SCREEN printing
Respect the Squeegee
Six areas to consider for optimal print quality and consistency.
IN THE STENCIL printing process, the squeegee blade often fails to get the recognition it deserves. Yet the squeegee is the item that does all the work and is the unsung hero. Consider a squeegee running in high volume on a 300mm board may put in between five and 10 miles per day of grueling aperture filling, and it becomes clear close attention to squeegee attributes may result in higher-quality results. With that said, here are my top squeegee awareness tips.

Material. In the early days of SMT, squeegee blades were predominately made from polyurethane (rubber), as the very first surface-mount printing processes used mesh screens. As the industry transitioned to metal-etched stencils and then laser-cut, stainless steel squeegees became standard. However, there are applications – such as heavily stepped stencils (say a 75µm step down on a 150µm-thick stencil) – where the compliance of a polyurethane squeegee is beneficial. The vast majority of squeegee blades today, though, are stainless steel. And not just any stainless steel; to be sure, a tremendous amount of IP and proprietary alloy formulation is in today’s sprung steel compounds used to manufacture high-quality blades. They keep a good sharp edge and provide excellent consistency for the pressure and force applied, which delivers the aperture filling necessary for a repeatable process.

Angle. For mainstream production, two common squeegee angles are employed in stencil printing operations: 45° and 60°. The years have seen much debate over which angle is best. A 45° angle will produce higher fill pressure for rolling the material and shear thinning it. “Great,” you say. Well, that depends. The challenge with high pressure at a tight angle is that, unless a perfect gasket (absolute seal of the aperture to the pad) is established, the likelihood that elevated pressure will break the seal and introduce smearing and bridging is fairly high. My general advice is to go with a 60° angle, as good pressure is created but not so much to introduce possible defects; it gives a slightly larger printing bandwidth. Like anything, there are exceptions: Applications that dictate a large volume of paste – such as backplanes with big components – and specialized processes like pin-in-paste can benefit from a 45° angle squeegee blade.

Overhang. Squeegee overhang is the amount of flexible blade that is visible outside of the assembly (holder); our company offers a 6mm and a 15mm overhang. With 6mm, pressure can be quite high, and the squeegee will not give or bend much at all, which is often preferred for boards that are ultra-flat and where absolute control is required, such as in semiconductor or ceramic printing. With SMT, and the variability of heavily routed boards, FR-4, and PCBs that may have a bit of warpage, 15mm is my go-to. It provides more flexibility, and the extra overhang depth permits a bit of pressure adjustment to create a customized angle for challenging assemblies.

Clamping. The mechanism by which the squeegee attaches to the holder can also be a variable. The incumbent is a system where the metal blade is clamped using conventional nuts and bolts. This makes changing out the squeegee a simple operation. A bonded squeegee is when the blade is glued to the holder. Like the polyurethane squeegee, this was the predominant design in the early days of SMT but is rarely used today. While this method does ensure excellent blade coplanarity and may be effective for some alternative applications or adhesive printing, conventional clamping is the most appropriate for the majority of SMT production.

Length. This is one of my soapbox issues. There are varying lengths for squeegee blades (our company supplies from 170mm to 535mm lengths), and it is imperative the length that matches the product is used. While changing blades may take a few minutes, it is well worth the time and effort to align the blade length to the product being printed. Do not assume a 300mm blade will split the difference and manage most products. Absolutely not. Printing with an oversized squeegee blade will most assuredly result in a reduction in print quality.

Coatings. In the early 2000s, squeegee blade coatings were all the rage. The truth is most users do not need coated blades. The idea was coatings would help solder paste material roll more consistently and permit it to drop off more cleanly at the end of a print stroke. Most materials roll fine without coated squeegee blades. Naturally, situations such as high-temperature operations or specific alloy-based materials have more difficulty rolling, and, in those rare cases, a coating may help. Otherwise, a highly polished steel blade is the way to go.

Close attention to all these tips, securing squeegee blades from a reputable screen-printing equipment or blade supplier, and examining squeegee blades frequently to ensure they are free of damage will do wonders for print quality and consistency. Respect the squeegee, reap the rewards.

Clive Ashmore headshot
CLIVE ASHMORE
is global applied process engineering manager at ASM Assembly Systems, Printing Solutions Division (asmpt.com); clive.ashmore@asmpt.com. His column appears bimonthly.