Companies expect different performance levels from laboratory and production printers and volumes differ. Screen printers available today can be split into two main categories, namely laboratory and production, which can be broken down even further.
Since a clear-cut equipment classification is near impossible, the best thing to do is to select a screen printer to match the desired application. The combination of the right paste material, the right tools and the right process is crucial in achieving the finest printing result.
Solder paste printing
Solder paste is used to connect the surface mount components (or terminations) to lands. In this process, the printer is crucial for achieving desired print quality.
While solder paste is dispensed automatically in the case of automatic printers, the paste is applied manually on the screen/stencil using a print squeegee at the end of the stencil when using manual or semi-automatic printers. The print squeegee presses down on the stencil during the printing process until the stencil bottom touches the top board surface. The solder paste is printed on the lands through openings in the screen/stencil while the squeegee navigates the entire length of the image etched in the metal mask.
The screen snaps off or peels away immediately behind the squeegee once the paste has been deposited and returns to its original position. The snap-off distance and squeegee pressures are two vital equipment specifications for good quality printing.
'On-contact' printing signifies no snap-off and occurs where an all-metal stencil or squeegee blade is used. 'Off-contact' printing is used with flexible metal masks and screens.
Rubber or metal?
Rubber or polyurethane squeegees and metal squeegees are the two most common squeegee types. Rubber squeegees should measure 70 to 90 durometer in hardness. With the use of finer pitch components, metal squeegees are also more commonly used. They are made from stainless steel or brass in a flat blade configuration, and are used at a 30° to 45° print angle. Some are also coated with lubricating material.
In the case of metal squeegees, the pressure is lower and they tend not to scoop paste from openings. Because they are metallic, they wear slower and require less sharpening. However, they do cost significantly more than rubber squeegees, and can cause stencil wear. For brilliant print quality, squeegee edges should remain sharp and straight and squeegee wear should be monitored vigilantly.
High squeegee pressure often results in smeared prints, while low pressure causes skips and ragged edges. High pressure could even damage the squeegee and stencil or screen. While excessive pressure will cause the squeegee to scoop solder paste from wide openings, resulting in insufficient solder fillets or cause the solder paste to bleed from underneath the stencil. To prevent underside bleeding, the pad opening must provide a gasketing effect while printing and is dependent on the roughness of the stencil aperture walls.
One needs to be aware of the ramifications of using different squeegee types in printed circuit assemblies, as both standard and fine-pitch components demand different solder paste volumes. A metal squeegee makes it easier to prevent variation in paste volume deposits, but the process requires a modified stencil aperture design to prevent excess paste deposition on fine-pitch pads.
Tried and tested or the latest rage?
The accuracy and smoothness of the stencil aperture sidewalls are vital components dictating print quality. The recommended aspect ratio between stencil width and thickness is 1,5 and is essential to prevent stencil clogging, as solder paste tends to remain in the opening if the ratio is less. It is further recommended that the area of pad divided by area of aperture walls should be greater than 0,66. The smoothness and accuracy of aperture walls is controlled in the process by which the aperture is made. There are three common processes for making stencils: chemical etching, laser cutting and the additive process.
Metal mask and flexible metal mask stencils are etched by chemical milling from both sides using two positive images. Electro etched stencil walls are often not smooth and are put through electro polishing - a micro etching process - or nickel-plating to achieve a smooth wall.
In laser cutting, the stencil is produced directly from the Gerber data. This improves the accuracy of openings as the data can be adjusted to change dimensions as necessary. A further benefit is that the walls can be tapered. Laser cutting can produce opening widths as small as 0,1 mm (0,004") with incredible accuracy, making it very suitable for ultra-fine-pitch component printing.
The third and most modern process for making stencils is most commonly called electroforming, where nickel is deposited on a copper mandrel to build the opening, then photosensitive dry film is laminated on the copper foil. The film is polymerised by ultraviolet light through a photo mask of the stencil pattern and a negative image is created on the mandrel, where only the stencil apertures remain covered by the photo-resist.
Nickel-plating around the photo-resist then grows the stencil until the desired stencil thickness and the photo-resist is removed. The electroformed nickel foil is separated from the mandrel by flexing - a key process step. Now the foil is ready for framing as in other stencil making processes.
Conclusion
The printing industry is competitive, it is costly and it is ever-evolving. Attention to detail - such as the viscosity of the solder paste, the pressure of a squeegee blade or clean openings on a stencil - is key to achieving the best print result possible. Make sure you stay informed and ahead.
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