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Stencil selection for perfect printing
10 August 2009
There are three fundamental stencil technologies available to the electronics assembler today; chemical etching, laser cutting and electroforming, but how should you choose between them? Martyn Penfold, Managing Director of The Solder Connection explains.
Chemically Etched Stencils offered longer life than their polymer screen predecessors, but with the advent of SMT fine line technology in the early ’90s, Laser Cut stencils, (pioneered by AlphaMetals) took over and laser is now the most available technology in our industry.
Laser Cut Stencils, cut from stainless steel using the original board CAD data, have been recommended for solder paste and surface mount adhesive printing applications, including:
• Component technology ≥ 20 mil / 0.5 mm pitch
• Prototyping to medium volume production runs (1-50K)
• Low to medium aperture counts (1-4000)
• Standard thickness requirements (150 - 300μM / 6-12mil)
For high end, high volume applications, electroforming has been the technology of choice in recent years.
Electroformed Stencils, (generally nickel) are highly durable and highly precise, designed for challenging applications.
Produced from modified CAD data, electroformed stencils have been recommended for high-end solder paste applications, including:
• Component technology ≤ 20 mil / 0.5 mm pitch, Area Array type packages and < 0402 devices
• Medium to high volume production runs (50-100K)
• Medium to high aperture counts (≥ 4000)
• Custom thickness requirements and ≤150μM / 6mil thickness
These general rules served the industry well for over a decade but recent developments in laser technology combined with a better understanding of the printing process has had the effect of transferring the emphasis away from the technology to the stencil suppliers themselves.
The accuracy of each aperture becomes ever more critical as miniaturisation continues. Experience indicates that reducing the size of a 12mil aperture in a 5mil stencil foil by a mere 8% (-1mil) reduces the volume of paste deposited by some 34%.
Clearly, Laser cut stencil suppliers, such as Alpha, using the latest technology are able to offer higher degrees of precision. Aperture accuracy to ± 6 microns is achievable with the latest machines, taking laser cut stencils into realms once dominated by E-Form stencils.
A supplier able to offer all three manufacturing technologies is better placed to provide optimum solutions. Laser cut stepped stencils and laser cut nickel stencils both have a role to play in optimising the printing process.
Modifying aperture shapes and changing sizes can help optimise the printing process and reduce the incidence of specific problems such as mid-chip solder balling. Proficient suppliers, such as Alpha, will modify the CAD data automatically while giving the option for customer specific variations.
Reducing foil thickness in certain circumstances can reduce or even increase the solder paste deposit volume, depending on the aperture size and area ratio. As devices become smaller, some of the truths we held about stencils no longer apply. For this reason alone, the experience and expertise accumulated by stencil suppliers will become ever more invaluable in the future.
In essence, do not select the stencil manufacturing technology – choose the stencil supplier.
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