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Clive Ashmore

Conventional stencil technology isn’t going to hold up for microminiature devices.

To say there are printing challenges with some of today’s finer-featured devices would be an understatement. Miniaturization has pushed printing technology to what I like to call the “bleeding edge” of capability, and it’s not stopping. You and I are to blame, really. We want it all from our mobile devices: incredible image resolution, connectivity anytime/anywhere, camera and video quality that rival professional lenses; the list goes on. Of course, to get these awesome features in a thin, portable package, components must become thinner and more compact, and component density per assembly must increase dramatically.

For printing, this dynamic presents unique challenges, many of which we have addressed in this space before. We’ve discussed how using the latest printing techniques and some innovative technologies, such as activated squeegees, stencil coatings, closed-loop offset feedback systems, advanced SPI tools, successful material deposition for devices such as 0.3mm CSPs, 01005s and even metric 0201s, has been achieved. And, these results have been realized using standard thickness, 100µm stencils.

For those who haven’t yet invested in some of these latest technologies, however, a newer stencil material is helping deliver on the challenging area ratios found at the world’s top mobile manufacturers. Fine grain stainless steel, as the name implies, has a finer grain structure than standard stainless steel (1 to 2µm, versus 10 to 25µm, respectively). This material can be used with conventional mesh-mounted stencils or with interchangeable foil systems. In fact, from a manufacturer’s point of view, using fine grain stainless steel stencils alters very little in terms of process and logistics. It’s simply a change to the base material. What fine grain does deliver, however, is cleaner, smoother and more uniform laser-cut apertures, which results in better paste release of area ratios below 0.66. In addition to the transfer efficiency benefits, there are other advantages to fine grain stainless steel stencils. The finer structure results in a smoother foil surface finish, which improves understencil cleaning performance. Stencil-to-board gasketing is better due to the flatness of the material, and, over the long term, this increases the life of the stencil. Traditionally, electroform or nickel materials have been sought for their robust release properties, but fine grain offers similar performance for certain aperture dimensions at a lower cost.

Because of the clear process advantages of fine grain stainless steel stencils, mobile device manufacturers are increasingly using them in production to enable more robust deposits for challenging devices such as 0.3mm CSPs. However, they are doing so with a stencil thickness of 80µm and not the more common 100µm.
With this thickness and without the other tools mentioned previously, there is still quite a bit of pressure on the manufacturing process to enable heterogeneous assemblies, but tight process control is yielding some success. It’s interesting to note that even an 80µm stencil made from standard stainless steel wouldn’t have you off to the races, but the fine grain material gives transfer efficiencies that are good enough to satisfy the miniaturized and larger material deposits in a single stroke.

Lurking on the sidelines, however, is the 03015 device. Will fine grain at 80µm be enough on its own to accommodate sub-0.45 area ratios? My gut says probably not. As mentioned previously, there are techniques for printing down to area ratios of sub-0.45 using an activated squeegee technology and a standard 100µm stainless steel foil. I suspect that, based on the successful results we’ve realized for metric 0201 printing, 03015s would benefit from this same methodology. I don’t believe that simply moving to an 80µm fine grain stainless steel stencil on its own will deliver on the challenging requirements for 03015s in a production environment. And, taking the stencil thickness down any further – even with a Type 5 solder paste material – won’t provide the results needed for heterogeneous assembly.

Honestly, I think when 03015s become mainstream (read: for mobile device manufacturing), it’s going to take everything we’ve got in the toolbox for a good printing outcome. We’ll need stencil coatings, activated squeegees, high-performance solder pastes and fine grain stencils.

It’s getting really interesting!

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.

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