A study of solder paste flow inside a sealed printing head

Abstract

In the assembly of Printed Circuit Boards (PCBs) using Surface Mount Technology (SMT), solder paste is deposited on the bond pads of the PCBs using stencil/screen printing technique. The last few years have seen the development and introduction of new printing mechanism to meet the miniaturisation challenge of electronic products. The most notable is the development of new printing heads such as the ProFlow and the Rheometric Pumping Head. Unlike the traditional squeegee blade, in these new printing devices the solder paste is contained in a sealed pressurised chamber, and is released during the printing stroke via an opening as the printing head passes over the stencil apertures. The flow profile of the solder paste inside such a chamber plays a key role in determining the volume of solder paste deposited onto the PCB pads. In this paper we investigate the paste flow inside such a chamber and its influence on the aperture filling. Our results show that the paste does not vertically fill the apertures, but has a horizontal velocity component in the printing direction. This horizontal velocity component will lead to insufficient filling of paste at the rear corner of the aperture. To counteract the influence of this undesirable velocity component, we propose to introduce a horizontal shaft perpendicular to the printing direction inside the chamber. During a printing stroke this shaft rotates inside the chamber in the printing direction and drives the paste near the bottom slot to flow against the printing direction. We present an analysis of the paste flow inside such a device and the nature of the aperture filling process. The main parameters that influence the paste flow are the diameter, the rotational speed and the position of the shaft. The key to obtaining sufficient and consistent paste deposits is to minimise the horizontal velocity component of the paste to ensure the paste fills into the aperture vertically, and to maximise the vertical velocity component of the paste to shorten the aperture filling time. The introduction of such a shaft is also expected to significantly reduce the pressure loading on the paste at the top of the chamber.