The Most Common Used LPI Soldermask Application Method – Silk Screen Printing
Semi-automatic screen print-applied LPISMis the most common used solder mask application method today. The silk screen process started to develop rapidly in about 1921. Now highly automated and has become established in a wide variety of decorative and industrial fields, including the printed circuit industry. Of all the available printing techniques, the screen process is well adapted to the requirements of printed circuit production as it can deposit resist on both non-etched and etched boards over a wide range of film thickness, depending on application.
The silk screen printing applies the LPI to the circuit board with a squeegee blade through a tensioned mesh. Ink deposit is controlled by varying the mesh count and print settings, speed, angle and pressure. A stencil-like image is supported on a fabric mesh stretched across a rectangular frame. During printing the frame is supplied with ink which is flooded over the screen. A squeegee is then drawn across it, with the ink flowing through the open cells of the mesh. At the same time the substrate is held in contact with the screen and the ink is thus transferred to it, from the open cells of the mesh.
LPI Silkscreen Printing
Non-uniformity of coating can result from the damming effect on the leading edge of the trace in the direction of the squeegee movement. There is compression of the screen on the crown of the trace which can result in extremely thin coating and the flooding effect in the space between the traces also must be taken into account. All affect the resulting impedance.
A working supply of ink is placed at one end of the screen. The screen is then lowered into position and a rubber squeegee is drawn across the stencil to produce the print.
On most automatic flat bed machines the base to which the substrate is applied is a vacuum. This prevents the laminate sticking to the screen when tacky inks are used.