New-generation wafer-bump-ing stencils from Tecan are production aids which ensure optimum solder paste release characteristics, resulting from mirror finish stencils with mirror finish aperture walls. These stencils are used to deposit solder paste directly onto silicon wafer. Following a reflow operation, the paste forms solder bumps required for interconnects. Using electroforming mandrels, the company produces highly accurate stencils which have apertures with exceptionally smooth side walls and a subtly narrower than normal trapezoidal cross-section. This ensures optimum paste release characteristics, improving the consistency of solder paste volume and hence, ball size and regularity. The stencils cater to pitches associated with components such as chip-scale packages (CSP), micro ball-grid arrays (BGA) and flip-chip designs. For example, a stencil for a 6-in wafer may require 300,000 apertures, or over 500,000 for an 8-in wafer, with hole diameters of 60 to 125 microns.
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