Process and Precautions for Silkscreen Pretreatment

According to the company’s own conditions, a large number of experiments have been carried out on the pre-treatment process, parameters and matters that should be noted in the operation, and the process and parameters have been optimized, which has achieved good results.

Process flow of pre-treatment:

Due to the chemical cleaning and nylon brush processing method, the specific process flow is as follows:

Water pre-soaking → pickling → water cleaning → brushing → high-pressure water washing → municipal water washing → squeezing water roller squeezing water → cold air drying → hot air drying

Specific operations and precautions for pre-processing:

(1) Water pre-soaking
The purpose of water pre-soaking is to wash away impurities on the copper surface and wet the copper layer, so that the pickling speed can be accelerated, so as to control the transfer speed of the entire machine. Because the transfer speed of the machine is too slow, the copper surface will be excessively ground.

(2) Pickling
Pickling is sprayed with 5-10% sulfuric acid solution, and the time is controlled within 1 minute. Pickling mainly removes chemical impurities on the PCB board surface and slightly etches the copper surface. Before operation, check whether the nozzle is blocked, so as not to spray in some places and affect the effect of pickling. Rinse the solution with city water after pickling.

(3) Brush the board
The number of meshes of the brushing machine is divided into 300 meshes and 500 meshes. Use 300 mesh to perform rough brushing first. The brush roller pressure of the rough brush should be controlled below 2.2 to prevent unacceptable scratches on the surface of the substrate and copper layer and affect the appearance of the printed board surface. Then use 500 mesh for fine brushing. At this time, the brush roller pressure should be controlled between 2.4 and 2.5. The fine brush increases the density of the brush marks on the board surface, so that the mechanical action between the solder mask and the board surface is enhanced. The function of the brush plate is to mechanically scrape off the dirt on the plate surface and roughen the copper surface at the same time. Before operation, open the water source to spray water and check the nozzle regularly to ensure the pressure of the spray water, which can reduce the pollution of the nylon to the plate surface and the hole, and also extend the life of the nylon brush roller.

(4) High pressure water washing

The pressure of high-pressure water washing should be maintained above 12.5bar. The pressure can be seen from the pressure gauge. In actual operation, this is not enough. The smoothness of the nozzle directly affects the actual pressure acting on the plate surface. Also, the high-pressure washing water is not circulating water, and the effect of circulating water is not good. High-pressure water washing is to wash away the copper powder after brushing the board, so as not to cause insufficient insulation resistance of the board and affect the conductivity of the board.

(5) Water squeezing rollers squeeze water

The brushing machine generally uses three sets of squeezing rollers. The function of the squeezing rollers is to absorb the water on the surface of the printed board after high-pressure water washing. The printed board after squeezing water should have no watermarks, otherwise, it is in the drying section. Oxidation that is invisible to the naked eye is formed. After the solder mask is printed, there will be a faint black layer, which can cause the solder mask to fall off in severe cases. The squeezing roller should always be kept in a semi-dry state (moisture by hand, but no water can be squeezed out), so as to have a good squeezing effect and prolong the life of the squeezing roller.

(6) Dry by cold air
The main function of the cold air drying is to blow out the water in the holes, so as to avoid the formation of new oxidation points on the board surface in the hot air drying section. When the board is dried by cold wind, the wind of the wind knife should be blown onto the board first, so as to avoid the formation of oxidation spots on the surface of the board by the water droplets in the hole, and at the same time prevent the board from jamming, so as not to affect the production schedule.

(7) Hot air drying
The temperature of the drying section is controlled between 75~85℃, and the hot air drying will dry the moisture in the board surface and the hole, so as to avoid insufficient adhesion of the ink after the silkscreen solder mask.