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The purpose of this paper is to study high-voltage interactions in polymer thick-film resistors, namely, polyvinyl chloride (PVC)-graphite thick-film resistors, and their applications in universal trimming of these resistors.

The authors applied high voltages in the form of pulses and impulses of various pulse durations and with different amplitudes to polymer thick-film resistors and observed the variation of resistance of these resistors with high voltages.

The paper finds that high voltages can be used for trimming of polymer thick-film resistors in both directions, i.e. upwards and downwards.

The research implication of this paper is that polymer thick-film resistors can be trimmed downwards or upwards practically using this method.

The practical implications of this paper is that one can trim the polymer thick-film resistors, namely, PVC–graphite thick-film resistors, in both directions, i.e. upwards and downwards, by using this method.

The value of the paper is in showing that high voltages can be used to trim downwards and also upwards in the case of polymer thick-film resistors. This type of trimming is called universal trimming, developed first time for polymer thick-film resistors.

An investigation of laser‐trimmed thick‐film resistors’ current noise was carried out. A large number of samples from two pastes with different P‐ and L‐cuts were prepared. The noise spectrum of the trimmed resistors was measured to obtain a relationship between the current noise and the cut parameters. A simple but comprehensive electrical model for the simulation of the current noise of trimmed resistors is also presented. The results from the noise simulation agree with the measured data.

Fired resistors exhibit variations which are minimised by abrasive and laser trimming. The latter may cause unstable behaviour which is further aggravated by thermal shock. The chemical structure of a thick film resistor is analysed with respect to mechanical stress, and the theoretical conclusion that the coefficient of thermal expansion of the resistor should be equal to or smaller than that of the substrate is verified experimentally. The thermal behaviour of ruthenium dioxide is examined and a range of CTE values are determined for materials of varying chemical composition. The relationship between CTE and post laser trimming stability is demonstrated on four thick film resistors which differ in thermal expansion. It is pointed out that formulations with high metallic content can absorb tensile stress by elastic deformation, thus minimising the formation or propagation of laser induced cracks.

This paper deals with the investigations carried out on the variation of current noise (1/f noise) in polymer thick film resistors, when they are subjected to pulse voltage trimming. The current noise is measured in terms of noise index (micro volts of noise per volt of DC applied, in a decade of frequency) using Noise Meter Model 315 C manufactured by Quan‐Tech of New Jersey. It has been found that current noise decreases as the resistors are trimmed to lower values. An attempt has been made to explain the decrease in current noise with the trimming of resistors.

An investigation involving the use of thermal imaging has been undertaken, both on trimmed thick film resistors and on wave soldered joints to surface mounted devices. The surface mounted devices were ‘zero‐ohm’ jumpers which in themselves did not generate appreciable heat if current was passed through them, but also at the same time allowed the passage of DC current flow through the soldered joints. Thermal imaging results have shown the degree of current bunching obtained for thick film resistors trimmed using air abrasion. Appreciable changes in heating and hence in current density are not observed until the resistor has been trimmed with a cut extending into approximately 50% of the total width. In the case of soldered joints on surface mounted devices, it was found that thermal imaging techniques could not easily detect appreciable temperature changes and hence the current flow in joints which had previously been subject to stress due to one test or another. As a result, measurements were made on artificially damaged joints in order to determine the degree of damage that was needed for thermal imaging to show any noticeable effect. The joints were cut with a miniature PCB saw (1 mm wide) and it was not until the cut extended through approximately 60% of the soldered joint height that any appreciable thermal effects could be observed. Conclusions are drawn from these observations on the applicability of thermal imaging techniques to the assessment of trimmed resistors and soldered joint analysis.

Diffusion patterning is a dielectric patterning technology, which is used in the screen printed thick film technology for higher density multilayer circuits. This technology is suitable for producing lower cost multichip modules and requires a low additional investment in conventional thick film technology production lines. Comparisons of via resolution capability of diffusion patterning versus conventional thick film technology are described and discussed. Preliminary experimental results obtained with a test circuit showed that 200μm lines and 200μm vias could be achieved with acceptable yield and with minimal modification to standard production lines. The electronic circuit for the pressure sensor was designed and realised with the verified technology as a low‐cost ceramic multichip module. A few results of an investigation of some thick film materials, which comprise the “set” of pastes for diffusion patterning technology, are presented.

The results of a characterisation carried out on thick film resistors (TFRs) with dimensions reduced below the usual limits are reported in this paper. The test vehicle was a purpose‐designed test pattern with resistors whose dimensions reached a limit of 0·3 mm in length and 0·4 mm in width. The proposed aim of the work was to look for dimensional limits where TFRs could still give acceptable performances, though, if possible, keeping unchanged the materials system and the process conditions which are used in the authors' thick film hybrids facility.

The effect of high voltage pulses on the resistance of polymer thick film resistors (PTFR) is studied. It is found that the resistance decreases with both the amplitude and duration of the high voltage pulses and with the number of pulses. The change in resistance is attributed to the dielectrophoretic motion of graphite granules in the PVC medium when high voltage pulses are applied to PTFR. A model has been proposed through which the percentage change in resistance can be estimated in terms of the number of pulses, duration of pulses etc. Based on these investigations, we suggest a downward trimming method for PTFR through the application of high voltage pulses. This method of trimming is a clean process and trimmed resistors are free from hot spots.

A finite difference method is presented to calculate the potential distribution in a trimmed resistor. The numerical results are used to study the influence of trimming on the power density distribution and the noise performance.

Thermal ageing experiments on various thick film resistor systems have shown that resistance change is caused by a number of different mechanisms with different time dependences. Three distinct types of behaviour have been identified: corrosion due to ambient attack; diffusion through resistor interfaces with conductor terminations or through the resistor top surface; and stress relief within the bulk of the resistor. Wherever possible the dominant mechanism has been identified and the activation energy and time dependence of the ageing process have been determined.