3 - Assembly of electronic circuits

The surface mounted components are assembled by reflow soldering technology in mass production. The first step is the solder paste deposition. This can be carried out with stencil printing. The basic tools used in printing are the squeegee and the stencil. The stencil is a metal mask which rests directly in contact with the surface of the Printed Wiring Board and the apertures in the stencil are formed according to the solder pads on the board. In stencil printing, the board is moved into contact with the stencil before the squeegee starts to move. When the squeegee has completed its stroke, the board and stencil are then separated vertically, which releases the paste from the stencil, producing well-defined edges to the print. It is usual, but not essential, for the stencil to remain fixed and the board to be raised for printing and lowered at a controlled speed afterwards.

The components can be placed into the deposited solder paste automatically with pick&place and/or collect&place machines. The pick&place machine picks and then places components one by one. The collect&place machines collect several components (from 4 to 32 pcs.) and then place them one by one.

The last step is the soldering of the components. This can be performed either with convection ovens or with vapour phase soldering.