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The soldermask process involves a plenty of steps. The general steps consist of Pretreatment - Coating - Drying - Pre-Baking - Registration - Exposure - Developing - Inspection - Final Curing.
It is hard to combine the copper layer and the solder layer together, if the copper surface is oxidized or too smooth. The weak combination cannot resist such as the thermal shock in the subsequent processes. In order to avoid this problem, there is a Pretreatment before coating. The oxidized layer should be degreased with the help of a pre-cleaning step. And the copper surface should be made rough, in order to enhance the adhesion.
Coating, called also screen printing, is to apply the soldermask color according to the requirement of customers to the circuit board.
After screen printing, the board should be drying at least for 15 minutes before pre-baking. It helps to reduce the load of the pre-baking by solvent evaporation.
The purpose of pre-baking in tunnel oven is to go on evaporating the ink solvent. Pre-baking temperature, time and ventilation control is extremely important. Too high or too long pre-baking temperature will cause the difficulty in developing.
After pre-baking there is a registration step with film.
Before exposure please check whether the polyester film and glass frame in the exposure frame are clean or not. Then turn on the power switch of the exposure machine, and then open the vacuum button to select the exposure program. Before the formal exposure, the exposure machine should work in vain for 5 times, in order to lead the exposure machine to enter the best working state. The exposed areas will be remained, while the unexposed areas will be washed out.
Developing is to remove the solder solvent on the pads in some shading parts. The solution for developing is 1% Na2CO3. The liquid temperature is usually around 28 to 32 Celsius degrees. Before the formal developing, you should improve the temperature of developing machine, and the solution can reach the predetermined temperature, so as to achieve the best developing effect.
After developing step, inspection is needed to check whether there is any defect image, and the burrs should be removed. Please pay attention to wear yarn gloves and hold the sides of the boards in the inspection process.
9. Final Curing
Put the qualified circuit boards into the tunnel oven for baking for the final thermal curing. Sometimes an additional UV curing will be needed for increasing the chemical and mechanical properties of the soldermask.