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High-resolution electroformed stencil manufacturing method for ultra fine pitch wafer bumping technology

: Pudas, M.; Manessis, D.; Patzelt, R.; Hagberg, J.; Leppävuori, S.; Vähäkangas, J.; Ostmann, A.; Reichl, H.

International Microelectronics and Packaging Society -IMAPS-:
15th European Microelectronics and Packaging Conference & Exhibition 2005. Conference programme & proceedings : June 12 - 15, 2005, Brugge, Belgium, IMAPS
Reston, Va.: IMAPS, 2005
European Microelectronics and Packaging Conference and Exhibition (EMPC) <15, 2005, Brugge>
Fraunhofer IZM ()

Wafer-level bumping technology using solder paste stencil printing methods relies heavily on stencil aperture wall quality for good paste release as well as on aperture dimensional accuracy and consistency. At the technological front of Ultra-Fine-Pitch (UFP) (<120 µm pitch) bumping, the challenges concerning manufacturability of laser cut stencils as well as the associated cost for opening a large number of apertures have driven us to initiate a feasibility study to explore the capabilities of electroformed technology for UFP stencils. Specifically, a comprehensive experimental matrix has been designed which incorporates peripheral and area array structures with respective pitches of 100, 80, 60, and 40 µm. The test designs had to be produced on both 20 µm and 30 µm thin foils manufactured from nickel with an additive electrolysis method. The finest apertures demonstrated had dimensions of 20x40 µm2 at 40µm pitch with very smooth inner walls. Depending on the process parameters, the tapering angle of the apertures has ranged from 3° to 9° wider on the temporary smooth substrate surface, which constitutes the wafer side for stencil printing. The bath-side of the stencil has been smooth with an average roughness value RzDin of 1300 Å, whereas the temporary substrate side has a roughness RzDin of 90 Å. This enables the bath side surface to be used as the squeegee side for stencil printing. Manufacturing-related issues are discussed and extensive cross sectional and SEM analysis of the apertures at UFP pitches is presented. Further development possibilities are suggested for even harder and smoother stencil surfaces. With appropriate tuning of the process parameters, 20 and 30 µm thick stencils were made.