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The resistivity of cured conductive ink films are dependent on a wide range of process parameters. An early indication of the resistivity that is likely to result following curing can enable these parameters to be optimised and, therefore, improve product quality. This paper aims to report on the use of alternating current (AC) impedance measurement techniques on curing printed ink films as a means of assessing the resistivity likely to be attained following the curing process.

Impedance measurements (100 Hz-10 MHz) were performed on curing conductive carbon ink films printed on polyethylene terephthalate substrates during convective heat curing. A jig was designed to incorporate the test structure in an convection oven such that the effect of cure on the structure impedance could be investigated.

The initial impedance was found to decrease with an increase in the measurement frequency. As the ink films were cured, the impedance magnitude across the 100 Hz-10 MHz range converged with the direct current (DC) resistance value. For a given ink, the ratio of initial AC impedance at 10 MHz to final cured resistance was found to be consistent, thus giving a method where final conductivity can be estimated before cure.

Data from printed ink resistance measurements are required to ensure the optimal conductivity of printed devices. However, after the printed structures are fabricated and cured, it is too late to optimise process parameters, leading to significant wastage. AC impedance measurement can give an indication of the final cured resistivity whilst the structure is freshly printed and still in its curing phase, enabling the printing process parameters to be adjusted to improve the resistivity of subsequently printed devices. Measuring AC impedance of printed ink structures in a production environment can, therefore, improve output.

As the transition away from lead‐containing solders gathers momentum, isotropic conducting adhesives (ICAs) are being considered as possible replacements for conventional SnPb solder in a range of applications. Consequently, the reliability of ICA joints is under scrutiny. The purpose of this paper is to report the effect of printed circuit board (PCB) and component finishes on the reliability of ICA joints.

Previous work by the authors identified a suitable test regime to generate relevant reliability data. In the present work, those tests are employed to investigate whether the finishes on the components and/or PCBs have any effect on the reliability of the ICA joints after exposure to damp heat conditions.

The effect of different finishes is found to be very adhesive material dependent. Two adhesives are studied, and for one material the joint reliability is relatively unaffected by changes in component or PCB surface finish. However, for the second material, and components with a high‐tin content‐plated finish, the joints display a less stable resistance. The surface finish on the PCB is found to have a smaller effect on joint reliability than the component finish, with results dependent on adhesive material type. Performance with one material exhibited little difference in reliability irrespective of the PCB surface finish. For the second material, the joint reliability performance with components having the electroless nickel/immersion gold finish, is not as good as that with components having the immersion tin or silver finishes.

The paper shows that surface finish is an important factor in determining the conductivity of ICA joints during exposure to the 85°C/85%RH environment. Systems containing tin are more prone to lose conductivity and, conversely, noble metal systems are more immune to degradation. This is a major concern as the industry is showing many signs that the component termination of choice will be pure tin.

Enclosed print heads have recently been developed as an improvement on the traditional squeegee methods for solder paste printing. They offer the opportunity of widening the printing process window and reducing process waste. Consequently, this work was undertaken to evaluate some aspects of enclosed print head printing, and it has been shown to be a robust process. A number of performance factors were established: with increased humidity the paste degradation was limited due to its sealed paste reservoir; the system also permitted successful intermittent printing over a 5 day period; printing is much more tolerant to distorted substrates than some squeegee blades, and hence improves printing on non‐planar surfaces; significant reduction in paste wastage occurs, since paste ageing is reduced.

The purpose of this paper is to present work on the incorporation of capacitors into printed circuit boards (PCB) as a method to measure moisture content and follow moisture diffusion under ground planes.

PCBs were manufactured of FR‐4 incorporating different arrangements and sizes of capacitors formed between the tracks on adjacent layers of the PCB. The boards were placed in an 85°C and 85 per cent relative humidity (RH) environment to absorb moisture before baking at temperatures of 80, 110 or 125°C with the capacitance periodically measured. The effect of ground planes with different densities of plated and non‐plated through holes (PTH) has been studied by placing capacitors between ground planes.

Parallel plate capacitors embedded within a PCB showed a 10 per cent capacitance increase going from a dry state to being saturated with moisture in an 85°C and 85 per cent RH environment. The slow ingression of moisture under the capacitance planes meant that the measured capacitance change did not reflect the moisture content of the remainder of the board well. Capacitor plates with slots for the moisture to penetrate were also investigated, with the increase in capacitance found to show good correlation with the increase in board mass. In investigating moisture under ground planes, either by decreasing the hole density or by plating the holes, the time for moisture to diffuse out of the board was found to increase due to the lower exposed area on the PCB.

The paper illustrates a method that can be applied to PCB manufacturing to assess the moisture content of a board prior to reflow.

The purpose of this work is to undertake a comparison of accelerated test regimes for assessing the reliability of solder joints, in particular those made using lead‐free solders.

Identical samples of 1206, 0805 and 0603 resistors were subjected to six different cycling regimes to investigate the effect of thermal excursions, ramp rates and temperature dwells.

The most damage to joints was found to be caused by thermal cycling between −55 and 125°C, with a 10°C/min ramp rate and 5 min dwells. Large thermal excursions were shown to give faster results without compromising the failure mode.

Similar degrees of damage in the lead‐free solder joints were experienced from thermal shock regimes with ramp rates in excess of 50°C/min. However, these regimes, although faster to undertake, appeared to cause different crack propagation modes than observed with the thermal cycling regimes. However, these differences may be small and thermal shock testing may still be used to differentiate between, or enable ranking of, the effects of changes to materials or processes on the reliability of the solder joints. Hence, it is envisaged that if a wide range of conditions are to be tested a first sift can be completed using thermal shock, with the final work using typical thermal cycling conditions.

The difference between the SAC (95.5Sn3.8Ag0.7Cu) and SnAg (96.5Sn3.5Ag) solder alloy results across all types of cycles showed very little difference in the rates of joint degradation.

This paper compares relative reliability (remaining shear strength) of three chip components soldered with two lead‐free alloys based on various thermal cycling conditions.

Rob Knowles has been appointed Technical Service Specialist by Metech‐Ronal to support the company's rapidly expanding Thick Film Materials business throughout Europe.