Stencil printer

Adhesives — In terms of equipment, any stencil printer capable of printing solder paste for a fine-pitch SATT assembly can be used for adhesive printing. In fact, adhesive printing does not require any special features from the stencil printer. Often, semi-automatic equipment is adequate. The most important aspect of the printer is programmable snap-off (or printing gap) capability between the... 

After PCB design has been determined and materials have been selected, the next step is to review and select the appropriate equipment. It is important to consider future production demands when evaluating systems. There are several key criteria that a stencil printer should meet ... 

FIG. 33 Representative SMT assembly line. The tools depicted in the photograph are (a) stencil printer, (b) turret-style placement machine, (c) line-pitch gantry style placement machine, and (d) reflow furnace. (Courtesy of Universal Instruments Corporation). 

The process parameters key to the stencil printing process include the squeegee type and hardness, print speed, print pressure, and print gap. An optimal stencil printer setup provides a clean sweep on the stencil surface and a repeatable solder paste deposition process. Shore A scale polyurethane squeegee hardness level between 85 to 95 is appropriate to achieve repeatable solder deposits for lead-free solder pastes. Print parameters such as the print speed, print pressure, and the print gap are adjusted to accommodate the variety of solder paste rheologies available. There are no specific equipment modifications necessary to stencil printer lead-free solder pastes. 

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... 

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. 

In volume production, the printer must reproduce this precise alignment at production speeds. To ensure it can meet application requirements, the alignment process must be capable of greater precision than is required to meet physical alignment specifications. Ideally, a closed-loop vision system should control the alignment process to enable alignment to multiple fiducial points on the stencil and substrate. 

Mechanical Stability. The printer s ability to provide accuracy and repeatability requires a robust design. Structurally, the machine must be rigid enough to prevent relative motion between any axis and the rest of the machine. This can cause variations in the system s response — resulting in assembly defects. Rigidity also is required to ensure that stencil movement in relation to the board is accomplished in planes that are parallel within narrow limits. 

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