Printed circuit board in the sun resistance welding process, is the screen printing after welding resistance of the printed circuit board with a photographic plate will be covered by the pad on the printed circuit board, so that it is not exposed to ultraviolet radiation in the exposure, and the welding resistance protective layer after ultraviolet light irradiation of a more sturdy attached to the surface of the printed circuit board, the pad is not subject to ultraviolet radiation, you can reveal the copper welding plate, so that in the hot air leveling of the lead on the tin.
1.Pre-baking
The purpose of pre-baking is to evaporate the solvent contained in the ink, so that the solder resist film becomes non-stick state. For different inks, the pre-drying temperature and time are different. Pre-drying temperature is too high, or drying time is too long, will lead to poor development, reduce the resolution; pre-drying time is too short, or the temperature is too low, in the exposure will stick to the negative, in the development of the solder resist film will be eroded by sodium carbonate solution, resulting in loss of surface luster or the solder resist film expansion and fall off.
2.Exposure
Exposure is the key to the whole process. If the exposure is excessive, due to the scattering of light, graphics or lines of the edge of the soldermask and light reaction (mainly soldermask contained in the light-sensitive polymers and light reaction), the generation of residual film, which reduces the degree of resolution, resulting in the development of graphics smaller, thinner lines; if the exposure is not enough, the result is the opposite of the above situation, the development of graphics become larger, thicker lines. This situation can be reflected through the test: exposure time is long, the measured line width is a negative tolerance; exposure time is short, the measured line width is a positive tolerance. In the actual process, you can choose "light energy integrator" to determine the optimal exposure time.
3.Ink viscosity adjustment
The viscosity of liquid photoresist ink is mainly controlled by the ratio of hardener to main agent and the amount of diluent added. If the amount of hardener is not enough, it may produce an imbalance of ink characteristics.
