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Understand screen printing, component placement

Posted: 26 Nov 2008     Print Version  Bookmark and Share

Keywords:placement machine  solder paste  screen print 

This paper discusses how boards are printed with solder and how components are securely loaded to PCBs using a placement machine.

Screen printing
In printing solder paste onto the pads and placing the component onto the solder, the most critical element is usually time. As soon as the paste is deposited onto the pad, volatile solvents begin to escape from the solder.

For this reason, the various components that will be placed onto the board are loaded into the placement machine before the printing of paste begins. If 20 components are going onto the board, and if the run is 1000 boards, then the kit–the total assemblage of components–numbers 20,000.

The placement machine is programmed and the component feeds are loaded with the kitted parts. The printing of the solder paste should not start until after the placement machine has been loaded because the printed solder dries quickly.

At this point silk-screen printing of the solder paste onto the pads can begin. The first few boards need to be examined very carefully to ensure that all pads have received full coverage. Two frequent errors at this stage are skips, where part of the pad received no solder, and misses, where the pad received no solder.

The supply of solder paste is often loaded manually into the screen print machine. A drawback is that manual loading requires the operator to observe the level of paste remaining. Newer machines use solder paste cartridges that are loaded automatically. Automatic loading makes operator observation unnecessary and ensures that the solder paste being printed remains fresh at all times.

The vertical dimension of the printed paste is critical. It is important to make the paste thick enough so that the leads of all components contact the solder. But the thickness of the paste is somewhat difficult to control. Measured by weight, the paste typically is 80 to 90 per cent metal–tin, silver, and copper, for example. The other parts–the solvents and oils–are much lighter but occupy a substantial portion of the paste's volume.

Furthermore, the solvents and oils are continually evaporating from the solder paste. This has two effects: the surface of the solder becomes harder and less tacky, and the middle of the solder sags downward. Eventually the surface becomes hard enough to trap the remaining solvents and oils inside. These trapped solvents become very important during reflow.

Component placement
When a board has been printed with solder, component placement should follow in short order. The board is loaded into the placement machine while the solder is still wet. This ensures that the components become tacked into position when they are placed on the solder. If the solder is allowed to become too dry, components may fall off.

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