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 Powder Spray Process. The typical powder spray process is: Pretreatment - Spray – Baking – Clean – Inspection - Package.
Powder Spray Process
Pretreatment quality directly affects the quality of the powder coating. If pre-treatment is not good enough, it will cause easy falling off of the coating and bubbling phenomenon. Therefore, we should pay great attention to the Pretreatment.
(1) Pre-treatment: For stamping work piece by chemical pre-treatment method. That is to say: oil removal → rust removal → cleaning → phosphate (or purification) and so on. Most rust or thicker work piece should remove rust by sand blasting, shot blasting and other mechanical methods. After a mechanical rust removal, the surface should be ensured clean and without dirt.
(2) Putty: According to the workpiece defect degree to scrap conductive putty, after drying smooth it with sandpaper.
(3) Protection (also known as shield cover): If some parts of the workpiece don’t require coating, so before preheating protective colloid can be adopted to cover up these parts so as to avoid the spray paint.
(4) Preheating: Generally There is no need for preheating. If a thicker coating layer is required, the workpiece can be preheated to 100 ~ 160 ℃, which can increase the coating thickness.
The workpiece should be aligned to the spray gun in the powder room for spray. Electrostatic generator will release high voltage electrostatic (negative) through the electrode needle of the spray gun muzzle. The high-voltage electrostatic makes the powder and compressed air and the surrounding air ionization electrode (negatively charged) mixed. After hanging the workpiece there will be a ground (grounding). The powder is attached to the workpiece with the help of electric force and air pressure, relying on electrostatic attraction the uniform coating is then formed on the surface of workpiece.
After spraying the workpiece is heated with the temperature 180 ~ 200 ℃ in the baking room, and insulate the heat for about 15-20 minutes, so as to melting, leveling, curing, thereby obtaining the surface effect we want. (Different powders need different baking temperature and time). This should be noticed in the curing process.
After the coating is cured, remove the protective material and burr.
After curing, the workpiece appearance should be inspected: whether smooth and bright or not, with or without particles, whether shrinkage or not and other defects. Besides, the thickness should be controlled between 55 ~ 90μm. The workpiece should be reworked or re-sprayed, if there are tainted spray, pinholes, bumps, bubbles and other defects.
Finished workpieces should be packed with flexible cushioning material, such as foam paper, air bubble film to isolate from each other to prevent scratching and wore.