Stencil printing equipment

A low-volume prototype screen- and stencil-printing machine is shown in Figure 6.18. This machine is equipped with a porous stone vacuum chuck that is specifically designed for handling tape layers. Unlike a prefired ceramic substrate, the green sheets of LTCC, HTCC, or MTCC are fragile and must be handled with care however, adequate force must be applied... 

The screen defines the pattern of the printed film and also regulates the amount of paste that is deposited. It is a very important part of the screen-printing equipment, and is essentially a stencil through which the paste is forced during the printing process. The most conunon type of screen consists of a frame, normally cast aluminum, onto which a finely woven mesh is stretched. 

The lead-free transition will have some impact on the stencil printing process. Printability of solder paste depends on flux formulation, but not for all alloy types. Experience has proven that there are no differences when printing lead-free solder pastes than when printing tin/lead solder pastes. Formal testing has verified that the printed volume of several lead-free solder pastes and tin/lead solder pastes were the same statistically when using the same stendl, printing equipment and boards (Figure 2). 

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. 

As with most processes, numerous variables affect the final product. For solder paste deposition using stencils, Chouta and Heck identified 39 variables relating to materials, 32 variables to processes and equipment, and 10 variables to personnel and environment. Among variables critical for the successful stencil dispensing of adhesives are stencil design, the adhesive material and its rheological properties, and the printing process. 

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... 

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