Stencil Printing: The Next Big Thing is Actually Very Small Print E-mail
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Written by Chrys Shea   
Friday, 29 March 2013 16:29

Next-generation materials make almost every stencil better.

Once in awhile, something really big comes along that makes a step function improvement in our PCB assembly processes. Stencil printing has enjoyed its share of milestone moments, and it’s currently on the brink of another. The science of surface energy modification is enabling a new level of print quality, and it is poised to make a significant impact on the process, on the same order of magnitude as electroformed stencils did in the 1990s or structured light SPI in the 2000s.

Commonly known as “nanocoating,” the process of applying a single-molecule layer of material to the stencil’s contact side dramatically lowers its surface energy and improves solder paste release. Several types of materials are classified as nanocoatings; the most popular is the two-part system introduced in 2010. Originally met with overt industry skepticism, the coating quickly proved itself in independent tests and developed a strong user base. I admit that I was one of the doubters who was quickly converted by the overwhelmingly positive data; the coating made almost every stencil tested perform better, and prompted an instant 5% overall print yield increase when implemented in production.1,2

The original formulators have now introduced a next-generation nanocoating product that improves printing performance, along with a new business model that makes it more affordable and more available. Eric Hanson, Aculon’s vice president of technology, presented the science of how and why the technology works at the technical conference, while Edward Hughes, Aculon’s CEO, discussed supply-chain improvements on the show floor. I was able to speak with both, and they answered many of the questions that process engineers and printing specialists have been asking each other for the past two years.

What is it and how does it work? The coating is known as a Self-Assembling Monolayer of Phosphonates, or SAMP material (sidebar). These films are literally one molecule thick. They don’t change the surface roughness or topography; they change the surface energy. SAMP surface coatings are engineered to produce specific properties; they may be hydrophobic (water repelling), oleophobic (oil repelling) or fluxophobic (flux repelling).

The ultrathin (<5 x 10-9m) coating prevents flux from wetting out on the stencil. Poor wetting equals poor adhesion, so in the solder paste tug-of-war of stencil-PCB separation, the stencil quickly loses its grip on the paste. The minute “strings” of paste that would stretch between the stencil and the PCB as the two separate break free from the stencil, rather than snap back to it, keeping its underside cleaner. The flux-repellent surface also prevents solder paste from wicking out around the edges of the apertures during the print stroke,3 further limiting bottomside paste buildup. The cleaner contact surface results in better gasketing, less squeeze out, and crisper print definition.

How can a user tell if the coating has worn off? This has been a major concern since its introduction. Nobody wants to wait for performance drops to signal time for reapplication. The solution is a dyne pen: a felt tip marker filled with dyne fluid. It’s a basic go/no go gage; if you swipe the marker across the stencil and the fluid beads up, the coating is still robust. If the mark doesn’t bead up, it’s time to reapply.

In the laboratory, more quantitative indications of surface energies are acquired by measuring the contact angles of oil and/or water on surfaces. Higher surface energies produce good wetting and lower contact angles; lower surface energies produce poor wetting and higher contact angles. Untreated stencil foils exhibit oil contact angles of approximately 10˚, whereas treated foils exhibit oil contact angles of approximately 87˚.

How long does the coating last? The coating is designed for durability. Users of the original formulation cite reapplication intervals of 25,000 wipe cycles or more, a very long time for many print processes. Given that one of the prime benefits of this treatment is extending wipe frequencies, 25,000 wipe cycles could become a very, very long time.

At Apex, Hanson presented results of multiple durability tests, including one that demonstrated robust coating presence after 100,000 abrasion cycles, outlasting vacuum deposited nanocoatings as shown in Figure 1.4



Will stencil cleaning solvents attack the coating? No. The new coating has been compatibility tested with one major manufacturer of stencil cleaning chemistry and is currently undergoing testing with another. Single application coatings have withstood 160 15-min. cleaning cycles in typical stencil cleaning chemistries with no degradation in repellency. That’s equivalent to cleaning it once every day for almost six months, which is more cleaning than most stencils see in their lifetime. If stencil cleaning processes use solvents that have not yet been tested and approved, chemistries with a pH <9 are recommended.

What’s the new performance improvement? In addition to reaching new levels of durability for nanocoatings, the new formulation is reported to improve fluxophobicity, as it is 50% more concentrated than the original. The extra “slipperiness” of the stencil should extend underwipe frequency intervals even farther than its predecessor. Users testing the new SAMP reported incredible – almost unfathomable – results: 40 prints per wipe on 0.5mm µBGAs, as shown in Figure 2. Hanson comments on the performance feedback: “We understand PCB assemblers’ needs for better print quality, and are extremely pleased that this product makes such a remarkable improvement in the overall process.”



