed for rectangular specimens with an underclad crack, which are composed of A533B class 1 steel and a clad material, to obtain the fracture mechanics parameter J-integral and the stress distribution ahead of a crack tip. The Q-factor proposed by O`Dowd and Shi has calculated from the stress distribution ahead of a crack tip, and the constraint effect of a crack tip due to a clad material or the effect of a clad material on the fracture toughness of a base material is discussed in terms of the Q-factor. Clad thickness, crack length, and the material property of a clad material are varied to examine their effects.

Evaluation of Stress Effects on Electrical Characteristics of N-Type MOSFETs: Variations of DC Characteristics During the Resin-Molding Process

Stress-induced changes in the electrical characteristics of a semiconductor device become a major concern in the production of semiconductor packages because the electrical characteristics are adversely affected by packaging (residual) stresses. The objective of our project is to evaluate the effects of stress on semiconductor devices. In this study, the shift of the DC characteristics of nMOSFETs during the resin-molding process was investigated experimentally. After a silicon chip including the n-type metal oxide semiconductor field effect transistors (nMOSFETs) was encapsulated in a quad flat package, the drain current variations and the transconductance shifts were measured. The drain current decreased during the resin-molding process while no significant shift in threshold voltage was observed. The experimental results were estimated adequately from the residual stress predicted by numerical and experimental analyses and from the stress-sensitivity of the nMOSFETs measured by the four-point bending method. Also, we tested the validity of an electron-mobility model that included the effect of stress. The electron-mobility model takes into account the variation in the relative occupancy of the electrons in each conduction-band energy valley. It was found that the effect of biaxial stress on the variation in electron-mobility can be qualitatively evaluated by the electron-mobility model but are quantitatively different from the experimental results. Several needed improvements to the electron-mobility model are proposed in this article.

Experimental Study of Uniaxial-Stress Effects on DC Characteristics of nMOSFETs

Stress-induced shifts of the direct current characteristics on n-type metal oxide semiconductor field effect transistors (nMOSFETs) were investigated experimentally. The stress sensitivities of nMOSFET characteristics were measured by the 4-point bending method, and the gate-length dependence of transconductance shifts caused by uniaxial stress was evaluated. As a result, it is shown that the gate-length dependence of transconductance shifts is attributed to parasitic resistance of the nMOSFETs. Also, this paper verified the electron-mobility model proposed in the previous study that includes stress effects in comparison with the experimental results. As a result, several improvements for the electron-mobility model are proposed in this paper. We describe the change of the conduction-band energy induced by the shear deformation of silicon. The shear deformation with a uniaxial stress along the direction of silicon should be considered in the change of the conduction-band energy.

Influence of uniaxial mechanical stress on the high frequency performance of metal-oxide-semiconductor field effect transistors on (100) Si wafer

The effects of uniaxial mechanical stress on the radio frequency performance of n- and p-metal-oxide-semiconductor field effect transistors (MOSFETs) are investigated up to 10 GHz. Under tensile stress, the gate transconductance (gm) increases in the n-MOSFETs while it decreases in the p-MOSFETs, whereas the results were vice versa for compressive stress. The total gate capacitance (CG) extracted from scattering parameters increases (decreases) under tensile (compressive) stress for both n- and p-MOSFETs, which is explained by the variation in the effective mass perpendicular to the Si/SiO2 interface. The cut-off frequency (fT) varies in inverse proportion to the CG variation.

Warpage analysis of an LCD panel under thermo-mechanical and hygro-mechanical stress

A polarizing plate, which is an important part of a liquid crystal display panel (LCD), is made by sandwiching an organic polarizer between protecting films. An organic polarizer is both a hygroscopic and orthotropic material. The hygroscopic swelling and drying shrinkage of the organic polarizer can cause the polarizing plate to crack and the liquid crystal display panel to warp. The diffusion coefficient and Henrys law coefficient were measured using a thermo-gravimetric analyzer (TGA) under controlled humidity, while the coefficient of moisture expansion (CME) was measured using a thermo-mechanical analyzer (TMA), also under controlled humidity. The thermo-mechanical and hygro-mechanical deformation of a polarizing plate was analyzed using the finite element method (FEM). This analysis was performed as follows. The distribution of the moisture concentration was analyzed according to Ficks law. The equation of Ficks law is similar to that of the transient heat conduction, and the FEM for the transient heat conduction was utilized for the transient diffusion analysis. The hygro-mechanical analysis was then carried out in a way similar to the thermal stress analysis. Thermal stress was analyzed separately using the FEM. Finally, the obtained hygro-mechanical strain and stress were added to the thermal strain and stress, respectively. The analyzed displacement of a polarizing plate using the CMEs of a polarizer and protecting films corresponds to the measured displacement. The warpage of a liquid crystal display panel sometimes causes light leakage along the frame of the display panel due to contact of the display panel with the bezel of the frame. The warpage was analyzed according to the thermo-mechanical strain and the hygro- mechanical strain.

Predicting Technique of Delamination at Adhesively Bonding Joints in a Flip Chip Package During Solder Reflow Process

Recently, adhesively bonding techniques such as the anisotropic conductive film (ACF) or the non-conductive adhesive resin are often used for connections in the chip size packages instead of conventional solder joints due to their reasonable cost and the ease of miniaturization. Adhesively bonding techniques expected to be a key technology for the chip size packaging and the system in package. However, the level of reliability for adhesively bonding techniques is still less than that for solder joints. The quantitative evaluation techniques for the reliability of adhesively bonding techniques are desired. This paper focused on the reliability of adhesively bonding joints in a flip chip package during the solder reflow process for other solder jointed devices. This paper presents a methodology for quantitative evaluation of the delamination in a flip chip interconnected by an ACF under moisture/reflow sensitivity tests. The delamination toughnesses between components in a flip chip based on the stress intensity factors were measured by fracture tests in conjunction with the numerical analysis developed in our previous study. Moisture concentration after moisture absorption was expected by the diffusion analysis using the finite element method. Then, vapor pressure in a flip chip during the solder reflow process was estimated. Finally the delamination was predicted by comparing the stress intensity factor of an interface crack due to vapor pressure with the delamination toughness. The delaminations in an actual flip chip package during moisture/reflow sensitivity tests have successfully predicted by the present methodology.Copyright