Christopher Hunt

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...

Characterization of the Conduction Mechanisms in Adsorbed Electrolyte Layers on Electronic Boards Using AC Impedance

The reliability impact of flux residues on electronic assemblies has been traditionally evaluated using Surface Insulation Resistance (SIR) measurement with DC voltages. An AC impedance technique has been investigated to provide detailed information on the conduction mechanisms and electrode reactions in the presence of flux residues on electronic boards. By evaluating different fluxes using a standard comb pattern the relative applicability of the AC impedance and SIR techniques has been made in terms of assessing the reliability of electronic boards. Impedance values at low frequencies, <1Hz, are close to the SIR results, and hence produce similar predictions of reliability. Importantly the AC impedance spectrum can be used to predict dendrite formation, although the technique itself does not actually promote dendrites. The AC impedance method can distinguish between ionic solution resistance of the thin water layer and impedance from interfacial electrochemical processes. At low contamination levels the solution resistance, between the copper comb electrodes, dominates the overall impedance. At high contamination levels the impedance from interfacial electrochemical processes becomes significant, and increases the potential of dendrite formation. This predictive capability could be developed into a non-destructive test method to provide a more detailed electrochemical characterisation and indication of future reliability.

The impact of temperature and humidity conditions on surface insulation resistance values for various fluxes

The surface insulation resistance (SIR) test has traditionally been performed by taking measurements at certain points during a seven‐day test under well established environmental conditions. The work reported here explores the influence of test temperature and humidity when using a typical resin flux, a weak organic flux and glycol based fluxes when sampling SIR patterns every ten minutes. Results indicate that some fluxes are very sensitive to the test temperature, with volatilisation of flux residues an important issue. The frequent monitoring of the results also permitted the detection of dendrites during the SIR test. The results clearly show the importance of selecting the correct testing conditions and the benefit of frequent monitoring.

Low cycle isothermal fatigue properties of lead‐free solders

Purpose – The purpose of this paper is to discuss a new method to measure the fatigue of single solder joints in shear, and hence calculate the joint strain energy density in each fatigue cycle.Design/methodology/approach – There has been a step change in the requirement to characterise solder joint reliability with the number of new alloys introduced as a result of the recent RoHS legislation. Experimental testing for every configuration is expensive and time consuming, and hence modelling has become more attractive. The accuracy of modelling predictions is limited by the accuracy of the materials data. The data for these new alloys must reflect the miniaturisation of electronics and that solder joints are loaded in shear, two aspects not well reflected in the existing SnPb data. The approach here has been to develop an instrument interconnect properties test machine, where the strain and stress can be measured directly for small solder volumes and in shear. A four‐point measurement system for resistance...

Fabrication of highly conductive stretchable textile with silver nanoparticles

A method has been developed for the deposition of conductive metals onto fibers within textile structures. The resultant fabric exhibited full metal coverage with good adhesion to the fibers. As well as being conductive and extremely flexible with little effect on its handle and drape properties. In order to make a conductive fabric, cotton was first mercerized followed by immersion in poly(diallyldimethylammonium chloride) solution. Silver nitrate was then reduced on the surface of the fabric which allowed formation of silver nanoparticles. Scanning electron microscopy studies of the conductive fabric confirmed the deposition of the polymer resulted in more uniform attachment of nanosilver to the surface of fibers. The fabric was then electroless plated to obtain a surface resistivity less than 0.2 Ω/square. This method can be used for woven, non-woven and knitted types of fabric. It can be applied on to the fibers before or after being made into a textile.

Determining conformal coating protection

Purpose – This work sets out to characterise the protective properties of conformal coatings and how they degrade.Design/methodology/approach – The approach dosed several commercial coatings with two different contaminants, a synthetic generic flux mixture of dibasic acids in both a solvent‐ and water‐based carrier, and sodium chloride. The protective properties were monitored using three complementary techniques: surface insulation resistance measurements, sequential electrochemical reduction analysis, and diffusion measurements.Findings – The experimental approach was verified and the SIR measurements were shown to be the most valuable. Coatings offered varying levels of resistance to the contaminants, with the silicone coating being the most resistant. The flux variants generally proved more harmful to the coatings, suggesting that flux diffusion through the coating exceeded that of NaCl and hence led to greater electrochemical corrosion. Flux transmission through the coatings was verified by the diffu...

On the factors affecting the dissolution of copper in molten lead‐free solders and development of a method to assess the soldering parameters

Purpose – The purpose of this paper is to investigate the various factors that influence the dissolution of copper in molten solder, paying particular attention to important parameters: temperature, solder composition and flow rate.Design/methodology/approach – To determine the dissolution rate of copper in lead‐free solders, a simple and automated technique is developed. This methodology provides repeatable measurements that allow the various experimental parameters to be isolated. Factors that greatly affect the dissolution rate of copper, such as soldering temperature, flow rate and solder composition, are taken into account. Particular attention is paid to the flow rate of the molten solder. In fact, different alloys at the same temperature can have considerably different flow rates, owing to their different viscosities at that temperature. The dissolution rates of copper in seven lead‐free alloys and the Sn‐Pb alloy are compared at 255, 275 and 300°C.Findings – It is observed that generally the sampl...

High-frequency vibration tests of Sn-Pb and lead-free solder joints

There are many applications in which electronics devices are subjected to vibration. Yet while there has been much research into the effects of low-cycle fatigue, there has been very little in the field of high-cycle fatigue, despite this being of interest in critical fields such as the aerospace industry. In consequence, this work has been aimed at studying lead-free solder alloys in a high-frequency environment, above 100 Hz, using SnPb alloy as a benchmark. Many previous studies on high-cycle fatigue have been focused on studying the effects on a full printed circuit board. That assessment method, however, is not suitable for characterising the solder performance, as the approach is geometry and condition specific. In this work therefore, a more general approach has been adopted, using a reproducible model solder joint as the sample to which well-controlled vibrations have been applied. This study highlighted the different behaviour of the various alloys when subjected to vibration, and in particular the superiority of the SnPb alloy, especially at higher frequencies. This method has been shown to be useful in characterising and ranking various materials, and tests could be easily repeated with a range of conditions, targeted at specific industrial applications. The method is versatile, and the equipment could easily be modified for testing at high temperatures.

An evaluation of the effect of ageing on the cleanability of solder flux residues

The efficiency of cleaning of flux residues after various periods of ageing was assessed by measuring the ionic contamination removed in an Ionograph 500 SMD. The flux residues were removed from bare boards, and boards with through hole and surface mount components. The effect of different ageing temperature was also investigated. The work has shown that there is a maximum time interval following assembly during which cleaning should be carried out. The ionic contamination of aged assemblies with through hole and surface mount components were cleaned with varying efficiencies. The surface mount components were more difficult to clean. The use of brushing and scrubbing proved particularly beneficial for the through hole components. A proprietary cleaner proved more effective than the generic alternatives considered.

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.