When testing the original formula with Vicor’s Ray Whittier in 2011, we didn’t test for wipe frequency, but found an enormous impact on print yields,5 as shown in Figure 3. We tested 13 pairs of stencils of varying composition and quality, and it boosted yields on all but two pairs. (Note that those were seemingly beyond help, originally producing only 0 and 20% yields.) The results were so dramatic that Whittier immediately implemented the coating as the Process of Record, applying it to all stencils in his operation. We have plans underway to test the new coating formulation on a similar test vehicle and quantify the performance differences.



Can it be used on any type of stencil? Yes, the coating bonds to metallic oxides, and is compatible with electroformed, electroplated or laser-cut nickel, as well as the many stainless steel alloys available. (As an aside, I just finished another big stencil study  in which laser-cut SS outperformed electroform for the third time in as many years.)

Does it actually coat the aperture walls, or just the bottom surface of the stencil? The product volume is formulated to treat both the underside and the aperture walls. The company developed an instructional video in which Hanson demonstrates the application process and offers tips for achieving optimum coverage (aculon.com/stencils.php).

Does it contain ionic materials that could pose reliability risks to solder joints? No. When properly applied, the cleaner/treatment chemistries will not contaminate downstream processes. While the cleaning solution does contain some ionic materials, they are removed during the rinsing step with water. The monolayer treatment step does not contain any ionic materials that could compromise subsequent processes.

What’s the cost per stencil? The cost of the two-packet system has been reduced from $40 per stencil treatment to $25. At $40, I thought it was a bargain because it was still less expensive than reworking a single BGA with a print-related solder defect. At $25, the deal just got sweeter and the ROI faster. One user quantified the payback period in wiper paper alone: about 170 wipes with standard paper, or one roll of the really good stuff. I want to apply it not only to my stencils, but also to my board support tooling and squeegee blade holders to make them easier to clean on changeovers.

The two-part coating system is available to stencil manufacturers and PCB assemblers, giving the latter the option to specify it on their stencils or apply it themselves. Because all parties have equal access and pricing, assemblers should expect a service fee if the stencil shop applies it. The application process is pretty straightforward; details are in the video.

With two years of limited distribution under its belt and a loyal following of users, SAMP-based stencil nanocoating is ready for prime time. It’s going to change the way most operations manage and control their printing processes over the next few years. But unlike some of the previous breakthrough technologies that advanced stencil printing, it doesn’t come with a hefty price tag or a steep learning curve. It’s $25, plug and play. This coating’s low price, widespread availability and ease of use may quickly make it the de facto standard for all SMT stencils.

What is SAMP Technology?

Self-Assembling Monolayer (SAM) materials are specially designed molecules that automatically organize themselves and bond to form engineered chemical structures, without requiring any catalysts or forming any byproducts. The molecules have head and tail groups; the head groups bond to a substrate, while the tail groups bond to each other and deliver the desired functionality.

Phosphonates (P) are organic compounds that bond readily with metal oxides to chemically “seal” surfaces, and are commonly used to inhibit corrosion or scale because they are extremely stable in harsh environments.

The SAMP molecule developed specifically for stencil nanocoating consists of a phosphonate head group that covalently bonds to the oxides on the stencil’s surface and a specially selected tail group designed to repel nearly all flux formulations. The molecules set up and form a single-molecule layer within seconds of their application. Familiar SAMP applications include surface treatments for sunglass and optical lenses, PDA screen cleaners, watch displays, nozzles and needles.

References
1. Chrys Shea, “SMT Stencils from a Production Perspective,” CIRCUITS ASSEMBLY, December 2011.
2. Chrys Shea, “What Used to be Old is New Again?” CIRCUITS ASSEMBLY, February 2012.
3. C. Ashmore, M. Whitmore and J. Schake, “Big Ideas on Miniaturization,” Proceedings of IPC Apex Expo, February, 2013.
4. R. Bennett, Ph.D., and E. Hanson, “Low Surface Energy Coatings, Rewrites the Area Ratio Rules,” Proceedings of IPC Apex Expo, February 2013.
5. Chrys Shea and Ray Whittier, “Evaluation of Stencil Materials, Suppliers and Coatings,” Proceedings of SMTA International Conference, Fort Worth, TX, October 2011.

Chrys Shea is founder of Shea Engineering Services (sheaengineering.com); This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

Last Updated on Monday, 01 April 2013 10:45
 

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