Rheological characterisation of solder pastes and isotropic conductive adhesives (ICAs) for microsystems assembly technology
The printing of solder pastes through very small stencil apertures required for flip-chip pitch sizes, is expected to result in increased stencil clogging and incomplete transfer of paste to the printed circuit board pads. The paste printing process depends on conditions such as good paste roll, complete aperture filling and paste withdrawal. The paste rheology is important in the printing process because the viscosity of the paste must be low enough for the squeegee to force the paste through the stencil apertures, but high enough to recover to its required shape and not flow beyond its stencilled area. Rheological measurements can be used as a tool to study the deformation or flow experienced by the pastes during the stencil printing process. In addition, the rheological measurements can also be used as a quality control tool in the paste production process for identifying batch to batch variation; and to reduce the associated printing defects in the paste printing process. The work reported here on the rheological characterisation of novel pastes used in Microsystems assembly applications is divided into four main parts. The first part concerns the study of the effect of wall slip on paste viscosity measurement. For this study, a novel technique was developed for flow visualisation experiments using video observation to characterise the pastes wall slip behaviour. The second part of the study concerns an investigation of the visco-elastic behaviour of the pastes, and the main focus of the work was to determine the linear visco-elastic region using oscillatory shear stress experiments. The third part of the work deals with the study of the thixotropic behaviour of pastes using both hysteresis loop and constant shear rate tests. The final part of the study concerns the printing trial of three different paste formulations (XP22, Tamura and Polysolder) paste using a UV-LIGA manufactured stencil recently developed at Heriot-Watt University. The work is part of EPSRC funded research project on Microsystems Assembly Technology for the 21st Century under grant GR/R09206/02, and was carried out in collaboration with five industrial collaborators and two other academic partners. The Individual Grant Review (IGR) for the MAT21 project was assessed as internationally leading' by the EPSRC committee. The extensive set of results from the experimental programme, in particular, on the aspect of key paste performance indicators has been adapted by one of the industrial partners (a paste manufacturer) for implementation as part of a quality assurance (QA) tool in their production plants in various part of the world, and the results have also been disseminated widely through journal publications and presentation at international conference.