Exploring Factors Affecting PCB Appearance Silk Screen Pretreatment

PCB brushgenerally uses three groups of water squeezing rolls. The role of the water squeezing rolls is to absorb the water on the surface of the printed board after high-pressure water washing. The printed board after water squeezing should have no water marks, or it will form invisible oxidation in the drying section

(5) Water squeezing roll

The brush generally uses three groups of water squeezing rolls. The role of the water squeezing roll is to suck up the water on the printed board surface after high-pressure water washing. The printed board after water squeezing should be free of water marks, otherwise, oxidation will form in the drying section that cannot be seen by the naked eye. After the solder resistance is printed, there will be a light black layer, which can also cause solder resistance to fall off in serious cases. The water squeezing roll should always be kept in a semi dry state (there is moisture when touching, but the water cannot be squeezed out), so as to have a good water squeezing effect and prolong the service life of the water squeezing roll.

(6) Cold air drying

The main function of cold air drying is to blow out the water in the hole, 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 air, the wind from the wind knife shall be blown to the board first, which can not only prevent water drops in the hole from forming oxidation points on the board surface, but also prevent the board from sticking, so as not to affect the production progress.

(7) Hot air drying

The temperature of the drying section shall be controlled between 75~85 ℃. Hot air drying is required to dry the moisture on the board surface and in the holes, so as to avoid insufficient adhesion of the ink after PCB silk screen solder resistance.

4. Analysis and Discussion of Common Problems in Pretreatment

(1) Dotted oxidation on copper surface

There are oxidation points on the copper surface, which will seriously cause the solder resistance to fall off during hot air leveling. Generally, after the PCB board is delivered for use, the lines with oxidation points are easy to break. This oxidation point can be seen when connecting plates. After finding problems, check the water squeezing roll and cold air drying section.

(2) Large area of copper darkens after printing

After printing, a large area of copper becomes dark, and the ink on the board looks inconsistent, which also affects the adhesion between the ink and the copper layer, so it should be reworked immediately. Before rework, the temperature of the wringing roll and the hot air drying section of the brush shall be checked to see whether the wringing effect is good, and whether the displayed temperature of the drying section is consistent with the actual temperature.

(3) Some places haven't been brushed

There are two kinds of places that are not brushed: the left side of the same printed board has obvious brush marks, while the right side is not brushed; Is there any brush on the middle part of the same printed board.

The first is caused by the non parallelism between the brush rollers, which is mainly due to the fact that the brush rollers are often placed to the right when the small plates are usually placed. After a long time, the brush rollers form a cone shape from the cylinder, and appear when the large plates are encountered. The second is caused by uneven board placement by the operator. At ordinary times, the brush roller should be checked frequently. The inspection method is: place a base plate on the lower (upper) side of the brush roller without opening the drive. If the brush roller is started, wear marks will be left on the base plate. When the wear mark in Figure 2 appears, the first phenomenon will appear in the process of brushing the plate; When the wear mark in Figure 3 appears, there are two phenomena. Under normal conditions, it is required that the edge lines of wear marks are two basically parallel parallel lines.

5. Maintenance of PCB equipment

In order to reduce the PCB production cost increased by pretreatment rework, the pretreatment PCB equipment should be maintained regularly. The maintenance of equipment is not to wipe off the external surface of the machine. The role of maintenance is to keep the machine in normal working condition.

Frequently check whether the brush roller is in the working state shown in Figure 4. If not, the sand plate with the same length as the brush roller should be used for renovation. At ordinary times, check whether the nozzle is blocked to avoid affecting the pre-treatment effect. The maintenance of the water squeezing roll is very important. When the water squeezing roll works in a dry and hard state, it can not achieve the effect of pre-treatment and shorten the service life of the water squeezing roll, which is also the reason to protect the semi dry state of the water squeezing roll.