M. Mundlein

Modeling of particle arrangement in an isotropically conductive adhesive joint

An isotropically conductive adhesive (ICA) is a composite material consisting of a nonconductive polymer binder and conductive filler particles. When the filler content is high enough the nonconductive binder is transformed into a good electrical conductor. This transition can be described by the percolation theory. We present a two-dimensional model to analyze the principal influences of the geometrical and electrical properties of the filler particles on the percolation threshold and the electrical resistance of an ICA joint. With this model, the arrangement of the particles within the joint is calculated by considering different types of forces. Taking into account the electrical properties of the particles, the electrical contact behavior is investigated. The goal of this study is to provide a deeper understanding of the changes of the macroscopic contact behavior due to different environmental impacts.

Electrical resistance modeling of isotropically conductive adhesive joints

An isotropically conductive adhesive (ICA) is a composite material consisting of a nonconductive polymer binder and conductive filler particles. When the filler content is high enough the nonconductive binder is transformed into a good electrical conductor. A 2D model is presented to analyze the principle influences of the geometrical and electrical properties of the filler particles on the probability of interconnection and the electrical resistance of an ICA joint. With this model the arrangement of the particles within the joint is calculated by considering different types of mechanical forces. Taking into account the electrical properties of the particles, the electrical contact behavior is investigated

Aspects concerning the optimization of a calorimetric flow sensor built up in thick-film technology

A calorimetric flow sensor for ultra-low velocities built up in a thick film on tube technology is presented. It comprises a heater and temperature sensors which are arranged symmetrically upstream and downstream with respect to the heater onto an insulated steel tube. The resulting temperature difference is acting as signal output. The performance of the sensor is characterized experimentally as well as by finite element analysis. Special attention has been spent on the position of the temperature sensors along the tube with regard to signal output resolution. Finite element analysis has been also applied to determine design procedures in order to increase the linear range of sensor characteristic.

Simulation of the aging behavior of isotropic conductive adhesives

Although, in numerous studies the changes of contact resistance during accelerated aging under elevated temperature and humidity were observed there is still a lack of understanding of degradation mechanisms. Some models illustrate connections between the contact behavior and parameters of the conductive filler. In addition to these considerations in our paper we describe the macroscopic behavior by considering the microscopic resistance change between the conductive particles themselves and between particles and contact pads. For this purpose, we calculated the electrical contact resistance with a two dimensional numeric simulation. Herein the conductive particles are modeled by randomly distributed ellipses placed between two parallel electrodes comparable to silver flakes as known from metallographic cross sections of our contact samples. The voltage and current distribution and the contact resistance as well are calculated by transforming this model into a resistor network. During a forced aging process different opposite effects occur in an adhesive joint. One effect is the increase of entire resistance due to a degradation of the interparticle contact. Another effect is the decrease of resistance caused by a post curing of the resin during aging of the adhesive joint. This behavior was experimentally determined in a previous investigation. In accordance to these effects we simulate the aging process of the joint by introducing different time dependencies of the interparticle resistance. The goal of this study is to provide a deeper understanding of the changes of the macroscopic contact behavior due to different environmental impacts.

Packaging concept for a miniaturized wirelessly interrogable temperature logger

We present the packaging concept of a miniaturized, biocompatible temperature logger. In a first application, the device will be cast in the palate piece of a removable brace to observe the wearing habit of the patient. For this purpose the temperature is measured with a pn junction and recorded using a micro controller which acts as a temperature logger and is capable of storing the thermal history from several months. This device is hermetically sealed using polymeric encapsulants. The data exchange is carried out using a wireless interrogating transponder using radio frequency identification device (RFID) technology. The function principle of this data transfer technology is explained briefly. Due to the limited space in the brace, an extremely high level of miniaturization is necessary. To reach this goal, the device is built-up as a multichip module using a single sided FR4 printed circuit board with a line width of down to 35 /spl mu/m. The entire prototype fabrication process, including details on miniaturization related difficulties, is described. Also, the paper describes the finding and evaluation of a biocompatible casting material as needed for a medical application. An easy way for the direct assessment of water absorption and its effect on the electrical function of the module is also presented.

