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Cost- effective LPI Applying Method - Air Spray

Generally the printed circuit board (PCB) is green, red, blue, black,ect. These colors are called solder mask. The solder resist has many advantages for PCB, such as the following:

1) Enable mass soldering techniques

2) Prevent solder shorts under components

3) Prevent corrosion to underlying circuitry

4) Plating resist for surface finishes

5) Prevent growth of metal whiskers

6) Insulate substrate from debris and environment

7) Assist with component placement, and so on.

By far the liquid photo imageable solder mask is the most common used soldermask. There are a variety of methods for LPI application, eg, silk screen printing, curtain coating, electrostatic spray, air spray. This article will introduce the LPI Air Spray.

Air Spray Gun

Air Spray Gun

Using air-spray the LPI is applied using single spray gun or multiple spray guns. This is a conventional spray technique where a thin liquid resist is intermixed with decompressing air at the nozzle of a spray gun and the resulting atomized droplets of resist and then carried by the force of the air pressure onto the circuit board. Air spray produces generally uniform coating with multiple gun spray systems having a tendency to form "stripes", due to overlap or interference between adjacent guns, across the board.

This application technique has been used in industry for over 70 years but has only recently been introduced into the printed circuit market. Compared with electrostatic spray, the spraying principle of air spray is slightly different and the equipments are less expensive. And the production rates, loss in overspray, and coverage over panels of electrostatic spray and air spray are similar. Equipments are available to coat circuit boards horizontally or vertically.

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