The Fundamental, Process & Equipments of LPI Electrostatic Spray
Almost every PCB has solder mask on surface, because solder resist has so many advantages for PCB, such as :
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.
Currently the most common used soldermask is liquid photo imageable solder masks. There are several methods for LPI application, for example, silk screen printing, curtain coating, electrostatic spray, air spray. This article will introduce the fundamental and process and equipments of LPI Electrostatic Spray.
Fundamental:
In the electrostatic spray technique LPI is applied from a rotating bell, which, aided by compressed air, atomizes the ink, and deposits it on the PCB. The principle of Electrostatic Spraying is based upon the physics principle of "Like charges repel. Unlike charges attract". The LPI is given a negative charge and the PCB is earthed, so the LPI is attracted to the board. The negative charge on each of the resist droplets helps to further reduce droplet size. However, despite electrostatic attraction there is some 10 - 20% loss of resist due to overspray. And the electrostatic effect tends to attract the LPI to the copper areas, resulting in less than perfectly uniform coating.
Electrostatic coating is a manufacturing process that employs charged particles to more efficiently paint a work piece. Paint, in the form of either powdered particles or atomized liquid, is initially projected towards a conductive work piece, using normal spraying methods, and is then accelerated toward the work piece by a powerful electrostatic charge.
Typical Powder Coating Process:
Pretreatment- Spray-Baking- Clean-Inspection-Package
Typical Powder Coating Equipments:
Powder room, high-voltage electrostatic generator, electrostatic spray gun, powder feeder, powder recovery, etc.
LPI Electrostatic Spray Equipment
