Hironori Tohmyoh

A governing parameter for the melting phenomenon at nanocontacts by Joule heating and its application to joining together two thin metallic wires

For cutting and joining extremely thin metallic wires, the issue of Joule heating in the wires is considered. The middle of a section of a thin Pt wire with a diameter of about 800 nm was melted locally by a direct current supply, and the wire was cut at a predetermined point by applying a force to shear the molten part of the wire. Furthermore, a constant current was applied to a system in which the free ends of two Pt wires were contacted, and the wires were joined together. A parameter, which governs the melting phenomenon at the point of contact of very thin wires, is proposed. It was verified that the conditions required for joining thin wires were able to be determined by the parameter that depends on the applied current, the length and cross sectional area of the wires, and a function of the geometrical quantities for calibrating the thermal conditions.

Dry-contact technique for high-resolution ultrasonic imaging

To accomplish a high-resolution ultrasonic imaging without wetting a sample, the efficiency of the dry-contact ultrasonic transmission is discussed. In this study, a dry-contact interface is formed on a sample by inserting a thin film between water and a sample, and the pressure is working on the interface by evacuating the air between the film and the sample. A model of dry-contact ultrasonic transmission is presented to assess the signal loss accompanied with the transmission. From the determination of the signal loss caused by the transmission using various films, it was found that the higher frequency ultrasound is transmitted effectively into the sample by selecting an optimum film, which can keep the displacement continuity between the film and the sample during ultrasonic transmission. Finally, ultrasonic imaging with the sufficient signal-to-noise ratio (SNR) and high lateral resolution was performed on the delamination in a package and the jointing interface of the ball-grid-array package without wetting the packages.

Acoustic resonant spectroscopy for characterization of thin polymer films

An acoustic resonant spectroscopy technique for measuring the acoustic impedance, ultrasonic velocity, and density of micron-scale polymer films is developed. The method, which is based on spectral analysis, observes the acoustic resonance between water, the film, and a tungsten plate with high acoustic impedance in the frequency range of 20–70MHz. The interface between the film being examined and the plate is vacuum sealed, enabling us to characterize the low-density polyethylene film with acoustic impedances as low as about 1.9MNm−3s and the poly(vinyl chloride) film as thin as about 8μm. The error in the film density measurements is found to be less than 1%, and the validity of the technique is verified.An acoustic resonant spectroscopy technique for measuring the acoustic impedance, ultrasonic velocity, and density of micron-scale polymer films is developed. The method, which is based on spectral analysis, observes the acoustic resonance between water, the film, and a tungsten plate with high acoustic impedance in the frequency range of 20–70MHz. The interface between the film being examined and the plate is vacuum sealed, enabling us to characterize the low-density polyethylene film with acoustic impedances as low as about 1.9MNm−3s and the poly(vinyl chloride) film as thin as about 8μm. The error in the film density measurements is found to be less than 1%, and the validity of the technique is verified.

Polymer acoustic matching layer for broadband ultrasonic applications

A polymer acoustic matching layer has been designed that can act as a frequency filter between water and a test sample, and a method for controlling the high-frequency components of broadband ultrasound by using the designed layer is described. Acoustic imaging of a silicon-bonding sample using a very-low-scale matching layer fabricated from poly(vinylidene chloride) is demonstrated, in which the air gaps between the layer and the sample are evacuated. The experimental results show that the layer, which was designed for signal amplification, works as a broadband filter, as predicted, as well as offering protection against water for dry acoustic imaging.

Design and performance of a thin, solid layer for high-resolution, dry-contact acoustic imaging

Compared to the usual water immersion case, more effective transmission and reception of high-frequency ultrasound through a thin, solid layer are reported. A theoretical model is presented to perform the signal amplification and the signal modulation toward the higher frequency components for getting the high-quality acoustic images without immersing the object to be imaged. Also, the thin, solid layers are designed from the theoretical model, and the transmission of high-frequency ultrasound is carried out through the layer/silicon interfaces with an applied pressure of about 0.1 MPa. The spectral intensity in the frequency range of 20 to 70 MHz remarkably improves compared with water immersion, and the peak frequencies of the spectra modulate the higher than water immersion. Furthermore, the solder joint inspection of a package is performed. The present dry-contact technique achieves the higher spatial resolution and the higher signal-to-noise ratio (SNR) than the usual water immersion technique, and clearly detects the defective joint without getting the package wet.

