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December, 2012 Production
Auto-Focus Mighty Scope Stay focused in on the task at hand Order # 26700-203
Step Stencils: Where and Why to Use Them
By William E. Coleman Ph.D., Photo Stencil, Colorado Springs, CO T $29500 00
he demand for step stencils is increasing. Why? Shrinking components. Step stencils offer significant flexibility in achieving the proper solder paste height and volume for the overall paste printing process. Steps are
available on the PCB side, squeegee side, or on both sides of the stencil. There are step-down and step-up stencils. Relief step-board side stencils eliminate the gap between the stencil apertures and the pads, allowing the stencil to cleanly gasket to the PCB. Step stencils are available in laser-based, NicAlloy-XT, and Efab electro-
formed. Mixed technology applications of solder paste printing for through- hole/SMT as well as solder paste/flux printing for flip-chip/SMT require spe- cial step stencil designs. Thick metal stencils with both relief etch pockets and reservoir step pockets are useful for glue and paste reservoir printing. Electroform and laser-cut step-up stencils for ceramic BGAs and RF
shields achieve additional solder paste height on the pads of these compo- nents. Special 3-D electroform stencils are an excellent solution for odd PCBs having some raised areas on the board.
HIGH RESOLUTION Digital Microscopes
zipScope 9M w/Polarizer Cut the glare, clear high-res images Order # 26700-302
Ceramic BGAs present a challenge since high melting temperatures prevent solder balls from melting at normal reflow temperatures. Coplanarity prob- lems can result in an open contact. Printing higher solder bricks (7-8 mils) on the CBGA pads prevents this. SMT components, like 0.5mm pitch QFPs, 0402 chip components, and R-Packs, won’t tolerate an 8mil thick stencil as aperture sizes are too small for good paste release. A stepped AMTX electroform sten- cil gives good release when 0201 chip components and 0.5mm pitch µBGAs are present.
Step-Up Stencil for Through-Hole Intrusive Reflow: When reflowing through-hole components along with SMT components instead of wave solder- ing, solder paste is printed on/in/and around the through-hole hole and pad
(annular ring).Three stencil alternatives achieve sufficient solder paste: l
Overprint the hole/annular ring with an oversized stencil aperture. l Step-up and overprint the hole/annular ring with an oversized stencil
aperture. l
Two-print stencil where the second print stencil is very thick and provides more solder paste for the requirement.
Relief Step Stencil with Relief Etch Pockets on the Contact Side of the Stencil. l
$19500 00 Order # 26700-312
depth is + of the stencil thickness, usually enough to clear the raised via. l
the stencil to sit flat on the PCB during printing. l
down clips. l
PCB. For good stencil/board contact a relief pocket is etched on the contact side of the stencil wherever there is a raised via. Typically, the relief pocket
Raised via pads on the PCB prevent the stencil from gasketing to the
to the board pads they can prevent the stencil from gasketing to the pads during printing. A relief pocket etched in the area of the bar code enables
Bar code identifiers are often attached to the PCB surface. When too close
the PCB and component pads during printing, can prevent gasketing to the board on the edge. A solution is a relief etch pocket in the area of the hold-
Step Stencil for Two-Print Operation for Mixed Technology. l
Board Hold-Down Edge Clips, if too close to the board’s edge when holding
PCBs with Additive Traces to correct design problems can add height to a board’s surface. A relief pocket around the additive trace solves the problem.
3D tand $4995 3D Stand 95
Mighty Scope 5M Inspect analyze and measure Order # 26700-209
two-print stencil operation is a viable solution. l
tures for the through-hole) with a normal step-up stencil doesn’t provide enough solder paste volume for proper intrusive reflow a thicker stencil must be used. Normal SMT components won’t tolerate a thick stencil (15-20 mils thick), and a step-down (20 mils down to 5 mils) is impractical. The
SMT/Through-Hole Mixed Technologies: When overprint (oversized aper-
or solder paste for flip chip components and solder paste for normal SMT de- vices. Both are placed and run through the reflow cycle. Normally, stencil thickness for flip chip printing is 1-2 mils; too thin for normal SMT printing. Two-print stencils fit this application. A thin, 1-2mil thick AMTX electro- form stencil is used to print either flux or solder paste on the flip chip pad sites on the PCB. A 5mil thick SMT stencil prints solder paste on all SMT pads. This stencil has relief pockets formed on the contact side anywhere flip chip flux or paste was previously printed.
Flip Chip/SMT Mixed Technologies: It’s sometimes desirable to print flux $24900 00
Thick Stencil with Deep Etched Reservoirs for Printing Glue for Component Attachment: Printing, rather than dispensing, glue reduces set-up time and shortens process time since glue bricks are deposited in par- allel rather than a dot at a time. It’s acceptable and useful to leave glue inside the stencil aperture. This allows printing different glue heights using the same stencil thickness. Large apertures release all the glue while small aper- tures release only a portion of it, useful when PCB components have different stand-off heights. A glue reservoir stencil can achieve up to 20-30 mils of glue height. The small apertures allow only a portion of the glue to release to the board where large apertures allow glue to be pulled down from the reservoir. A 3-D AMTX Electroform Stencil, 5 mils thick with a 95 mil high relief
4595 Platt Rd., Ann Arbor, MI 48108 734-973-0099
aveninc.com sales@aveninc.com
pocket formed in the stencil, solves the problem of printing solder paste on SMT pads with a flexible connector 90 mils above the board surface obstruct-
ing a normal stencil from making contact with the board. Contact: Photo Stencil, 4725 Centennial Blvd., Colorado Springs, CO 80919 % 719-599-4305 fax: 719-599-4334 Web:
www.photostencil.com r
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