Lars Rebenklau

Evaluation of soldering processes for high efficiency solar cells

Solar cells have to be connected electrically to each other in a solar module. During the last years, new connection techniques with low stress have been developed, such as selective soldering. The industrially most frequently used selective soldering techniques are induction soldering, contact soldering, laser soldering and spot light soldering. The aim of this work is to evaluate the characteristics of solder joints, which are produced by the use of selective soldering techniques. Therefore adhesive strength tests, electrical characterizations and x-ray analyses have been performed. In this process one front-side metallization, a SnAg-solder and a SnPbAg-solder were used. The characterization method will be demonstrated on the spot light soldering technique. In addition the influence of the solder joint quality on the resistance of the whole solar cell will be demonstrated and discussed.

Reliability and effective signal-to-noise ratio of RuO 2 -based thick film strain gauges: The effect of conductive and glass particle size

For RuO2-based thick film resistors used in high-sensitivity and -reliability strain gauges, crucial quality indicator besides long-term stability is the effective signal-to-noise ratio, which incorporates both piezoresistive strain response and current noise produced in the resistive film. Previous research in this field has shown an influence of both conductive and glass particle sizes on the microstructure and electrical properties of the films, however, most of these studies did not account for effects on resistor reliability and effective noise behaviour. The aim of this work is a systematic approach to revealing correlations between strain sensitivity, current noise and stability of resistance on one side and resistor microstructure on the other. For this purpose, various thick film resistor pastes have been prepared from RuO2- and glass powders of different particle size distributions and concentrations. It has been shown that the conductive and insulating particle sizes effect resistance and its temperature behaviour, the effective signal-to-noise ratio and the ageing stability of resistance, and that the development of a thick film paste suitable for strain gauges requires an application dependent tradeoff between these properties.

Evaluation of long time stability of solder joints on Ag thick film conductors on Al 2 O 3

The long time stability of solder joints on Ag-based thick film pastes has been studied. Therefore shear strength, electrical resistance, temperature coefficient of resistance (TCR), optical inspections of solder surface and cross section analyses have been performed. Samples of 1206 and 0805 SMD-components mounted with SnAgCu, SnPbAg, SnBi and SnCu on Ag/Pd or Ag/Pt conductors were prepared. To detect the influence of intermetallic phase growth, high temperature storage according JESD22-A103C at 150°C (125°C for SnBi-solder) up to 500 h has been applied. Shear fatigue life of the interconnection joints was studied with temperature cycling according JESD22-A104D between +125°C/-40°C for up to 1000 cycles. Differences in shear fatigue and electrical characteristics will be evaluated and discussed.

Wettability of rapid thermal firing silver-Pastes

Photovoltaic technology is evolving rapidly with new concepts and ideas to systematically improve module efficiency. Cell metallizations based on thick film pastes have to be fired less than one minute. It has already been shown that the solder interconnection of these rapid thermal firing pastes is critical in terms of wettability [1]. Main problems are solder leaching of commercially available pastes and paste adhesion to the silicon substrate. The correlation between paste composition and solder wettability has not been investigated yet. The aim of this work is to evaluate the wettability of rapid thermal firing pastes in dependence on paste composition and firing temperature. Pastes with spherical silver particles and lead free glass composition have been developed and fired with an industrial standard rapid thermal firing profile between (760...840) °C. Thick film surfaces and cross sections have been inspected. Solder wettability was evaluated with solder dip and solder reflow tests. Correlations between resulting thick films and their wetting or leaching characteristics have been investigated to improve the solder interconnection for rapid thermal firing solar cell metallizations.