Stencils Go 3D Print E-mail
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Written by Clive Ashmore   
Thursday, 31 May 2012 14:38



Or why cavities might be good for you after all.

Those of us raised in the era of SMT printing see the world as flat. The printed circuit boards we’re used to dealing with are nice, level layers of laminate and copper. So, from a printing point of view, it’s pretty straightforward: flat board comes in; flat stencil gaskets to board; board gets printed, done. (Well, in the age of miniaturization and high-speed production, it’s not really that simple, but you get the idea.)

By and large, the flat board scenario accounts for 90%-plus of the processed PCBs in the market. But, there are those applications that are a little more “Christopher Columbus”-like and are definitely more topographically complex. For those assembly specialists involved with these niche applications, the world is certainly not flat. Substrates for automotive, industrial and power devices tend to divert from the standard FR-4 PCBs that we’re generally accustomed to. With these applications, it’s not uncommon to find PCBs with preforms, features or structures already attached to them prior to component assembly. Examples might include structures such as heat sinks, metal or aluminum reinforcement bars attached to the side of the PCB for strengthening or chassis attachment, and some LED products. These elements are part of the board fabrication process and, therefore, require adaptation during the assembly phase.

With these boards, the SMT production engineer now has substrates with positive Z-axes and, therefore, has two choices: print on the structures or print around them. Historically, this would require two separate processes: either two printing steps or a printing and dispensing combination. Anyone who has worked with dispensers knows that dispensing paste can be a delicate balancing act. Particularly with highly miniaturized devices, dispensing just the right volume and consistency is challenging at best. Plus, incorporating a dispense unit to address dimensionally complex boards means added capital expenditure. A third option is the use of preforms, where flux is applied and the solder preforms are placed on top of the flux. But, again, this requires additional equipment and adds a process that increases cost. Add to this the lack of ability to address fine features and the traditional options aren’t that appealing.

New 3D stencil technology, however, helps resolve many of the historical challenges associated with printing positive (or negative) Z-axis boards. 3D stencils are electroformed stencils that are nickel-formed and grown around a mandrel with apertures that can be either grown or laser-cut. The 3D stencil effectively allows printing on substrates with different levels by enveloping existing board topography and enabling printing in the positive or negative Z-axis direction, thereby streamlining production and reducing cost by enabling a single process (Figure 1). Processing with a 3D stencil requires a specially cut squeegee that conforms to the shape of the stencil – either in the positive or negative direction. Envision a squeegee with slits cut into it, much like a comb; this is what a 3D stencil-compatible squeegee looks like.



It’s worth mentioning that those brick-like solder paste deposits that we all have modeled in our minds are not achievable with 3D stencil – well, at least not at this moment in time. The volume, accuracy and repeatability (the “process holy trinity” of solder paste printing) are achievable, but the deposit topography is more conical.

As a key enabler for next-generation technology and packaging, 3D stencils have the potential to vastly alter previous perceptions of printing capability. In fact, our company is already working with several manufacturers that are putting fine-pitch devices onto boards that are not flat. Though far from mainstream at the moment, we have successfully proven 0201 and 01005 3D stencil print capability in a lab environment and fully expect robust performance in a production setting as well.

As mentioned, it’s not only positive Z-axis product where 3D stencils have shown effectiveness, but also with negative Z-axis PCBs. Commonly referred to as cavity printing, negative Z-axis designs are emerging as an effective technique for putting more functionality into a very small, defined footprint. The main driver for this, naturally, is mobile technology, but cavity designs can also be found in devices that have pre-set size limits – products such as hearing aids, for example. There is a certain amount of space, and all the function has to fit into that space, end of story. By placing components into recesses (cavities) in the board, the required space constraints can be met, and some designers might argue that better clock speeds result as well because the I/O is closer to the action.

With 3D stencils, the design is really only limited by the imagination, as they can be manufactured to conform to any shape or topography. And, with device space limitations continuing to be even more restrictive, 3D stencils may just be the shape of things to come.

Clive Ashmore is global applied process engineering manager at DEK International (dek.com); This e-mail address is being protected from spambots. You need JavaScript enabled to view it . His column appears bimonthly.

Last Updated on Friday, 01 June 2012 14:43
 

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