Written off laser-cut stainless steel stencils? Not so fast.

I’ve been sharing my takeaways from last summer’s Big Stencil Study with Ray Whittier of Vicor. If you missed the first installment of my overview (see December 2011), here it is in a nutshell: We tested 13 pairs of stencils, made from four different materials/manufacturing methods in two thicknesses, supplied by four vendors. Each pair of stencils was theoretically identical; one had a do-it-yourself nanocoating applied and one didn’t. We used a pretty challenging test vehicle that packed almost 15,000 apertures into a 3 x 7" area. We found that electroformed stencils and foils disappointed us; bottom-side DIY nanocoating didn’t help paste release, but worked absolute wonders for overall print yields, and quality varied immensely from one supplier to another.

Moving on to more findings.

The measured variations in aperture size were appalling. We asked for 0.0108" circular apertures for our µBGAs; we actually got them as small as 0.0094". On a 0.0045" foil, that takes the area ratio down from an already unattractive 0.6 to a totally ugly 0.5 – unacceptable. Ironically, the extra-small apertures were from the only vendor of three that could hold a decent thickness tolerance on its electroformed foils. How frustrating! Of the 17 nickel stencils tested, only two met their size spec, but those two produced terrible yields. I’ve been told it is possible to get good electroformed stencils, but at this juncture, I would need a mountain of data to even consider trying them again.

Twenty years ago, electroforming was a breakthrough technology that enabled a lot of finer-pitch printing, especially with the early no-clean solder paste formulations of the era. At that time, electroformed nickel outperformed the incumbent laser-cut or chemical-etched stainless steel (SS) hands-down with respect to paste release. That was then; this is now. SMT layouts are denser; pastes are more printable, and it would appear that both the SS stencils’ machine and materials suppliers have stepped up their game and retaken a leadership position.

The SS stencils we tested outclassed the nickel in every performance category. The materials exhibited the best thickness uniformity with nearly zero variation. The aperture sizes and locations were more consistent than the electroformed products (including both fully electroformed and laser cut nickel). Their transfer efficiencies were better, and their print yields were higher. When compared to the nickel stencils in our tests, SS had no downsides or performance tradeoffs.
This new generation of laser-cut SS is not the stuff of the 1990s that was so easily bested by the e-forms. Nope, these next-gen SS stencils are proving to be the truest tooling we’ve yet to experience. Two different SS alloys were put to the test; one has been on the market for several years and claims to be “optimized” for laser cutting; the other is a more recent market entrant that boasts a refined grain structure. Both performed head-and-shoulders above the nickel products, and their performance was so similar that we could not actually discern a statistically significant difference between the two.

The brand of laser cutter did not matter, but the type of laser did. The top-performing stencils were produced by two different vendors. Both used the same two SS materials from the same supplier, but each used different laser cutters from competing equipment manufacturers. While the machines were different, both were the newest models, employing the latest and greatest fiber lasers. This type of laser boasts numerous advantages over its predecessor, including a finer beam to produce smoother walls and tighter corner radii. It’s also got a lot more parameters that can be programmed and controlled, giving the stencil manufacturers more opportunity to dial in their cutting processes for individual materials.

New SS plus new lasers equals a new value proposition for SMT assemblers. The previous best-in-class stencil (for this particular SMT operation) was laser-cut nickel. Nickel stencils, whether fully electroformed or laser cut, are more expensive than stainless. We now have an opportunity to get more performance at a lower cost. That doesn’t happen very often.

While we’re on the topic of value … you don’t always get what you pay for, unless you buy from the lowest-cost provider. While the least-expensive vendor’s stencils produced results so unexpectedly poor that we had to change the scales on all our charts to plot them, the most-expensive vendor’s stencils did not actually produce the best results. In fact, the best performance came from the mid-priced vendors – and two companies actually tied for the honor! Both winners are companies focused on stencil manufacturing, staffed with extremely proficient specialists, and fully invested in modern equipment and materials. The prices they quoted were fair, reasonable and in the middle of the range.

Don’t ask us who the two best vendors were; we’ll never tell. The advice we will provide, however, is:

• If you’ve written off laser-cut stainless steel, give it another chance.
• Use a supplier that specializes in stencils or PCB assembly tooling.
• Make sure the supplier is using name brand, high-quality material.
• Ask about their cutting machine. You can verify it is the latest model on the machine vendor’s website.
• Never, ever purchase stencils based on price alone.

If a year ago you asked my opinion on the best stencil technology, I would have said laser-cut nickel. But over the past 12 months, I’ve worked on two extremely well planned and executed studies, with two different top-tier SMT assemblers located on opposite coasts of the US. Both studies independently reached the same conclusion: The new generation of SS, cut on the new generation of lasers, is the new best-in-class when it comes to accuracy, repeatability, transfer efficiency and overall performance.

It may have taken the better part of two decades for laser-cut SS to retake the lead in SMT stencil technology, but these recent developments beg the question: Will electroforming answer back with similar performance improvements? We certainly hope so. When suppliers compete, customers win.

Chrys Shea is founder of Shea Engineering Services (sheaengineering.com); chrys@sheaengineering.com. She wrote this article on behalf of Christopher Associates (christopherweb.com).

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