Authors: Fan Gao Ph.D. and Dinesh Amin Company: Kester Inc. Date Published: 10/14/2018
Abstract: Can expired solder paste be used without an issue? Solder paste consists of powder and flux components that start chemical reactions immediately after blending. Therefore, during manufacturing, packing, transportation and storage, solder paste is continuously changing over time. Additionally, during usage varying temperature and humidity conditions can influence the reaction rate. Flux formulation is the key factor affecting the reaction between solder powders and flux, which will further impact the solder paste shelf life. Besides solder ball, slump and viscosity measurement, rheological analysis is one of the best methods to quantify the change of solder paste. In this paper, an accelerated shelf life prediction method is introduced to study the status of solder paste in a very short period. Several rheology test methods are developed to investigate the change of solder paste from different perspectives, including yield stress, shear sweep, strain sweep (oscillation mode) and axial force. Each rheological experiment has its limitation, and the responses are influenced by the test method used. This paper examines the accuracy and precision of rheological experiments through four different solder pastes. Both no-clean and water-soluble type pastes are included to illustrate the environmental contribution to the paste degradation. This study forms the background for a better understanding of the correlation between rheological properties and shelf life of solder paste exposed to extreme conditions.