The influence of pre-treatment of the hottest wire

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The influence of silk screen pretreatment on the appearance of printed boards

[Abstract] this paper discusses the influence of silk screen pretreatment on the appearance of printed boards, and optimizes the parameters of pretreatment. How to prevent some common problems before silk screen printing is discussed

[Key words] optimization of silk screen pretreatment to prevent pressure

1. Overview

with the rapid development of PCB manufacturing technology, users now not only have higher and higher requirements for the internal quality of PCB (internal quality refers to hole resistance, copper foil thickness, on-off test, solderability, etc.), but also have higher requirements for the appearance of PCB, such as uniform ink color, no impurities There are no foreign matters and oxidation points on the surface of the copper layer. The influence of silk screen pretreatment on appearance quality is discussed below

the effects of silk screen pretreatment on the appearance of printed boards are mainly as follows: the copper under the ink is oxidized, and there is hard damage (obvious scratch on the copper layer or substrate) after pretreatment. It is found that the ink color on a large area of copper foil is uneven after resist welding. The quality of pretreatment directly affects the appearance quality of the printed board, slightly causing rework and affecting the production progress. The imported brand period is adopted for the delayed delivery bearings, which reduces the reputation of the company; In serious cases, the board may be scrapped. Finally, the "orders" of the company are reduced, which directly affects the economic benefits of the company

to gradually reduce the losses caused by pretreatment and improve the competitiveness of the company. It is necessary to strictly control the pretreatment process

2. Purpose and method of pretreatment

the ultimate purpose of pretreatment is to make the copper surface free of oxides, grease and impurities, and have a certain roughness. The board impurities after pretreatment have a certain roughness. Clean the pretreated board with zero ion 100A ion concentration tester to see the concentration of impurity particles in the pretreated board. The ion concentration should be lower than 1.5 μ G/cm2, and the surface state of the copper layer is shown in Figure 1. Figure 1 shows the surface state of the copper layer magnified by 200 times, with a relatively uniform concave convex structure and no oxidation points, so as to enhance the mechanical adhesion with the ink, avoid foaming and large-area falling off, and also do not affect the conductivity of the printed board

there are a variety of pretreatment methods. The following are commonly used in industry: nylon brush grinding, chemical cleaning, alumina/pumice powder spraying, alumina/pumice powder plus nylon brush, chemical cleaning plus nylon brush

3. Pretreatment process flow and precautions

according to the company's own conditions, a large number of experiments have been carried out on the pretreatment process flow, parameters and precautions in operation, and the process flow and parameters have been optimized, which has achieved good results

3.1 pretreatment process, Therefore, the specific process flow is as follows:

water pre-treatment → acid pickling → water cleaning → plate brushing → high-pressure water washing → municipal water washing → water squeezing roll water squeezing → cold air drying → hot air drying

3.2 specific operations and precautions for pretreatment

(1) water pre-treatment

the purpose of water pre-treatment is to wash away impurities on the copper surface and wet the copper layer, so as to accelerate the speed of acid pickling and control the transmission speed of the whole machine, Because the transmission speed of the machine is too slow, the copper surface will be excessively ground

(2) pickling

pickling uses 5~10% sulfuric acid solution for spraying, and the time is controlled within 1 minute. Pickling mainly removes the chemical impurities on the plate surface and causes slight micro corrosion on the copper surface. Before operation, check whether the nozzle is blocked, so as not to spray in some places and affect the pickling effect. Rinse off the solution with municipal water after pickling

(3) brush plate

the mesh of the brush plate machine is divided into 300 mesh and 500 mesh. Rough brushing shall be carried out first when using 300 mesh. The pressure of the brush roller for rough brushing shall be controlled below 2.2 so as not to cause unacceptable scratches on the surface of the substrate and the copper layer and affect the appearance of the printed board. Then fine brush with 500 mesh. At this time, the pressure of the brush roller should be controlled between 2.4~2.5. Fine brush will increase the brush mark density of the plate surface, so as to enhance the mechanical effect between the resistance welding and the plate surface. 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. Open the water source to spray water before operation, and regularly check the nozzle to ensure the spray pressure, which can reduce the pollution of nylon to the plate surface and hole, and prolong the service life of nylon brush roller

