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52 SMT Magazine • January 2015 most assemblers will be using a manual squee- gee. Some machines have a built-in guide that aids in applying the right amount of solder paste. There are several key points to applying paste properly: • Angle of attack • Squeegee pressure • Squeegee speed Once the paste is applied, the stencil is lifted off the circuit board, either vertically, or pivot- ed. There is no practical difference in either of the two lift methods. Semi-automatic Systems In a moderately high-volume environment, several of the functions can be automated to some extent, making the process more efficient, more repeatable, and allowing a higher yield at lower cost. The following are typically found in semi-automatic stencil printers: • Auto open/close for board loading and unloading, which can reduce operator fatigue • Controlling the squeegee pressure • Controlling the movement and speed of the squeegee • Visual alignment/assist options to facilitate positioning fully automatic Systems In fully automatic systems, everything is managed without operator intervention. Sys- tems may include automatic board loading/ unloading and auto fiducial alignment for X, Y, Z and Θ positioning. Again, you get what you pay for. If you're running 2,000 boards/ day, more automation may provide signifi- cant dividends in terms of quality yield, re- duced labor costs, and more control over your process. If you're spending the kind of money a fully automatic system commands, you will need to perform some serious due diligence on special capabilities that can't be covered in a short article. Construction Regardless of the volume, you're looking for the best accuracy and repeatability you can get, so once fixturing is set up, you don't want to waste time recalibrating. Accuracy and re- peatability are directly related to fine-pitch ca- pability, and pitch accuracy is directly related to machine stability. For this reason, construc- tion of the holding plates is critical. Machines constructed with sheet metal are not nearly as stable as those made using a machine-ground holding plate with welded frames. This assures that nothing moves from one board to the next so you can expect repeatable accuracy from board to board. HOW TO SELECT a STEnCIL PrInTEr continues Figure 1: adjustable camera on vision machine. Figure 2: Machine-ground holding plate. smt quiCk tiPs