Cheng-Chi Tai

Thickness and Conductivity of Metallic Layers from Pulsed Eddy Current Measurements

Coatings and surface treatments find a wide range of technological applications; they can provide wear resistance, oxidation and corrosion protection, electrical contact or isolation and thermal insulation. Consequently, the ability to determine the thickness of coated metals is important for both process control and in-service inspection of parts. Presently ultrasonic, thermal, and eddy current inspection methods are used, depending on the circumstances. A number of commercial instruments for determining the thickness of nonconducting coatings on metal substrates are based on the fact that the impedance change of the coil decreases exponentially with the distance of the coil from the metal (the lift-off effect). However, these instruments are not suitable for determining the thickness of metal layers on conducting substrates.

Pulsed eddy-current measurement of a conducting coating on a magnetic metal plate

A new measurement method has been developed using the pulsed eddy-current (PEC) technique to determine the thickness or conductivity of metallic coatings on a metal substrate for the case when either the coating or substrate is magnetic. We demonstrate this technique for nickel layers (25-200 µm) over copper substrates, copper layers (25-200 µm) over nickel substrates and zinc layers (50-400 µm) over magnetic steel substrates. The theoretical current difference was calculated using the method developed by Tai et al (Tai C C, Rose J H and Moulder J C 1996 Rev. Sci. Instrum. 67 3965-72). All current difference measurements were recorded with a PEC instrument. The determination of coating thickness or conductivity of the metals is based on the comparison of the data taken with air-core coils and theoretical calculation using closed-form solutions. The physical phenomena of the PECs interactions with the coated magnetic metals are also discussed.

Characterization of coatings on magnetic metal using the swept-frequency eddy current method

A measurement technique using the swept-frequency eddy current (SFEC) method for determining the thickness, conductivity, and permeability of metallic coatings on metal substrates for the case when either coating, substrate, or both are magnetic was developed. This technique involved using the empirically determined permeability of the metals as input to the model calculation. This technique is demonstrated for nickel layers (25–200 μm) over copper substrates, copper layers (25–200 μm) over nickel substrates, and zinc layers (50–400 μm) over steel substrates. The electrical impedance was measured for air-core coils in the presence and absence of the layer using a 4194A Hewlett–Packard impedance analyzer. An analytic closed-form solution for calculating the impedance of a cylindrical air-core probe over a layered metallic half-space is presented. The determination of coating thickness and conductivity and permeability of the metals is based on the comparison of the data taken with air-core coils and theore...

Nondestructive analysis of interconnection in two-die BGA using TDR

Nondestructive analysis (NDA) is one of the most important tasks that is performed during the industrial characterization of integrated circuits (ICs) because even a tiny defect or failure in the IC packages could be disastrous from the standpoint of quality control. To detect an interconnection failure in IC packages, a time-domain reflectometry (TDR) analysis system was developed. An open-end fixture (OEF) was employed to detect the rapid rise of edge signals from the package and to monitor them under the two parameters of time interval and reflection voltage. We developed a simple and effective electrical NDA system based on the TDR technology that can evaluate the interconnection of ball grid array (BGA) packages. The TDR-measurement results can determine both the failure location and type based on the aforementioned parameters for a two-die BGA package.

A correlated empirical mode decomposition method for partial discharge signal denoising

Empirical mode decomposition (EMD) is a signal processing method used to extract intrinsic mode functions (IMFs) from a complicated signal. For a measurement with two or more correlated inputs, finding and capturing the correlated IMFs is a critical challenge that must be confronted. In this paper, a new correlated EMD method is proposed. The cross-correlation method was employed to determine dependence between the IMFs. To verify feasibility, an analysis was performed on simulated test signals and practically measured partial discharge (PD) signals collected from several acoustic emission sensors. At the surface of the gas-insulated transmission line, the PD signal arrived at the AE sensors with varying time delays and unique mechanism vibrations. Following an abnormal detection using the standard-deviation variation, the PD signal and the background signal of each sensor were applied using the correlated-EMD method. A twice correlated-EMD calculation was applied to the signals for the purpose of noise elimination. In addition, the unwanted low-frequency IMFs induced from the EMD calculations were excluded. The experimental results reveal that the correlated-EMD method performs well on both selecting and denoising the correlated IMFs. The results further provide analysis on correlated-input applications with a precise signal completely induced from the disturbance.

