The screen printing process is an integral part of the printed circuits boards’ manufacture in the electronics industry. Following the screen printing process, as mentioned in a previous article (“Surface mount devices: the pick-and-place machines”), the components are placed on the board thanks to a placement machine. The board then passes through an oven that will carry the molten paste to permanently weld the component onto the board. This solder paste is deposited by a screen printing machine. The support on which will be applied solder paste has openings corresponding to areas of deposition. The paste is pushed into the openings by means of a scraper and is thus applied on the circuit board.
Functioning of a screen printing machine
There are 3 types of screen printing machines: manual, semi-automatic and automatic. The last one will be mainly used for industrial production for the manufacture of a large number of printed circuits boards.
The main purpose of the machine being the deposition of the solder paste, it should be perfectly applied to the corresponding locations. The stencil (equivalent to a colander to deposit only where desired) must be therefore centered . It is possible to do it manually but the most efficient machines use an « optical » automatic centering with a camera, allowing thus saving time. The camera can also « check » the quantity of solder paste deposited on the printed circuit. In this way, it helps to prevent possible defects (lack of solder paste, short-circuit …).
The screen printing method is used to cover the tracks of the printed circuit board with solder paste and this thanks to a stencil. Only terminal locations of the components will be covered. The next step will be to solder these components through a furnace by melting the solder paste previously applied.
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