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News Release from: Tecan
Edited by the Electronicstalk Editorial Team on 22 December 2004

Stencils smooth transition to lead-free processing

A combination of proven design features will ensure solder paste printing stencils offer the best possible performance when faced with present-day and future challenges facing PCB manufacturers.

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A combination of proven design features present the most efficient way of ensuring solder paste printing stencils offer the best possible performance when faced with present-day and future challenges facing PCB manufacturers, says Dorset based market leader, Tecan. During the electronics industry's transition to lead-free manufacturing an increasing number of the company's stencil design features are proving beneficial by satisfying problems such as poor deposition, paste quality and post-print cleaning. Particularly, with lead-free solder pastes, the company's photo-electroformed (PEF) stencils offer advantages such as optimum paste release, paste-roll activation (PRA) and no-clean surfaces.

In custom combinations these features can maximise yields and minimise post-print cycle rework for low and high-volume multicomponent PCB manufacturers.

Overall, this can make PEF stencils far more cost-effective than laser-cut alternatives.

Core to the successful deposit of the appropriate amount of paste on every pad, every time, is the cross-sectional shape of the individual aperture - which should ideally be trapezoidal and with smooth side walls, says the company.

Also, PEF stencils are produced in nickel which inherently has a lubricational quality, further enhancing paste release.

The manufacturing technique is being increasingly recognised by the industry as the optimum and most cost-effective method of producing such stencils in a single manufacturing process, without the need for subsequent post-forming cutting or smoothing operations.

A PEF stencil can be cost-effectively fabricated with areas of different thickness across its surface.

This allows a multitude of different components to be catered for, such as fine and ultra-fine-pitch SMT components, BGA and uBGA discreet devices, power components and connectors - and increasingly to individual silicon die with multiple super-fine termination pads.

In each case, the relationship between the aperture size and the stencil thickness can be optimised for the most advantageous paste quantity and release characteristics Ongoing development and optimisation has led to the implementation of the proprietary paste roll activation (PRA) technique, across the company's range of enhanced stencils, including PEF, precision laser and laser-formed variants.

PRA is effectively a micro-fine pattern created on the squeegee side of the stencil which has been designed to significantly improve paste roll.

The PRA wave pattern assists in activating and 'conditioning' the paste to improve its rheology.

This paste conditioning also extends the printing life of the paste.

Effective paste roll is an important prerequisite for successful solder paste printing and the PRA pattern encourages paste roll within 5 to10mm of the start of the printing stroke, thereby reducing the run-on and run-off distances which ultimately increases the print area.

The PRA pattern delivers a better aperture filling capability than non-PRA stencils and is proven to be particularly beneficial for fine pitch applications in both leaded and lead-free production.

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