Authored By:
Rita Mohanty Ph.D., Speedline Technologies, Franklin, MA
S. Manian Ramkumar Ph.D., CEMA, Rochester Institute of Technology, Rochester, NY
Chris Anglin, Indium Corporation, Clinton, NY
Toshitake Oda, Bon Mark Co. Ltd., Japan
Summary
The demand for product miniaturization, especially in the handheld device area, continues to challenge the board assembly industry. The desire to incorporate more functionality while making the product smaller continues to push board design to its limit. It is not uncommon to find boards with castle like components right next to miniature components. This type of board poses a special challenge to the board assemblers as it requires a wide range of paste volume to satisfy both small and large components.
One way to address the printing challenge is to use creative stencil design to meet the solder paste requirement for both large and small components. Examples of stencil design include step stenciling, dual printing, over-size apertures, etc. The stencil printing process, at its most basic level, involves pushing solder paste through a stencil (with various size apertures) by a squeegee blade. As the squeegee blade and the stencil are in constant contact with the paste during the printing process, their surface characteristics play an important role in the printing process. The most important attribute of a stencil is its release characteristic. In other words, how well the paste releases from the aperture. The paste release, in turn, depends on the surface characteristics of the aperture wall and stencil foil. The recent introduction of a new technology, nano coating for both stencil and squeegee blades, has drawn the attention of many researchers.
As the name implies, nano-coated stencils and blades are made by conventional method such as laser-cut or electoform then coated with nano-functional material to alter the surface characteristics. This study will evaluate nano-coated stencils for passive component printing, including 01005. Various print experiments will be conducted using different stencil technology, stencil thicknesses, aperture size, aperture orientation, aperture shapes, and selected paste type, with optimal print parameters to understand the effect of chosen factors on the print quality. Print quality will be determined by visual inspection and 3D measurement of the paste deposit to understand the volume transfer efficiency.
Conclusions
- An extensive experimental work was conducted using four different types of stencil technology and a wide range of area ratios. Based on current study and preliminary data analysis, we can make the following conclusions. In general, TE increases and STD decreases with an increased AR for passives, with the exception of 0402. The anomaly observed for 0402 needs further investigation.
- Based on the TE analysis, nano-coated stencils show a slightly better performance as compared to those without nano coating, for 01005 components. This is consistent with other reported results.
- Among the three types of nano-coated stencils, the advantage appears to be AR dependent.
- The stencils with dishing edges provide higher volume transfer, but inadequate control of the volume transfer.
- For ARs between 0.45 to 0.55, Laser with Nano stencil seems to provide the highest TE, while for ARs between 0.56 to 0.65, Polished E-form with Nano seems to be the better choice. Further investigation is required to confirm this finding.
- Based on the preliminary analysis, rounded corner apertures appear to provide slightly higher TE compared to square corners. This is consistent with the theory presented earlier. Also, there was no significant difference between the 8x10 rectangular apertures and the 9x9 square apertures.
- Finally, the Cp analysis for various AR and stencil types shows a distinct advantage of electroform with nano coating over other stencil technologies. Initial results indicate that with electroform with nano stencil may have the capability to be effective for AR of 0.48 and above. This finding needs to be validated with additional experimental work.
Initially Published in the IPC Proceedings
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