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Screen Printing

Clive Ashmore

An RFID tag can log everything from storage location to print strokes.

Outside of sheer printing machine capability, the stencil is arguably the next most important element of the printing process. Stencil material, thickness, aperture integrity, sidewall smoothness (or lack thereof), and tension all play a role in the quality of the solder paste deposit. And, like all consumables, metal stencils have a lifetime: They do not last forever. Unless a stencil is damaged, tension loss is the factor that most often determines when a stencil has run its course. A properly tensioned stencil enables a good, solid release of the paste deposits onto the board. Alternatively, a stencil that has lost tension and has begun to “sag” may result in defects such as “dog ears”1, bridges, or insufficient paste on pad, to name a few.

Today, stencil tension is more important than ever. Historically, when stencil thicknesses averaged 200µm, one was far more likely to retire a stencil from damage than from wear. Now, however, with the exceptionally thin 60µm foils required for miniaturized designs, tension loss can occur sooner, as repeated stencil pressure during the print stroke eventually reduces stencil elasticity. As has been addressed in this column, there is a proven correlation to changing tension and the output of the printing process.2

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Clive Ashmore

Multitasking platforms are becoming the standard.

While productivity – manufacturing more product, more efficiently in less time – has been center stage in electronics assembly for decades, today’s razor-thin margins, coupled with the requirement for limited human intervention, have put an exclamation point on managing output proficiency. (This is especially true as the world restricts building access and maintains safe personal distances.) An optimized stencil printing process, as I’ve said many times, comes down to depositing the right volume in the right place at the right time. These are the three pillars of the print operation. Ultimately, for maximum productivity, a manufacturing operation needs a stencil printer that is always available and, when it is available, efficient and reliable.

It wasn’t long ago the bottleneck on the production line was usually the placement machine, so the stencil printer was generally available and had plenty of time to run the print routine. With recent modular approaches to manufacturing line setups, however, this is no longer the case. Placement platforms have exponentially improved speed. The printer now must maintain a much faster pace; this starts with mechanics and cycle time. In mass production settings, getting a printer down to a core cycle time of five seconds has become a necessity.

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Clive Ashmore

Do thinner boards require a different transport mode?

Just when we think we have reached the limit on shrinking substrate thicknesses, tighter pad spaces and higher component densities, the industry says, “Not so fast!” Today’s mobile phone boards average a remarkable 0.6mm thickness, with as many as 1,000 components packed into a 20mm x 80mm space. Over the past five years, advanced equipment sets have accelerated transport, tooling, vision systems, inspection capabilities and platform controls, all of which have certainly made producing high-quality products with ultra-small dimensions possible. However, in the stencil printing world, even more may be required to ensure maximum board stability during the print operation.

Traditionally, the mode of transport – bringing the PCB or pallet into the machine – has been achieved on some form of rubberized belt. This will no doubt continue as the solution for the assembly line. Inside the printer, however, not only is the board brought into the machine on the belt, but the substrate is clamped to the belt to hold it stationary, present it to the stencil and print. This has worked very well for years and is fine for multiple product builds. For mobile phones and other handheld products, however, current and future dimensions dictate a new paradigm. What are 600µm-thick phone PCBs today likely will continue to get thinner and, even at their current architectures, are susceptible to any type of undulation or extra pressure. Clamping thin, small boards or pallets to a rubber belt can result in movement, twisting or bowing at the substrate edges and potential print accuracy issues. There are flat belt options, which have been the interim solution for thin board printing, but the belts are still constructed from rubber and not completely rigid. Finally, belts are subject to wear; they eventually lose elasticity and require replacement. Without proper maintenance, even greater instability can occur.

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Clive Ashmore

Is a wet cycle necessary after every print? Maybe not.

In the previous installment of screen printing hacks, we discussed some proven workarounds for alignment issues. This month – and based on some recent customer observations – the advice centers on understencil cleaning, how lack of control can adversely impact this sub-process of printing and the overall result, and a few suggestions for correcting the problems.

Here’s the backstory: A customer printing very small dimensions – 200µm square apertures with spaces of 130µm, on average – was experiencing sub-4 Sigma results on some NPI designs. Transfer efficiency was low, and there was a large standard deviation across devices and the PCBs, so a lot of inconsistent paste-on-pad volume. Our team developed new stencil designs and tested them in a lab environment with our SPI, yielding excellent results. After making some machine calibration adjustments onsite at the customer and integrating the new stencils, however, there still wasn’t tremendous uptick in the process; improvement was observed but not at the expected level. Let the troubleshooting continue! We turned our attention to the cleaning process.

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Clive Ashmore

Not sure how to tackle an alignment issue? These tips might help set you straight.

In today’s slang, a “life hack” is any trick, shortcut, or proven workaround for a given task that increases productivity and efficiency. And, as regular readers will be acutely aware, this column’s focus is all about improving printing productivity and efficiency. So, I thought a series of “screen-printing hacks” might be helpful to engineers, no matter the level of experience. Throughout 2020, this space will periodically delve into various screen-printing hacks; an issue central to a good print outcome will be identified, and I’ll cover some ways to get the process back on track if something goes awry. The first installment of our screen-printing hack series is alignment.

The goal for the printing process is 100% alignment; the solder paste must align with the feature (the pad) on the substrate. When the solder paste inspection (SPI) system indicates this is not the case and offsets (paste not centered on the pad) are present, an alignment issue is most likely the culprit. Where do you start? Here’s a list of the most common causes and potential fixes; i.e., hacks:

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Clive Ashmore

Achieving printing nirvana is largely dependent on solder paste material, print speed and deposit release.

Ahhhhh … screen printing utopia. We process engineers strive for this existence. In a perfect process, printed solder paste would emerge from the stencil as exact replicas of the aperture shape: nice, flat, brick-like deposits. And, while modern printers and advanced materials get us close, solder paste is still, well, solder paste. The materials are not inks; they have a grain structure that is getting smaller in size and distribution and is suspended in flux. Try as we might, with these particles, there will be material undulation at best, and flat paste surfaces will likely never be a certainty.

With printing, we must be pragmatic. It’s not a digital process, and many variables come into play. The goal, of course, is to fill all the apertures on the stencil fully with solder paste to obtain the best deposit shape and volume possible. This is easier said than done, as the range of aperture sizes across a stencil can be broad, with 1mm square, 300µm and 200µm openings next to one another. Each of those apertures – from the very large to the very small – must be filled. Since printing with different thickness stencils is a nonstarter (generally and practically speaking), compromise is required, and that challenges our utopian ideal. Squeegee pressure, stencil thickness, print speed and separation speed must be balanced to accommodate variations in required deposit sizes. When all inputs aren’t optimized and in perfect balance, solder deposit shape differences can have the potential to introduce process problems. Known in the printing world as “dog ears” on square or rectangular deposits and “witch hats” on circular deposits (FIGURE 1), these solder paste deposit peaks may be defect bugbears, especially in the world of high-density, miniaturized assemblies.

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