Screen printing stencil

Wood Miop, S.P., An overview of screen printing stencil systems, mesh properties and mesh colour in relation to print quality. Professional Printer, Vol. 45, No. 1, January/February 2001, pp. 34-36. 

The next critical component for this material was the development of the flux and activator system for the paste. This proved to be a key factor in the success of this project. As the wettability of lead-free alloys is lower than eutectic Sn-Pb, a suitable flux system was critical to the successful introduction of the MD s conversion to lead-free. In addition to the flux system having the properties to promote wetting, the requirements for screen printing, stencil life, tackiness, and so on were also important when evaluating a solder paste system. Flux technology is discussed in... 

Image Carriers for Stencil Processes. There are two stencil processes in general use screen printing and stencil dupHcating. Screen printing used for art reproduction is called seriography. 

Fig. 1. Diagram to show preparation of screen-printed electrodes. (A) Polyester card support, (B) screen-stencil, (C) ink suspension, and (D) squeegee (this is a variation after Fig. 3 in ref. 7).

This process can use either solid stencils or screen (mesh) stencils (see Figure 16.12). The latter is called screen printing and was originally known as silkscreen, as silk-type screens were employed. Present day screens are made of nylon, polyester or steel and are relatively low in cost, 45-75. Each screen may give 15,000 to 50,000 impressions. The ink is forced through the screen by a squeezer. Prior to this action the screen is separated from the item to be printed by a lift-off gap. This gap is reduced by the pressure of the squeezer whereby contact is made between the screen, ink and item. Screen inks are relatively thick, and are either air or hot air dried. The screen process can produce coarse half tones with thin inks. 

The ink is ultrasonically (or mechanically) broken up in to small particles, some of which are then charged, and deflected to the substrate being printed via a screen or stencil, while the remainder are condensed and circulated back to the ink reservoir. A speed of 600+m/min (based on a single line of dots) is possible. Up to six lines can be printed (see Figure 16.14). 

The main advantage of stencil printing over screen printing occurs in applications where very small areas of paste have to be deposited. For components with pitches equal to or smaller than 0.65 mm, the stencil printing process is the only viable way for printing solder paste. Therefore, stencil printing has replaced screen printing in most cases. 

In surface mounting of components, both solder paste and adhesive need to be dispensed. The solder paste is usually screen printed or stenciled first, followed by selective dispensing of the adhesive between the solder pads. The prime function of... 

Of the mass-transfer dispensing methods, screen printing and stencil printing are the oldest and most widely used. Screen printing has been used for over 40 years in the electronics industry to apply thick-film conductors, resistors, and dielectrics in fabricating circuits on ceramic and plastic-laminate substrates. Screen printing is also used as a batch process for depositing electrically conductive and insulative adhesives to interconnect devices on thin-film and thick-film hybrid microcircuits. 

Screen, stencil printing Surface mount, hybrid, or MCM/ stencil or screen printer Fast, multiple dot size. Dots as small as 3-mils diameter on 5-mil pitch can be printed Stencil printing requires a new stencil per design complex cleaning procedures for small aperture stencils cannot be used with other components on boards... 

Adhesives used for screen or stencil printing in surface-mount applications are generally electrically insulative types whose functions are mechanical attachment and thermal dissipation. However, electrically conductive, silver-filled epoxies have been used for many years as ohmic contact adhesives to interconnect bare-chip devices in hybrid microcircuits and are used as solder replacements for surface mounting of components on printed-circuit boards. Regardless of their... 

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