Packaging of a thin-film sensor for transepidermal water loss measurements

The water content in the superficial part of the human skin is one index to evaluate the skin health. In the past we have developed a packaging concept which brings a sensor chip carrying a comb-shaped electrode system into intimate contact with the skin. The electrical impedance between these electrodes can be used as a measure for the water content of the skin. However, we have recently found that the same sensor chip used with a slightly altered packaging concept can measure the transepidermal water loss (TEWL), which is another indicator for the skin health. In our paper we report on the interconnection and packaging technique of this advanced measuring system for the evaluation of human skin. Furthermore, we present some first measurement results. The basic structure of the sensor consists of a thin film interdigital electrode system deposited on a ceramic substrate by rf-sputtering. Thin wires are guided through funnel-shaped holes arranged in the center of the contact pads. A CO/sub 2/ laser with a wavelength in the medium infrared range is used to produce these holes. The wires are bonded to the contact metallizations by using conductive adhesives. In the case of TEWL measurements the sensor chip does not touch the skin. The chip is placed in a distance of a few millimeters away from the skin. The electrical impedance of the electrode system depends on the stream of water molecules emitted from the skin. This kind of measurement does not suffer from pollution of the sensor chip by the oily substances of the skin surface. Therefore, the results are more reproducible compared to classical skin impedance measurements.

Thermal Analysis of Multilayer Printed Circuit Boards with Embedded Carbon Black-Polymer Resistors

The pressure towards increasing miniaturization and interconnection density forces the printed circuit board (PCB) manufacturers to integrate electronic components into multilayer PCBs. Goal of this work was a thorough thermal investigation of the thermal behavior of embedded carbon black-polymer resistors with special attention on dimensions and sheet resistance values relevant for the industrial application in multilayer PCBs. For this purpose demonstrator boards with embedded resistors were fabricated. Two independent temperature measuring methods were applied: calibrated embedded resistors used as miniaturized temperature sensors and thermography. The calibration process including an accelerated aging process in order to obtain stable temperature/resistance characteristics is crucial and described in detail. Due to the small dimensions of the resistors (width in the 100 mum range) it was decisive to consider the anisotropic nature of the glass fiber reinforced epoxy resin laminate. Although this kind of material is used for PCB manufacturing since decades no reliable data were available to describe its thermal behavior sufficiently accurate. For this reason we established the thermal conductivity of the applied substrate materials independently in the lateral direction by reverse modeling and in the perpendicular direction in a separate measurement. Using these data the thermal performance of embedded resistors and the influence of design parameters of the multilayer is investigated by thermal simulation. Comparisons of calculated and measured temperatures showed an outstanding agreement.

Partial discharge current measurement in high permittivity dielectrics and their meaning for quality control

Dielectric breakdown is one of the major problems of dielectrics with high permittivity. There are several intrinsic factors affecting breakdown strength of BaTiO/sub 3/ such as porosity, grain boundaries, and domain wall instability. But in general, the dielectric breakdown is caused by partial discharge within pores and cavities in the insulating ceramic body. The permittivity in these enclosures is usually much lower than the one of the BaTiO/sub 3/ matrix. Therefore, under operation of the ceramic capacitor the voltage across the pores may exceed the gas breakdown threshold and causes partial discharge in the pores according to the Paschens law. Although, the momentary effect of these discharges typically caused by currents in the microampere regime is little, their meaning for the long-term reliability of the component is crucial. In this paper we describe a new experimental set-up for determination of partial discharge phenomena in BaTiO/sub 3/ capacitors. The partial discharge current is monitored using a bridge circuit containing two branches with the same nonlinearity. In this way the basic harmonics of the bias current can be suppressed and the sensitivity of the interesting current peaks due to partial discharge is increased. Using this set-up we demonstrate the impact of mechanical defects on the partial discharge inception field strength. Moreover, the same capacitors were analyzed by means of metallographic cross sections. The ceramic-structure and fractures in the dielectric body due to unfavorable process parameters during manufacturing are analyzed. As a surprising result we show that capacitors even with significant fractures do not necessarily exhibit partial discharge even at an electrical field beyond 2 kV/mm. For this reason we conclude that conventionally applied electrical tests are insufficient for quality control.

Electrically Conductive Adhesives

Electrically conductive adhesives are being used in electronic packaging for several decades. A brief review of the dynamic development of conductive adhesives under the influence of the miniaturization, the adaptation of environmental friendly manufacturing processes is presented.

Evaluation of a novel transepidermal water loss sensor

Recently we have developed a novel microsensor to measure the transepidermal water loss (TEWL) of human skin. The sensor is based on an interdigital electrode system covered with a highly hygroscopic salt film. In this paper we present a new method to evaluate and calibrate the sensor device. Different TEWL values are imitated by a variable humidity source. We demonstrate the very high sensitivity of our TEWL sensor. The output signal represented by the conductance of the sensor varies over six orders of magnitude in the relevant TEWL range between 2 and 60 g/(m/sup 2/h). Our precise calibration method is very important for the reliable medical diagnosis of skin diseases.