Effect of Abandon Time on Print Quality and Rheological Characteristics for Lead-Free Solder Pastes used for Flip-Chip Assembly

The lack of understanding relating to the correlation between paste characteristics and print performance presents a challenge with regards to accurately predicting the outcome of the printing process. One such paste characteristic, the abandon time, is critical in reducing defects on a modern assembly line, due to the requirement of achieving an acceptable print after a delay in production. This study concerns the investigation of the abandon time for different lead-free solder paste formulations used for flip- chip assembly. The objective of the study is to determine the length of time a paste could be plausibly left on the stencil before the quality of print degrades. In the study, a series of viscosity and oscillation shear tests were carried out to aid with understanding the nature of the pastes. Results show that for all the pastes, there is a degradation of the pastes elastic and viscous behaviour after a period of 50 hours, thus demonstrating a breakdown in paste structure, which can be correlated to the deterioration of pastes printing quality. The results also show that increasing the length of the abandon time leads to an increase in the incidence of paste bridging, which is correlated to the decrease in storage and loss modulus from the oscillatory tests. The utility of the results of the study is in assisting paste manufacturers and process engineers in the identification of the process window with respect to the paste property changes with various abandon time scenarios, such as those experienced within a typical industrial production line.