SMTA International Conference Proceedings

Rheology of Flux and Solder Paste

Authors: Fan Gao, Ph.D., Yanrong Shi, Ph.D., Kyle Loomis, Bruno Tolla, Ph.D.
Company: Kester Inc.
Date Published: 9/17/2017   Conference: SMTA International

Abstract: Solder pastes present a complex rheological behavior, due to the dynamic nature of the system formed by solder particle dispersion into reactive flux media. Optimized rheological parameters are needed to enable solder pastes flow into stencil apertures of various sizes, while minimizing slump after releasing. The rheological property of a solder paste is not only important for product design, but also critical for quality assurance. The viscosity measurements commonly adapted in the electronic assembly industry only provide one data point under very specific condition, which does not reveal the whole picture of solder paste flow behavior. Therefore, pastes in the same viscosity range can present dramatically different performance on the printer. In this paper, the rheological properties of both solder flux and paste have been studied using a parallel plate rheometer. Different test methods have been developed to characterize flow properties, such as viscosity, stress, modulus etc. under flow and oscillation modes. This fundamental approach is complemented with a series of engineering studies involving a state of the art printer and statistical data analysis. This study builds the fundament for a better understanding of the correlation between solder flux/solder paste rheological property and printing performance in challenging conditions. It is found that flux is a predominant factor contributing to the rheological performance of solder pastes, with the same alloy powder and metal loading. The key parameters tested under certain rheological methods may provide some indication of the fine aperture release capability.

Key Words: 

Solder paste, flux, rheology, printing, fine aperture release

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