Inductance of a coil on a thick ferromagnetic metal plate

We study the frequency-dependent inductance of a small air-cored coil of wire placed flat upon various ferromagnetic metal plates. The change in the complex inductance of the coil, measured with an HP 4193A impedance analyzer, is reported for frequencies between 1 kHz and 1 MHz. The metal plates consisted of commercially pure (99.7% and 99.9%) Ni, commercially pure (99.9%) Fe, and a suite of medium carbon steels. For the steel plates, inductance changes were consistent with a simple half-space model that treats the metal as a continuum defined by a conductivity /spl sigma/ and a relative initial-permeability /spl mu/ where these material parameters are isotropic, local, and uniform throughout the plate. The inductance changes for Ni and Fe could not be fit to the half-space model for any values of /spl sigma/ and /spl mu/, but were consistent with a model that assumes a thin (/spl sim/10 /spl mu/m) surface layer with a significantly reduced permeability-a dead layer. We tested the existence of the hypothetical dead layer in several ways. We found that the inductance increased when the surface was chemically etched (presumably eroding the dead layer) and decreased when the surface was mechanically polished (presumably increasing the dead layer). We also found that the inductance of the Fe and Ni samples decreased substantially over the course of days and months when exposed to air.

Electrical characterization of BGA test socket for high-speed applications

This paper discusses the electrical characterization of a high-density and high-speed BGA (ball grid array) socket. The socket is built from copper-clad pogo pins and pinned in a 32/spl times/32 mm array with 400 pins on a 1.27 mm pitch. It provides power/ground array, and signal pins designed to offer 348 signal lines. The socket-base dielectric has a dielectric constant of about 4.2. The BGA test socket discussed in the paper is being targeted for high-speed testing fixture applications and their pins are closely spaced. Extensive TDR (time domain reflectometry) measurements and IPA510 simulations have been used to extract their significant electrical properties such as differential skews, characteristic impedances, reflection characteristics, and crosstalk. The bandwidth of the BGA socket from the 3 dB return loss criteria was calculated to be around 5 GHz for time domain measurements.

The information home appliance control system - a Bluetooth universal type remote controller

Along with the advancement of technology and the evolution of the information home appliances, such as TVs, air conditioners, sound and stereo systems, wireless phones, and so forth, the remote controllers dazzling our eyes still have not reached a unified model or style. In view of this fact, we propose an environment controller combining the Bluetooth technology. Along with Bluetooth transmission via radio wave, we can break through those space barriers existing in our housing structure comprising multiple separating objects. The system design is as follows: microcontroller and Bluetooth module. The receiving end can comprise multiple systems, including TVs, phones, air conditioners, sound systems, etc.

Modeling the surface condition of ferromagnetic metal by the swept-frequency eddy current method

Many ferromagnetic metals have, either by design or by happenstance, a very thin surface layer of reduced magnetic permeability. Such a layer might occur intrinsically or it might be induced by surface treatments such as case hardening, shot peening, or surface alloying. We studied the frequency-dependent impedance of a small air-cored coil of wire placed flat upon ferromagnetic metal plates. The impedance changes for nickel and iron could not be fitted to the half-space model for any values of conductivity and permeability. It is likely that the permeability varies continuously with depth in the near-surface region. Hence, it is possible that a multiple-layer model would yield a better representation of the data with a layer depth closer to that observed. In this paper, we illustrate that the multiple-layer model improves the agreement between the measured data and the approximate analytic solution. The results show that the permeability of pure nickel and pure iron decay exponentially from the base to surface.

Finite Element Method Simulation of Photoinductive Imaging for Cracks

In this paper, the numerical simulations of photoinductive imaging (PI) method have been performed using the finite element method (FEM) with the 2D transient to characterize corner cracks at the edge of a bolt hole. The PI imaging results have higher spatial resolution in the area of the defect in 2D models as compared with the conventional eddy current (EC) images. The FEM simulation results of 0.5-mm rectangular defects are showed and analyzed. The dependencies of PI signals on EC frequencies and temperature of the thermal spot are also examined. The results demonstrate that the PI method is applicable to examine the geometric shape of corner cracks.