A study of solder paste rheology for the Alternative Assembly and Reflow Technology (AART) process

Solder paste performance related properties such as stencil print quality, viscosity, thixotropy, and slump resistance are of great importance in the pre-reflow stages of the electronics assembly process. This paper focuses on the study of the theological behavior of solder paste and its correlation to process performance during the various steps of the Alternative Assembly and Reflow Technology (AART) process. This technology aims to integrate the Printed Circuit board (PCB) assembly process for both through hole and surface mount components. Four solder pastes were considered in this study and their theological characteristics were identified through flow and oscillation tests. The oscillation tests provided the linear viscoelastic characteristics of solder paste whereas the flow tests revealed information on the yield stress as well as the degree of shear-thinning and thixotropy of the solder paste. Pastes with high elastic properties and yield stress exhibited good hot slump resistance. Furthermore, extensive shear thinning of the paste facilitated the filling of the Plated-Through-Hole (PTH) sites. Recommendations are provided for tailoring the properties of a solder paste to meet the needs of the AART process.