Predicting Scooping and Skipping in Solder Paste Printing for Reflow Soldering of SMT Devices

Abstract

This paper examines the rôle that the squeegee plays in the solder paste printing process. Although the printing of solder paste is only one stage of many in the surface mount assembly process, it is crucial to deposit the correct amounts of solder paste cleanly onto the substrate. The amount of solder paste deposited affects the reliability and strength of the reflowed solder joint. Surface mount component lead pitches are continually being reduced due to the requirements of packing more and more components into a given space on the PCB, and this necessitates a proper understanding of the printing process and in particular of the squeegee which plays an important part in determining paste heights and the occurrence of defects. The paper outlines a model which predicts scooping and skipping in the stencil printing of solder pastes used in the reflow soldering of surface mounted devices. The model is based on the forces acting on the squeegee blade, which determines the paste flow pattern ahead of the squeegee, and on the stencil aperture geometry. The paper also examines the interactions between the paste properties and squeegee material properties. These interactions produce printing defects such as scooping, skipping and wet bridging. Results of an experimental comparison of different types of squeegee blade used in the stencil printing of solder pastes for reflow soldering in SMT, as well as the experimental results for squeegee deformation into stencil apertures, were used for validating the model. The empirically enhanced model which results takes into account the force on the squeegee due to solder paste flow and some of the non‐Newtonian properties of the solder paste. The main utility of the proposed model is the control of solder paste printing quality.