screen printing

clive ashmore

Spring Cleaning

Getting your printing house in order this season – and beyond.

As you read this, hopefully spring will have sprung wherever you are. It’s an agreeable time of year and one of renewal, refresh, and the annual “spring clean.” In that vein, I thought it would be an excellent opportunity to remind readers to take this same approach regarding the printing process, equipment and components. Maintaining all elements of the stencil printing operation is vital for good results, but these essential maintenance tasks are often overlooked in busy factories with even busier staff. If you’re reading this, tick these items off your printing spring cleaning list and watch the process flourish.

Squeegee mechanism. Check the deflectors! First, using paste deflectors is highly recommended. They are simple tools mechanically set to the edge of the squeegee to prevent material from spreading beyond the active print area, drying, and being integrated into the paste roll. Setting them correctly is important, as I often see them set too high to be effective or so low that they damage the stencil. The height should be just above the stencil, at the point where squeegee pressure is applied. Ensure they are correctly positioned, or invest in automatic paste deflectors. And use them! You’d be surprised at how many operations don’t.

Second, ensure the squeegee blades are damage-free with the “rule of thumb” test. For a good print result, squeegees need a reasonably sharp edge. If the blade is bullnosed or there are nicks, get rid of it. Run your thumb across the blade’s edge (it won’t cut you; it’s not razor-sharp) and expect to feel a true edge with no breaks. If that’s not the case, the blade edge is rounded or nicked and needs to be taken out of service.

Stencils. Again, if there is any damage, do not use the stencil. Any coining or damage around the artwork means the stencil is a no-go. In the modern micron world, where a 100µm print height is standard, don’t risk a bad result from a damaged stencil. Second, confirm the integrity of the fiducial alignment marks. Over time, the contrast can diminish or become impregnated with solder paste. Once fiducial finders can no longer locate the complete edge, it’s game over and accept scores will start to drop. Have a look with a loop, and if the fiducial doesn’t look right, discontinue that stencil’s use.

Also important is stencil tension. Automatic tensioning systems help mitigate against lost tension. For the traditional and incumbent mesh-mounted stencil, however, it’s a fact that it will lose tension over time. If the stencil can move easily in the z-axis or has visibly less tension than a newly sourced stencil, it may cause issues with separation and lead to bridging defects. Better to replace the stencil.

Solder paste. Like the food in the fridge, check the “use by” dates on solder paste materials. Follow the basics of stock rotation, not reusing material from shift to shift, and other standard – and common sense – material protocols for the best result. This is especially important as the industry moves toward smaller particle sizes (Type 5).

Understencil cleaner. The cleaner needs cleaning, too. Elements such as the solvent dispenser can become blocked by dissolved fluxes that wick back through the fabric onto the dispensing bars, so check that all dispense openings are clear. In addition, subpar understencil fabric can put lint and debris in the apertures and present problems. Use high-quality fabric for the best results. Finally, high-volume sites that often employ the understencil cleaner after every print should also check the vacuum ports to ensure no blockages.

Tooling. The. Tooling. Must. Be. Flat. It’s that simple. Operators should also inspect the tooling block for damage and dried material. Correctly labeling the tooling is important as well, especially if your operation creates its labels. There are often engineering change orders (ECO) to tooling that may be undetectable to the naked eye, so ensure the proper tooling is used and retired tooling blocks are moved out of rotation. Verify, verify, verify.

Machine. Last, but certainly not least, is the machine itself. Is it calibrated? If the machine’s calibration maintenance is out-of-date, calibrating the equipment should always be step one, before any other troubleshooting commences. Other things to check include the rising table, which can become the repository for material drips off the squeegee if the equipment isn’t fitted with a catch mechanism, and the clamping system. The clamping system sometimes gets bent or damaged, preventing the PCB from being tight and flush to the belt.

These simple but often skipped or overlooked maintenance checks should become routine – and not just in the spring, obviously. Hopefully, the headline got your attention, though, and got you thinking about procedural fine-tuning for a more robust printing operation. Article ending bug

Clive Ashmore is global applied process engineering manager at ASMPT (; His column appears bimonthly.