470 IEEE TRANSACTIONS ON COMPONENTS, PACKAGING, AND MANUFACTURING TECHNOLOGY-PART A, VOL. 17, NO. zyxw 3, SEPTEMBER 1994 Squeegee Deformation Study in the Stencil Printing of Solder Pastes zy S. H. Mannan, N. N. Ekere, I. Ismail, and E. K. Lo Abstract-We report on the results of an experimental compari- son of different types of squeegee blade used in the stencil printing of solder pastes for reflow soldering in SMT, concentrating on paste heights (scooping) and printing defects. We show how our experimental results for squeegee deformation into stencil apertures lead to the construction of a model for squeegee deformation. The model takes into account the force on the squeegee due to solder paste flow and some of the non-Newtonian properties of the solder paste. An explanation is proposed for the differences in paste heights between apertures of different orientations. zyxwvutsrqpon Index Terms- Squeegee, stencil printing, surface mount tech- nology, solder paste, scooping. I. INTRODUCTION HIS paper examines the role that the squeegee plays in T the solder paste printing process, where 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, and the squeegee plays an important part in determining paste heights and the occurrence of defects. Some work has already been done in this area in connection with the printing of conductor inks through screens instead of stencils, but using the same type of squeegee system [ 1]-[5]. We have previously indicated how that work might be adapted to printing through a metal mask stencil [6], and we will use the existing models for ink screening in this paper. Fig. 1 illustrates the role of the squeegee in the printing process. The squeegee acts on the solder paste to form a paste roll, and near the squeegee tip high pressures and shear rates are generated. However, as shown in the diagram, the actual filling region occurs several millimeters ahead of the squeegee tip, which is similar to what occurs in screen printing [3]. As the squeegee passes over the aperture it may dip into the aperture and scoop out solder paste, reducing the volume of solder paste deposited and also forcing the paste underneath the stencil, perhaps leading to wet paste bridging between adjacent pads. Squeegees are usually made of a polyurethane material, and the hardness is often quoted on the Shore A scale. In order to reduce scooping harder squeegees are being used (90-94 hardness on the Shore A scale rather than 70-75). Composite squeegee blades, combining advantages of soft Manuscript received March 22, 1994. The authors are with the Department of Aeronautical and Mechanical IEEE Log Number 9401761. Engineering, University of Salford, Salford, M5 4WT, U.K. Fig. zyxwvuts 1 \ SUBSTRATE The roll of the squeegee in solder paste printing. POLYURETHANE COMPOSITE SQUEEGEE^)^ POLYURETHANE METAL Fig. 2. squeegee, and a simple polyurethane squeegee. The geometries of a metal squeegee, a composite polyurethane and hard squeegees may consist either of a hard tip and soft backing or vice versa (Fig. 2). Another approach has been the development of metal squeegees with lubricated edges. A comparison of metal squeegees with polyurethane blades has been reported [7], and those results will be included here for completeness. 11. SQUEEGEE BLADE COMPARISON Six different squeegee blades were tested; one was a metal blade, two were composites, and the other three were ho- mogenous polyurethane blades. The hardnesses and Young's moduli of the polyurethane blade are shown in Table I. The squeegees were tested under identical conditions of pressure and speed. A high pressure was selected in order to emphasize the scooping of blades. A brass, chemically etched Ni coated stencil (152 mm thick) and a semiautomatic printer were used. The squeegee blades had a nominal contact angle with the stencil of 60". The heights of the printed paste deposits were measured, and level of defects recorded, for a variety of aperture widths, lengths, and pitches. A summary of average heights, and variability in paste heights from the same aperture and from different apertures and differences in heights between perpen- dicular and parallel apertures (Fig. 3), is shown in Table 11. 1070-9886/94$04.00 zyxwvuts 0 1994 IEEE