Patented Vibrating Squeegee Technology

Manufacturers may have different approaches to the inspection and alignment phases of the print cycle, but the one constant always been has the print stroke itself. The conductive material is applied to the lands of a printed circuit board (PCB) through an apertured mask, or stencil. The conventional print stroke is made with a rubber or metal squeegee traveling either parallel or perpendicular to the front edge of the circuit board (Figure 1). Several new approaches to the print stroke recently have been introduced, with the specific intent of producing accurate, well-defined solder pads at increased rates of squeegee travel, thus lowering total machine cycle time.

Most of the new techniques are similar, in that they involve changing the relative angles between the squeegee and apertures in the stencil. Whether the angle of the squeegee is altered, or the orientation of the board and stencil is changed, net results are the same (Figures 2, 3 and 4). One problem in particular always has been achieving consistent results on fine-pitch pads that are perpendicular to each other (for example, QFPs). By moving the squeegee across the apertures at a 45° angle, more uniform deposits can be achieved. However, by print- ing the board at this angle, the print stroke is lengthened considerably. The time gained by speeding up the print stroke is lost by having to travel greater distances on each pass. The net result often is no real improvement in cycle time. Also, by orienting the squeegee and stencil at this angle, much more space is required to print the same board in a parallel or perpendicular manner. This effectively reduces the maximum board size capability of machines using these techniques and can actually cause stencil and/or squeegee damage when the squeegee rides up and over the bonded area of the stencil.

Vibrating Squeegee Method

A new approach to stencil printing is use of a vibrating squeegee. This apparatus oscillates along a single axis perpendicular to the direction of squeegee travel. Resulting analysis of the squeegee stroke will show a “zigzag” motion, which is an extremely effective method for filling and leveling stencil apertures evenly, regardless of size or shape (Figure 5). By filling each individual aperture from multiple directions, consistency of solder definition and deposition is achievable at a high rate of squeegee speed.

The vibrating squeegee method can produce a superior print at speeds up to 500 per- cent greater than conventional methods, especially on fine- pitch devices. This, coupled with the fact that the squeegee still follows the shorter perpendicular print stroke, results in lower machine cycle times and increased production throughput.

To effectively deposit material onto the PCB, the paste must roll as the squeegee moves across the stencil apertures. With no-clean solders and other high-viscosity materials, achieving this “rolling effect” often has been a difficult task. Rolling occurs when adhesion of the solder paste to the stencil is greater than the cohesion of the paste itself. Until now, the angle of attack of the squeegee blade solely was responsible for producing a rolling motion in the material by forcing it into contact with the stencil, therefore increasing the adhesive bond. Vibration of the squeegee excites the paste on a molecular level, and actually reduces its cohesive bond, causing it to roll freely. The reduction of the cohesive bond in the printed material also extends its functional service life as it begins to dry. This ensures the printability of materials previously considered unusable due to high viscosity, resulting in a substantial decrease in the amount of wasted solder. In one recent demonstration, the vibrating squeegee successfully printed a no-clean solder paste with a viscosity of 1.8 million centiPoise.

By vibrating the squeegee blade, the adhesive bond between the paste and the squeegee is greatly reduced, and the squeegee will tend to shed the paste, further improving paste roll. As the squeegee passes over an aperture, reduced adhesion between squeegee and paste makes the deposit less likely to be dragged (scavenged) out of the aperture. At the end of the print stroke, as the circuit is separating from the stencil, vibration of the squeegee is transmitted to the stencil, allowing the solder deposits to release from the apertures freely with almost no deformation. Vibration of the squeegee also allows it to separate itself from the printed material as it is raised off the stencil at the end of the print stroke. This makes the entire bead of paste available as the next board enters the machine to be printed. The benefit of this new technology is that the user does not have to reprogram or reconfigure the whole production line to see a substantial improvement in production capacity.

Stencil/screen printing equipment, like all other machinery in the production line, has undergone a steady evolution since the first days of SMT in the early ’80s. From introduction of the semiautomatic printer to emergence of the “fully automatic” printer, the continuing goal has been maximization of print quality and production capacity. Technological advancements have given manufactures the ability to fine-tune the print process for each individual product, while reducing the need for operator intervention in the production cycle. The squeegee apparatus oscillates along a single axis that is perpendicular to the direction of squeegee travel. The resulting analysis of the squeegee stroke (Figure 6) shows a “zigzag” motion, an effective technique for filling and leveling stencil apertures evenly, regardless of size and shape. Tests have shown that the vibrating squeegee holds a significant advantage over conventional methods in this area. “Scavenging” also is largely eliminated. By employing the vibrating squeegee, consistency of solder definition and deposition is achievable at a high rate of squeegee speed. This results in an overall lower machine cycle time and increased production throughput.

Vibration reduces the friction between the squeegee blade and the stencil, result- ing in longer blade and stencil life. The vibration of the squeegee serves to improve the “rolling” effect of the printed medium. This extends the functional service life of the material as it begins to dry, substantially decreasing the amount of wasted solder. The improved rolling effect ensures printability of materials previously considered unusable due to high viscosity.

Conclusion

As the surface mount industry continues to strive for zero defects, the stencil print- ing process must lead the way. The vibrating squeegee is one illustration of how, by developing new techniques for materials deposition, it will continue to do so. SMT Stephen K. Brodeur, Program Director, may be contacted at Milara Inc., 4 Marc Road, Medway MA 02053; 508.533.5322; E-mail: sbrodeur@milarasmt.com

The images above and below show actual photos of a 200mm silicon wafer printed with Milara's vibration squeegee technology. ALL bumped pockets show uniform paste fill (figure 8) with the resultant reflowed spheres within these pockets (figure 9).

The vibration squeegee has proven its technological viability over and over in the semiconductor industry. For the past 8 years, companies such as Flip Chip Technologies (Kulicke and Soffa), Advanced Semiconductor Engineering, Silicon Precision, Amkor and Mitsubishi have realized the quality of having bumped wafers completely filled with paste to allow 99.96 percent reliability. Other companies such as Micron Technologies are utilizing our SemiTouch wafer printer and vibration technology to bypass the expense and tedium of creating pockets for paste and going directly to the wafer by actually printing bumps instead of filling. This method greatly reduces cost however, printing accuracy and reliability MUST be at it peak in order to attain reliable throughput.