SMTA International Conference Proceedings


STENCIL DESIGN FOR MIXED TECHNOLOGY THROUGH-HOLE / SMT PLACEMENT AND REFLOW

Author: William E. Coleman
Company: Photo Stencil
Date Published: 9/12/1999   Conference: SMTA International


Abstract: This paper will review stencil design requirements for printing solder paste around and in Through-Hole pads / openings. There is much interest in this procedure since full implementation allows the placement of both Through-Hole components as well as SMD’s and the subsequent reflow of both simultaneously. This in turn eliminates the need to wave solder or hand solder Through-Hole components.

Material type, pin type, lead length, and stand-off height of the Through-Hole components will be reviewed. Board design issues including Through-Hole hole size, pad size, board thickness, and solder mask type will also be reviewed. Three stencil designs will be considered: (1) Single thickness stencils with oversized stencil apertures for overprinting solder paste in the Through-Hole pad areas; (2) Step stencils with oversized stencil apertures for overprinting solder paste in the Through-Hole pad areas; (3) Thick stencils (15mil to 25 mil thick) for printing solder paste in the Through-Hole pad areas. The latter thick stencil is the second stencil in the Two-Print stencil process.

The selection of stencil designs depends on several factors: Total solder volume requirement to fill the Through-Hole around the pin and form a proper fillet; Board thickness; Pin diameter; Through-Hole opening; Position of the Through-Holes in the board layout; Pitch of the Through-Hole; Surface finish of the solder mask. Several examples will be reviewed with the recommended stencil designs.



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