Awni Qasaimeh

Stability of ITO Thin Film on Flexible Substrate Under Thermal Aging and Thermal Cycling Conditions

Indium-tin-oxide (ITO) is a transparent conductive thin film that is widely used as a top conducive layer in photovoltaic solar cells. However, ITO is sensitive to environmental conditions and the electrical conductivity degrades as a consequence of harsh environmental conditions. Furthermore, the thermal expansion coefficient mismatch between the ITO film and the substrate creates stress/strain on the film when the package is subjected to fluctuating temperatures. This could create micro-cracks and consequently damage the film. Therefore, this study was designed to study the effect of the thermal cycling and thermal aging on the ITO thin films to simulate the effect of continuous high temperatures and fluctuating temperatures that may be applied on the thin films during the usage. In this study, two sets of experiments were conducted on a 60Ω/□square sputter-deposited ITO on 127 μm heat stabilized Poly Ethylene Terephthalate (PET) substrate. The first set of experiments contained four samples which were thermally aged at 100°C for 30 days and the other set of experiments contained another four samples which were thermally cycled for 900 cycles. The thermal profile consisted of a high temperature of 100°C, a low temperature of 0°C, dwell time of 10 minutes, and ramp rate of 10°C/min , as depicted in Fig. 1. The initial results showed that the ITO thin film is not stable in the thermal aging experiment and the electrical resistivity gradually increased for all samples until the end of the 30 days. The degradation happened during the thermal cycling as well. However, SEM images show that the morphology of the ITO surface is stable under both conditions. Energy-dispersive X-ray (EDX) spectroscopy analysis showed stability in the ITO thin film in terms of composition. XRD spectra confirmed the improved crystallinity for the thermally aged films, which corresponded to the increased transmission in the visible region.

Recrystallization behavior of lead free and lead containing solder in cycling

The present work addresses the effects of thermomechanical history on the recrystallization behavior of lead free and backward compatible solder joints. 30 mil SAC305 balls were reflowed onto BGA pads using either a SAC305 or a eutectic SnPb paste. Systematic variations of the rate of recrystallization with precipitate coarsening as well as with cycling and annealing parameters were characterized and correlated with observations from thermal cycling experiments on lead free BGA assemblies. The introduction of a few percent of Pb has been seen to not only affect the distribution of secondary precipitates but also add minute inclusions of Pb within the individual Sn dendrites. These also act as barriers to dislocation motion but do not coarsen as rapidly as the precipitates, and we find recrystallization in cycling to be strongly delayed.

Effects of variable amplitude loading on lead-free solder joint propoerties and damage accumulation

Current extrapolations of accelerated test results to life in long term service may be greatly misleading when it comes to lead free solder joints. Notably, realistic service conditions are almost never well approximated by cycling with a fixed amplitude. However, recent results have demonstrated the consistent breakdown of common damage accumulation rules. In isothermal cycling tests the remaining life, after a step-down in amplitude, was invariably shorter than predicted by such a rule, while a step-up tended to have the opposite effect. The present work offers a mechanistic explanation for this and a basis for the development of a practical approach to the assessment of life under realistic service conditions. Realistic BGA joints were cycled individually in a micromechanical tester, monitoring the solder stiffness and the inelastic energy deposition. Cycling was seen to first cause rapid hardening, followed by leveling off in a “cyclic saturation” stage and eventually the initiation and growth of a crack until failure. A temporary increase in amplitude during cycling caused a lasting reduction in hardness, and thus enhanced inelastic energy deposition and damage evolution, afterwards as well. This would dominate during repeated increases and decreases, eventually shortening the remaining life dramatically.

Solder joint reliability in isothermal varying load cycling

Realistic service conditions of electronic assemblies tend to involve significant variations in loading. However, reliability prediction of solder joints relies on conducting accelerated tests with fixed amplitudes and extrapolating results to service conditions. This can be very misleading without proper constitutive relations and without understanding the interacting effects of cycling parameters under realistic service conditions. In this work, the effects of amplitude variations and strain rate on solder joint fatigue life are studied. Individual SnAgCu solder joints with two different Ag contents (SAC305 and SAC105) were tested in low cycling shear fatigue under single and varying stress amplitudes with different strain rates. Work accumulation and the evolution of solder deformation properties during cycling were measured. The results showed that cycling with a higher strain rate at fixed stress causes less damage per cycle and thus longer fatigue life. Alternating between high and low stress at a fixed strain rate leads to damage acceleration and rapid failure. In addition, alternating between low stress at a high strain rate and high stress at a low strain rate leads to ongoing increases in the rate of damage at the mild amplitude and thus relatively rapid failure. In comparing SAC305 with SAC105, SAC305 is more fatigue resistant than SAC105 in single and varying amplitude cycling. However, the effect of strain rate on both alloys is almost the same.

On the evolution of the properties and microstructure of backward compatible solder joints during cycling and aging

A number of groups have published accelerated test results comparing backward compatible (mixed) solder joints to pure SnPb and lead free ones, but the question remains as to the actual reliability under realistic long term service conditions. We are addressing this in a comprehensive fundamental study of the evolution of the microstructure and resulting joint properties and performance for mixed solder joints. The present work reports results of reflowing 30 mil SAC305 balls onto Cu coated BGA pads with different amounts of SnPb paste, aging and/or cycling the joints and inspecting the microstructure by cross polarizer microscopy and SEM. The addition of small amounts of Pb was found to affect the solidification during cool-down from reflow, and thus the initial microstructure, in a profound fashion. The shapes and distributions of secondary precipitates were, as expected, different from those observed in ‘pure’ SAC305 joints and a large number of very small Pb inclusions were distributed within the Sn dendrites. As a result the mixed joints started out harder and did not soften as rapidly during thermal cycling and aging. In fact, an initial break up of elongated Ag3Sn precipitates first led to a slight hardening and strengthening. In addition thermal cycling induced recrystallization of the Sn was delayed and acceleration factors were lower. However, the strongly reduced sensitivity to aging suggests that mixed joints may compare more favorably to their ‘pure’ lead free counterparts in long term service than reflected by accelerated test results.