Solder wettability and solder joint reliability of rapid thermal firing thick film pastes

Interconnection of high efficiency solar cells is a very critical process step in terms of metallurgy and long-term stability. Especially warranty requirements of up to 30 years are very hard to fulfil with the industrially used solder interconnection of photovoltaic thick film pastes. These pastes require firing conditions that are completely different from standard hybrid packaging applications and are constantly under development to improve contact formation between paste and substrate. However, the influence of these paste changes on interconnection aspects has not been investigated in-depth until today. Therefore, this work focuses on analysing correlations between paste compositions, solder wettability and solder joint reliability. Thick film pastes with different silver particle sizes and glass contents were produced and fired under different industrial relevant conditions. Solder wettability was investigated by use of the wettability index and aging effects were discussed on shear force or peel force measurements after high temperature storage or temperature cycling.