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The Short Scoop
By Rachel Miller-Short
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Adapting Stencils to Manufacturing Challenges in 2015
Now that we're well into 2015, we can take a look back at the 2014 stencil market and explore our expectations for 2015.
In general, 2014 was a solid year with year-over-year growth in volume. Component sizes continued to get smaller, with 01005 components no longer a rarity, but a regular occurrence. Shrinking components have been accompanied by shrinking spaces between the components. This combination of small components and spacing constraints has continued to challenge stencil manufacturers. Combine that with boards that contain both 01005 components and standard size components, and the stencil manufacturers have had to head back to the lab to find ways to accommodate these challenging parameters.
In the solder paste printing process, the squeegee blade delivers solder paste into the stencil aperture as it travels across the stencil surface. When the board separates from the stencil the solder paste in the stencil encounters a competing process: solder paste will either transfer to the pad on the PCB or it will stick to the inside of the aperture walls. The smaller the area ratio (the area of the aperture opening divided by the area of the inside aperture wall; the generally accepted guideline is area ratio > .45, depending on technology), the more difficult it is to achieve complete paste release.
To achieve good paste release with apertures small enough to accommodate 01005 components, manufacturers have experimented with different stencil materials and technologies. The material the stencil is made from determines the size of the aperture and the smoothness and exactness of the aperture walls. The need to satisfy the more rigorous printing requirements of finer pitch components leads to a natural hierarchy of stencil technologies based on the application.
The hierarchy advances through the technologies, starting with laser cut and reaching to electroformed and electroformed NiEx stencils, which are used for very fine pitch SMT and wafer bump applications. It was discovered that the smoother walls associated with electroform stencil technology provide better paste transfer and that good paste transfer can be achieved at lower area ratios. The electroformed product lines can address additional application needs such as .1 mil increments in stencil thickness requirements. In fact, good paste transfer has been achieved with area ratios as low as .42 with electroformed stencils.
For 2015, we will see the gamut of stencil types. Laser cut stencils will be used for more R&D and simple low-volume products where the components are larger; the solder paste deposit doesn’t have to be as exacting, and where the boards are relatively uniform. Laser cut stencils have become quite commercialized and can be purchased in many places at a relatively low price. They fill the need for companies that want off-the-shelf stencils they can get quickly.
Editor's Note: This article originally appeared in the March issue of SMT Magazine.
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