Roland Reicher

Designing the performance of a thick-film laser power detector by means of a heat-transfer analysis using finite-element method

Abstract Besides the usual application of thick-film technology to produce circuits of high reliability, this technology is also attractive for the fabrication of sensors. A laser power detector has been realized utilizing the thick-film technique. A special detector configuration provides an exact laser power measurement independent of the position of beam impingement on the detector area and independent of the intensity distribution of the laser beam. The detector, which actually operates as a heat-flow meter, consists of a thermopile circularly applied on an AlN substrate. The geometrical proportions of the detector have been accommodated to an NdYAG laser with a given specification by means of thermal heat-transfer analysis using the finite-element method (FEM). Based on the FEM simulation of steady state and transient temperature distributions on the detector area induced by laser beam impingement, the performance and the operating range of the detector have been specified.

A wireless SAW-based temperature sensor for harsh environment

A SAW-based temperature sensor system is presented. Much effort was placed in the development of a high temperature resistive transponder compromising of both a SAW device and an antenna stable up to 400 /spl deg/C for at least a few days. The system solution also includes a low-cost reader unit employing the FSCW (frequency stepped continuous wave) principle operating in the ISM band from 2.4 GHz to 2.4835 GHz. Spare space on the SAW device has been employed also to implement an ID for identification purpose. Hence, this solution also allows for tagging of objects exposed to harsh environments.

A new measuring method to determine material spectral emissivity

Emissivity is a measure of how well a real surface can radiate energy as compared with a blackbody. This characteristic radiative number is usually determined by means of optical pyrometry. By contrast an indirect measurement method has been developed which enables the determination of the normal spectral emissivity of various materials at a specific wavelength. A heat flow induced in a test body by the absorbed irradiation of a laser beam may be correlated with the spectral emissivity of its surface. The theory of the measuring principle is discussed and the feasibility of the method evaluated by means of practical experiments utilizing a thermopile built up using a thick film technique.

Development of Glass Frit Free Metallization Systems for AIN

Currently only glass bonding thick film conductor systems are commercially available for metallizing AlN-ceramic. The glass phase formed between metallization and ceramic impairs the high thermal conductivity of AlN. A new glass frit free metallization system has been developed utilizing the bonding mechanism of active brazing to provide the adhesion of metallization onto the ceramic. Aspects of paste preparation range from the derivation of the metallic powder to the selection of an appropriate printing vehicle which must decompose completely during the firing process under an inert atmosphere. The adhesion strength of the new paste system with the alternative bonding mechanism has been evaluated and contrasted with that of standard thick film pastes.

Ceramic patch antenna for high temperature applications

For a high-temperature stable SAW device, serving as both an RFID-tag and a temperature sensor, a hermetic case has to be provided to protect the SAW device from moisture, contaminations and harsh environments. Furthermore, for wireless readout a high-temperature stable antenna has to be connected to the SAW device. In order to fulfil both purposes adequately an alumina based ceramic package was designed accordingly and successfully tested up to 400degC

A fritless copper conductor system for power electronic applications

Abstract Copper coated aluminium nitride (AlN) ceramic is a preferred substrate material utilized for high power, high frequency electronic applications. The use of metallized ceramic requires bonding of the metallization onto the substrate. AlN, however, is chemically inert to Cu. At present time the direct copper bonding (DCB) technology is used to manufacture such structures. In this project, for reasons of simplicity and economy and as alternative to the standard DCB-technology, the metallization on the AlN ceramic is built up by active metal brazing of a screen printable AgCuTi thick film ink. The mean advantage of this technology is, that the metal/ceramic braze compound can be processed in a conventional belt furnace at inert firing atmosphere usually available at hybrid microelectronics industry. Aspects of paste preparation ranging from the derivation of the metallic powder to the assortment of an appropriate organic screen-printing medium were investigated. The physical properties of the phases occurring at the interface between the TiCuAg metallization and AlN ceramic and their contribution to the thermal conductivity and the thermomechanical performance of the metallized substrate have been characterized theoretically as well as by experiment and compared with different commercial glass frit containing copper thick film metallization systems.

Improving reliability of thick film initiators for automotive applications based on FE-analyses

Abstract Finite element method (FEM) has been proved as a useful tool to design thick film initiators for automotive safety unit applications as well as a method of rapid prototyping of such devices. Besides, it contributes to improve the reliability of the system since potential reasons of failure will be highlighted and as a result suitable remedies can be applied. Parameters like the thermal characteristics of the pyrotechnics, pore inclusions in the explosive, physical properties of the involved thick film materials as well as diffusion effects occurring at the termination ends of the initiator element have to be taken into account during the design process with regard to reliability aspects. Practical ignition tests have been conducted with thick film initiator elements and an explosive composed of Sb/KMnO 4 in the order to prove the efficiency of numerical simulation for the design process.

Preventing failure of soda lime cover glasses by design optimization

Abstract In order to quantify the requirements of headlamp cover glasses with regard to impact and thermal shock resistance, special tests have been devised by the automobile industry. The development of cracks and flaws which are responsible for the mechanical strength of glass is governed by the specific processing conditions. It is therefore difficult to predict the mechanical performance of a glass on the basis of a theoretical model. Based on a modified Hertizian theory a numerical analysis has been carried out to optimize the resistance to impact of the glass. The actual temperature distribution on a reference cover glass of the desired final shape is taken as the boundary condition for the computer simulation of the thermal shock resistance. The results of the numerical analysis are compared with results achieved by experimental tests.

Thick film initiator elements - an alternative to resistive wire initiators for automotive safety units

Safety units in cars such as airbags or seat belt pretensioners protecting passengers of cars against risks of injury in the case of an accident are equipped with detonators. They are typically activated by electro-explosive devices where resistive wire elements are generally considered as well established initiator systems. Crash sensors in cars induce, in the case of an accident, a current pulse in this resistive initiator element. As the device is heated up to a critical temperature the explosive detonates and consequently the airbag or the retention system will be released. Thick film technology has been evaluated for the realisation of appropriate resistive ignition elements as an alternative to the conventional wire elements, which are considered to be capable of meeting future requirements of the automotive industry. The performance of thick film initiator elements is evaluated on the base of numerical simulation utilizing finite element method.

Thick‐film strain gauges on alumina, zirconia and steel substrates

Strain gauges can be realised by printing and firing thick‐film resistors on ceramic substrates that are usually based on alumina. However, sensing elements made on some other substrates – tetragonal zirconia or stainless steel – would exhibit some improved characteristics, either due to a lower modulus of elasticity or a higher mechanical strength. As thick‐film resistors are developed for firing on alumina substrates their compatibility and possible interactions with other kinds of substrates have to be evaluated. The sheet resistivities and noise indices of the resistors were comparable, whereas the gauge factors were lower for the dielectric‐on‐steel substrates. The temperature coefficients of resistivity (TCR) of the resistors on the ZrO2 and dielectric‐on‐steel substrates were higher than the TCRs on the alumina substrates, which was attributed to the higher thermal expansion coefficient of the tetragonal zirconia and the stainless steel.