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Written by Clive Ashmore   
Wednesday, 30 April 2008 19:00

Why a 55° squeegee angle enhances print quality.

Screen Printing Arguably, it’s not the hottest topic at the local pub or around the water cooler, but it is a subject that has been debated among engineers and printing specialists for a long time. While screen printers have seen dramatic advancements in capability during the past 10 years, becoming remarkably accurate, repeatable and yes, fast, some basic elements remain that must be examined when analyzing total print quality: namely, squeegee angle and squeegee material.

Among screen-printer OEMs and process engineers, there has been passionate discussion on what squeegee angle delivers the best results. The only thing we all agree on, it seems, is that a normal, metal squeegee will do the job (now, even that is being challenged – keep reading!). Historically, at least in the Americas, many printer suppliers recommended running squeegees on a 45° angle as standard. At our company, we took a different approach. Based on our research, we always have contended a higher squeegee angle, in the range of 50° to 55°, is much better from a repeatability standpoint. And, now, with new data in hand, we have proved this is absolutely the case.

Here’s why: Changing the angle of the squeegee affects the process capability. When squeegee angle is set at a range between 40° and 45°, the volume of the paste deposited is increased, but the repeatability decreases. The blade moves over an open aperture and the hydrodynamic pressure forces the solder paste material into the opening. If this transfer force is too high (created by the lower squeegee angle), the solder paste compresses and goes underneath the blade, causing a “wake” effect. This extra compression gives higher deposit volumes, but sacrifices repeatability as it leads to high variability of deposit-to-deposit volumes. With unequal deposit volumes, coplanarity challenges and, ultimately, stressed interconnects and long-term reliability issues result. If, however, the squeegee angle is increased to between 55° and 60°, the deposited volumes move closer to the nominal volume and process repeatability is increased. In our most recent studies, the higher squeegee angle produced better deposit-to-deposit consistency than the lower blade angle, as well as delivered a more desirable “brick”-shaped paste deposit.

This deposit-to-deposit consistency is what we always have maintained is critical to a robust, high-yield process. The challenge, though, is ensuring adequate paste volume. Excellent inputs – properly manufactured stencils, robust solder paste materials, proper print parameters – all will help ensure proper material volume is deposited. But, as we all know, those inputs aren’t always optimal. Our most recent research, therefore, focused on methods by which deposit-to-deposit repeatability and proper paste volume could be realized. What we discovered was that by moving the blade position to a higher angle (between 50° and 55°) and using a chromium-coated metal squeegee, both desirable results can be achieved – repeatably. The chromium coating effectively reduces friction between the paste and blade, thus improving filling of small apertures and resulting in a higher volume of paste deposited. The higher print angle, as stated, ensures repeatability across deposits on a single board and from board to board in a production run.

Clearly, this revelation is important for every assembly being built, as repeatability and solder paste volume are critical for reliable interconnects on all products. But, mention the “M” word – miniaturization – and print repeatability and consistent paste volumes become even more critical, but also more challenging. Our testing using a 0.4 mm CSP showed the 55° chromium-coated squeegee gave excellent results. Further research will be conducted in the near future on even finer pitches.

Consider how many interconnects are printed in a given year: The impact of slight alterations to some print parameters can be huge. By my rough calculations, there are close to 60 trillion – yes, trillion – interconnects printed each year. That’s a lot of opportunities for failure. So, we’re not just talking about the quality of one or two products; we’re talking about the quality of 60 trillion interconnects. Amazing to think changing the squeegee material and print angle slightly can avoid trillions of failed interconnects. Surprising, but true.

Clive Ashmore is global applied process engineering manager at DEK (dek.com); This e-mail address is being protected from spambots. You need JavaScript enabled to view it . His column appears bimonthly.
 

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