K. Pitt

A simple method for accurate loss tangent measurement of dielectrics using a microwave resonant cavity

A simple yet rigorous method has been developed to enable the loss tangent of dielectrics, having a known relative permittivity, to be accurately measured using a waveguide resonant cavity. The novel method eliminates the need for any physical measurement, either on the cavity or dielectric sample under test. The only electrical parameters that need to be measured are resonant frequencies and Q-factors of a reference cavity and those of the same cavity loaded with the dielectric sample. One of the advantages of the new technique is that dielectrics, of arbitrary shape, can be characterized at very high microwave frequencies. The new method has been verified through measurement over X-band.

Chemical constitution and conduction mechanisms in thick film resistors

This paper re-examines the data produced in a study of the behaviour of thick film resistors on dielectrics. It uses the published data to investigate the physicochemical nature of the conduction in six combinations of resistors on dielectrics and alumina. The results appear to confirm earlier observations that the presence of barium oxide in thick film resistor glass matrices has a major impact on the conduction mechanism. It is suggested that the cause is the totally different nature of two ions of similar size, possibly giving rise to localized regions of barium-lead compounds.

The influence of materials properties on the design of MCM and microstrip structures

This paper examines the influence of the materials on the electrical properties of conductors for both high speed MCM structures and microstrip applications. Calculated figures for attenuation in microstrip lines are compared with published results. It is seen that the nature and physical structure as well as the geometrical shape of a conductor track all strongly influence the microwave performance. However, it is also clear that real materials have been developed for these applications which closely approach the behavior predicted for ideal conductors. It is proposed that microwave performance can be predicted by modelling the d.c. conductor materials and extrapolating to high frequencies.

A method for the prediction of microwave properties of thick film conductors from physical measurements and d.c. conductivity

It is theoretically possible to calculate electrical losses in conductors at high frequencies using standard microstrip equations. To do this it is necessary to know the length, width and surface roughness of sample tracks. The sheet resistance at d.c. is replaced with a new term, surface resistance which is the bulk resistivity divided by the skin depth at the frequency under consideration. The paper looks at the relationship between these predictions and measured results. A wide range of commercial thick film conductors has had its physical properties measured and a comparison made with the figures for idealized structures. The paper contains a discussion on the differences between real and ideal materials. It was seen that the predictions were close to the measured results at lower frequencies, but that there was divergence as the frequency approached 15–20 GHz. For low-cost applications it is generally considered that economically viable conductors will be unsuitable for microwave applications. However, the paper discusses whether some of the lesser used conductors, such as fritted silver, could have an application in low-cost microwave devices, for example alarm systems.

Influence of the contacts and firing process on the properties of thick film resistors on alumina and dielectrics

This paper describes the behaviour of thick film ruthenium resistors when printed and fired onto standard 96% alumina substrates and onto alumina substrates covered with a dielectric film during long-term thermal ageing. It was observed that the behaviour of the resistors on alumina and dielectric depends upon the firing process of the resistors and the kind of terminations. An analysis of the effects of thermal ageing was made as well as some SEM observations of diffusion processes between the resistor and its terminations.

Simulated thermal cycling of surface components on FR4 printed wiring boards using standard tin-lead solder

Thermal cycling is used as a diagnostic test for electronic equipment. It is particularly severe on solder joints that are under both compressive and tensile strain during a cycle as a result of differential thermal expansion. The electronics industry document IPC-SM-785 addresses this problem and provides means whereby joint failure can be predicted as a function of the type and number of cycles used. The method can be applied to both non-compliant ceramic chips and compliant packages on FR4 board. This paper describes the applicability of SM-785 for both compliant and non-compliant packages. Due to the change in metallurgy of solder at temperatures approaching −20 °C, the approach is restricted to temperatures where near eutectic tin–lead solder is in its non-elastic form. A new approach will be needed when lead-free joining becomes mandatory.

A ROBUST LOW COST THICK FILM SYSTEM FOR THE CONSUMER MARKET

This paper describes the fabrication and test of a new low cost thick film system using base metals. The resistor pastes were developed using non-precious metal oxides and contain molybdenum trioxide and other additives. The conductors are either silver or nickel. The dielectrics contain barium oxide, but, unlike ruthenium based resistors, on related dielectrics, there is very little difference in resistor behavior on alumina from that on the dielectrics investigated. Long term stability tests show that the new resistor family, while less stable than those based on ruthenium, is suitable for many low cost applications. Results suggest that high temperature ageing tends to minimize contact effects and that they are compatible with both silver and nickel terminations.