(4) high pressure water washing

the pressure of high pressure water washing should be kept above 12.5bar. The pressure can be seen from the pressure gauge that this is not enough in actual operation. The smoothness of the nozzle directly affects the actual pressure acting on the plate surface. In addition, the water for high-pressure washing is not circulating water, and the effect of circulating water is not good. High pressure water washing is to wash off the copper powder after brushing the plate, so as not to cause insufficient insulation resistance of the plate surface and affect the conductivity of the plate surface. The operator should check whether there is copper powder on the plate surface when connecting the plate, so as to solve it as soon as possible

(5) water squeezing roller water squeezing

generally, three groups of water squeezing rollers are used in the plate brushing machine. The function of the water squeezing rollers is to suck up the water on the printed board surface after high-pressure water washing. The printed board after water squeezing should have no trace, otherwise invisible oxidation will be formed in the drying section, and there will be a layer of light black after the solder resistance is printed, which can also cause solder resistance to fall off in serious cases. The water squeeze roll should always be kept in a semi dry state (there is moisture by hand, but the water cannot be squeezed out), so as to have a good water squeeze effect and prolong the service life of the water squeeze 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 plate surface in the hot air drying section. When the board is dried by cold air, the wind from the windward knife shall be blown to the board first, so as to avoid the formation of oxidation points on the board surface by water drops in the hole and 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 ℃, and the hot air drying shall dry the water vapor on the plate surface and in the hole, so as to avoid insufficient adhesion of the ink after the silk screen solder mask

4. Analysis and discussion of common problems in pretreatment

(1) there is point like oxidation on the copper surface

there are oxidation points on the copper surface, which will cause the solder resistance to fall off during hot air leveling. Generally, the lines with oxidation points are easy to break after the PCB is delivered for use. This oxidation point can be seen when connecting the plate. After finding the problem, the squeeze roll and the cold air drying section should be checked

(2) after the printing is blocked, large areas of copper are dark.

after the printing is blocked, large areas of copper are dark, and the board ink looks inconsistent in color, which also affects the adhesion between the ink and the copper layer, so it should be reworked immediately. Before rework, check the temperature of the squeezing roller and hot air drying section of the brushing machine to see whether the squeezing effect is good and whether the displayed temperature of the drying section is consistent with the actual temperature

(3) some places are not brushed.

some places are not brushed. There are two types: there are obvious brush marks on the left of the same printed board, but there are no brush marks on the right; Whether the middle part of the same printed board is brushed

the first one is caused by the non parallelism between the brush rollers, which is mainly caused by the fact that the brush rollers are often placed to the right when the small plate is placed. Over time, the brush rollers form a round platform shape from the cylinder, and appear when the large plate is met. The second is caused by uneven placing of the board by the operator. At ordinary times, the brush roll should be checked frequently. The inspection method is: place a base plate directly below (above) the brush roll, do not open the drive, and start the brush roll, which will leave grinding marks on the base plate. When the wear mark shown 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 will be two phenomena. Under normal circumstances, it is required that the edge lines of the wear marks are two parallel lines that are basically parallel (Fig. 4)

5. Equipment maintenance

in order to reduce the production cost of pretreatment and rework, the pretreatment equipment should be maintained regularly. The maintenance of the equipment is not just to wipe off the external surface of the machine, but to keep the machine in a normal working state

always check whether the brush roll is in the working state as shown in Figure 4. Instead, use a sand plate with the same length as the brush roll for renovation. At ordinary times, check whether the nozzle is blocked to avoid affecting the pretreatment effect. It is very important to maintain the water squeeze roll. When the water squeeze roll works in a dry and hard state, it does not have the effect of pretreatment. It also shortens the service life of the circuit board when measuring the resistance on the road. This is also the reason why fengdanyu, deputy director of the equipment Development Department of the Central Military Commission, should protect the water squeeze roll in a semi dry state

source: PCB information

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