Analysis of Solderless Press-Fit Interconnections During the Assembly Process

This paper deals with typical mechanical problems that are encountered in a solderless press-fit assembly process. First, the elastic-plastic properties of two types of press-fit pins and the friction coefficients of the pins in thin plated through holes are determined both experimentally and by three-dimensional finite element analysis. The elastic-plastic properties of the press-fit pins are determined by small-scale testing under three-point bending. The coefficients of friction of the pins in the through holes are successfully determined from the load-displacement relationships of the pins during press-fit assembly processes. The validity of the parameters that are determined is clarified by inserting the press-fit pins into holes of different diameters. By comparing the damaged areas of the printed circuit boards after assembly and the numerically obtained stress distributions, the failure stress of the boards is determined. Finally, both the retention force of the pins and the degree of damage to the printed circuit boards after assembly are predicted by numerical analysis.

Forming microstructures by controlling the accumulation and discharge of Al atoms by electromigration

We present a mechanism for controlling the accumulation and discharge of Al atoms using electromigration, by which long thin Al wires of lengths up to 336??m and diameters ranging from 0.8 to 4??m were fabricated. The experimental samples were Al lines formed on a TiN layer and covered with a SiO2 passivation layer. A slit in the Al film and a hole through the oxide or through the oxide and the Al film at the anode end of the line were used to control the accumulation and discharge processes. It was found that the position of the bottom of the hole, which was either at the SiO2/Al or Al/TiN interface, significantly affected these processes. A hole introduced down to the Al/TiN interface prevented cracks from forming in the line by relieving the compressive stress caused by accumulating atoms, and helped to extend the lifetime of the line. Guidelines for effectively controlling the accumulation and discharge of Al atoms were drawn, and based on these guidelines, other Al microstructures requiring the accumulation of a large number of atoms were fabricated after several tens of minutes of current supply.

Effect of Warpage of Flip Chip Packages Due to the Underfill Encapsulating Process on Interconnect Reliability

In flip chip packages, it is common practice for interconnects to be encapsulated with a liquid underfill material. This paper describes the effects of different underfill processes, i.e., the conventional capillary-flow underfill and two no-flow underfill processes, on slip chip packaging. The warpage of the package was examined, and the value of this during three different underfill encapsulating processes was measured. In addition, the interconnect reliability of the bump bonds after thermal-cycling was evaluated using a test circuit. The warpage of the package before curing varied depending on the assembly process, but that after curing was almost the same for all the processes studied. It was found that the interconnect reliability is closely related to the differences in the warpage arising from the assembly process, and that the smaller change in warpage introduced by the curing process gave a higher interconnect reliability for the bump bonds. Based on these.findings, lower curing temperatures are considered to be more effective for improving the mountability of the package and the interconnect reliability.

Numerical analysis of the electrical failure of a metallic nanowire mesh due to Joule heating

To precisely examine the electrical failure behavior of a metallic nanowire mesh induced by Joule heating (i.e., melting), a previously developed numerical method was modified with regard to the maximum temperature in the mesh and the electrical resistivity of the nanowire. A sample case of an Ag nanowire mesh under specific working conditions was analyzed with highly accurate numerical results. By monitoring the temperature in the mesh, the current required to trigger the melting of a mesh segment (i.e., the melting current) could be obtained. The melting process of a mesh equipped with a current source during actual operation was predicted on the basis of the obtained relationship between the melting current and the corresponding melting voltage in the numerical melting process. Local unstable and stable melting could be precisely identified for both the current-controlled and voltage-controlled current sources in the present example.

Heat conduction model for nanowire applications

A heat conduction model for determining the temperature of a nanowire through which a current is passed is proposed. In the model, the total Joule heating arising in the nanowire due to the current is considered until the system reaches a steady state. It is important to determine the positions of the “cold points” in the system, where the temperature is left unchanged by the current. The current required to cut a nanowire is predicted from the model. The mechanism for Joule heat welding is classified depending on the positions of the cold points in the metallic nanowire system.