Martin Wickham

The impact of thermal cycling regime on the shear strength of lead-free solder joints

Purpose – 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.Design/methodology/approach – 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.Findings – 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.Research limitations/implications – 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 th...

Finite difference modelling of moisture diffusion in printed circuit boards with ground planes

Abstract A finite difference model has been applied to diffusion modelling of molecular moisture diffusion in Printed Circuit Boards (PCB) that have ground planes. Capacitance measurements between two ground planes within a PCB during moisture uptake and removal allowed diffusion coefficients to be determined. These diffusion coefficients were then used to look at the effect of ground plane width, hole density and meshed ground plane dimensions on bake time. The effect of plating holes on the bake time has also been investigated. Results show that whilst a few hours are sufficient to bake PCBs with no ground planes, days, or even months are required to remove moisture from within ground planes due to the extra perpendicular distance the moisture must diffuse. The results demonstrate the importance of storage conditions of the PCB, as once moisture has diffused into a board with ground planes; it is often not feasible to remove it.

Fine pitch stencil printing using enclosed printing systems

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.

Obtaining the moisture content of printed circuit boards from capacitance measurements

Purpose – 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.Design/methodology/approach – 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.Findings – 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 tha...

Measuring the effect of substrate and component finishes on the reliability of isotropic electrically conductive adhesive joints

Purpose – 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.Design/methodology/approach – 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.Findings – 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 mate...

Impedance and resistance of carbon ink during cure

Purpose 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. Design/methodology/approach 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. Findings 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. Originality/value 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.

Development of a High Temperature Interconnect Solution as an Alternative to High Lead or Gold Content Solders

Abstract A collaborative research programme between project partners Microsemi, the National Physical Laboratory (NPL) and Gwent Electronic Materials (GEM), has successfully developed innovative materials specifically designed to offer an alternative for high Pb or Au content materials to increase the operating temperature of electronic assemblies. Currently, for electronic assemblies to operate at high temperature, they must use a high lead solder or a very expensive gold based solder to withstand these temperatures. The ELCOSINT project has developed an inexpensive lead-free alternative for joining high temperature electronics suitable for operating at temperatures above 250°C utilising standard surface mount assembly processes. This paper summarises the work undertaken by the authors to develop and better understand this new family of electrical interconnection materials. The project brought together a materials supplier (GEM – Gwent Electronic Materials), an end-user (MSL - Microsemi) and an technolog...

Third harmonic measurement in printed electronics

The purpose of this research paper is to investigate the defects detecting technique in printed electronics by the third harmonic measurements. Various types of defects were introduced on the samples and the third harmonic signal was measured using a component linearity tester (Radiometer CLT1). The relationship between the defects in the printed samples and the third harmonic signal and the third harmonic ratio was identified.

Transitioning to Lead-free: the Effect on Low Cycle Fatigue of Contaminating Solder Alloys

As RoHS comes into force, the impact on industry is to force it to struggle with mixed inventory, conventional tin-lead and lead-free. There are many stories circulating in the industry where the label does not correspond with the components. Consequently there is real concern of mixing different alloy systems that will have unpredictable results in terms of low cycle fatigue performance. Some workers have shown that lead at the 1 to 10% level may not be evenly distributed throughout the solder joint (Oliver, et. al., 2002). Segregation of contaminates at the joint level may well result in structural weakness. What is the effect of this on thermal fatigue. The work reported here will describe experiments where the lead level in solder joints was controlled by altering the plating on component terminations and using controlled solder compositions. Microstructural examination verifies the segregation of lead. The built assemblies were then thermally cycled between -55 and 125degC for 2000 cycles to assess this effect on reliability. The work has indicated that there should be few solder joint reliability problems when mixing SnPb and LF components and solder alloys (with Pb contamination in the range 1 to 10%). Very few thermal cycle fatigue failures were experienced other than within two component groups. Ball grid array components did fail generally, in the rings of balls adjacent to the edge of the silicon die within the package. However, the failures in these devices were largely restricted to SnPb alloy dominated systems, i.e. SnPb terminated components soldered with SnPb or SAC alloy solder pastes. Uncontaminated SAC systems or those systems contaminated with low levels of Pb showed fewer failures and thus must be considered more reliable. Indeed, the system showing greatest thermal cycle fatigue in BGA components was the SnPb terminated EGAs with SnPb solder. All other systems were shown to perform better. The other component type to show significant failures, were the QFP components where failures were confined to Sn-plated components with SAC solder. The process window for SAC alloy soldering is narrower than for equivalent SnPb processing and small additions of Pb may help widen the process window, improving the reliability for these soldering batches. It is therefore probable that these differences in QFP component reliability are batch related. Hot peel tests were also run to simulate problems that may occur is secondary wave operations where the fillet strength collapses and components can detach with little force at temperatures above 180degC This paper will discuss these results and the likely impact on the industry and the necessary precautions

Survey of rework practices in the UK electronics assembly industry

A survey of rework practices of the UK electronics assembly industry has been undertaken. Rework of electronics components in the UK was found to be dominated by the use of soldering irons. Many companies did not control tip temperatures, rework times, background heating or the number of rework operations which could be undertaken on any one component. Few companies followed the recommendations of the component manufacturers aimed at preventing damage to thermally sensitive components. It was found that those companies which did control rework temperatures, did reduce rework temperatures for surface mount components. In general, rework temperatures were much higher than those recommended by the component manufacturers. A great deal of cleaning was undertaken after rework, with most companies doing some form of local cleaning. Most companies reused components even if only on an occasional basis.