Stencil technology boosts lead-free PCB production

A Tecan product story

Edited by the Processingtalk editorial team Jan 1, 2004

Enabling the transition to lead-free production, innovative stencil design features, improved squeegee blade technology and added component protection have been announced by stencil specialist, Tecan

Enabling the transition to lead-free production, innovative stencil design features, improved squeegee blade technology and added component protection have been announced by stencil specialist, Tecan.

The next-generation solutions, specifically designed to improve the surface-mount technology (SMT) process, help to simplify the transition to lead-free manufacturing.

Improvements to solder paste deposit consistency and resultant solder fillets are achieved by employing multi-level stencils, polished electroformed squeegees and reflow reflector shields.

The company lead-free production solutions are based on extensive experience, practical involvement in joint research initiatives with the world's leading manufacturers and from its active participation in recognising the future requirements required to successfully implement lead-free techniques.

At recent practical seminars, in the UK and Scandinavia, where active customer participation was welcomed, an "optimised" multi-level stencil significantly outperformed a traditional single thickness stencil.

The enhanced stencil solution simultaneously demonstrated its capability to match the paste volume, and hence solder fillet requirements, for a diverse and yet typical component population, including 0201 discrete devices, microBGA, 0.400mm fine pitch and pin-in-hole-reflow (PIHR) connector requirements.

The experiences have allowed the company to clearly identify the issues critical to facilitating successful lead-free implementation, which can be broadly categorised into six main headings: PCB base material/ pad finish, Component type/ surface finish, Solder Paste alloy/ wettability, Reflow profile/ method and Resultant solder fillets, which are a function of stencil thickness and aperture design.

In addition to these issues, the company suggests that optimum paste release is also critical to consistently achieving the paste volumes required.

In response, it has developed highly-polished electroformed squeegee blades that help to overcome the rheology problems associated with less dense pastes.

These specialised squeegees reduce paste hang-up when the squeegee has completed the print-stroke.

They also work in conjunction with a unique micro-fine pattern, which is evident on the squeegee-side of the company's innovative stencils.

This micro-fine pattern improves paste roll characteristics to ensure that apertures with even the smallest of dimensions are filled.

It also facilitates a reduction in print stroke distance before the first printed aperture.

Adoption of lead-free soldering does involve temperatures above those currently used today.

The melting point of Sn, Ag, Cu (SAC) alloys is approximately 32C higher than the eutectic Tin/ Lead solders used today.

The temperatures required to ensure SAC alloys reflow effectively will be in the range of 220C to 260C.

Traditional leaded components can be used successfully within a lead-free environment by utilising PIHR techniques - where the paste is printed both onto the component pads and into the holes before placement and insertion - provided they are capable of withstanding the reflow temperatures.

Some "plastic" connector bodies, crystal oscillator modules and other temperature sensitive components have, however, until recently been excluded from this process to avoid thermal damage during the reflow process.

Tecan has recently developed "reflow reflector shields" which simply fit over the component, offering protection from the higher temperatures with reductions of up to 40C within the shield walls, thereby ensuring the survival of the component.

PIHR offers a cost reduction through the removal of one process step and improved solder joint quality.