Factors Affecting PCB Appearance

Factors Affecting PCB Appearance
With PCB manufacturing technology, customers are not only interested in the internal quality of PCB, but also have higher requirements on solderability for the hole resistance thickness of copper foil on/off test. They also have higher requirements on the appearance of PCB, such as uniform ink color, no impurities, and no foreign matters and oxidation points on the copper layer surface The influence of screen pretreatment on appearance quality is discussed below
The influence of screen pretreatment on PCB appearance mainly includes: the copper under the ink has oxidation, there is hard damage after pretreatment (copper layer or substrate has obvious scratches), and the ink color on a large area of copper foil is uneven after printing resistance welding. The pretreatment quality directly affects the appearance quality of PCB board, slightly causes rework, affects the production progress, delays the delivery date, and reduces the company's reputation; In serious cases, the Board of Directors may be revoked. Finally, the company's "orders" are reduced, which directly affects the economic benefits of the company.
In order to gradually reduce the loss caused by pretreatment and improve the competitiveness of the company. The pretreatment process must be strictly controlled.
1. Purpose and method of pretreatment
The final purpose of pretreatment is to make the copper surface free of oxidized grease and impurities, and have a certain roughness. The impurities in the pretreated plate have a certain roughness. A zero ion 100A ion concentration tester shall be used to test the cleanliness of the pretreatment plate to check the concentration of impurities in the treatment plate. The ion concentration should be lower than 1.5mG/cm2, and the surface state of the copper layer is shown in Figure 1. Figure 1 magnifies the surface state of the copper layer by 200 times. The concave convex structure is relatively uniform without oxidation points, so as to enhance the mechanical adhesion with the ink, avoid blistering and large-area falling off, and do not affect the conductivity of the PCB.
There are many methods of pretreatment. At present, there are mainly the following types in industry: nylon brush grinding brush chemical cleaning treatment aluminum oxide/pumice powder spraying aluminum oxide/pumice powder plus nylon brush chemical cleaning plus nylon brush.
Circuit board

2. Pretreatment process flow and precautions
According to the company's own conditions, pretreatment process parameters and operating precautions, a large number of experiments were carried out, and the process and parameters were optimized, and good results were achieved.
3. Pretreatment process flow
Due to the chemical cleaning and nylon brush treatment methods, the specific process flow is as follows:
Water pre leaching - pickling - water washing - brushing - high pressure water washing - municipal water washing - pressurized water roller pressing water - cold air drying - hot air drying
4. Specific operation and precautions of pretreatment
1) Water prepreg
The purpose of water pre leaching is to clean impurities on the copper surface and wet the copper layer, so as to speed up the pickling speed and thus control the transmission speed of the whole machine. The copper surface will be excessively ground due to the slow transmission rate of the machine.
2) Pickling
Pickling is to spray 5-10% sulfuric acid solution within 1 minute. Pickling mainly removes the chemical impurities on the steel 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. After pickling, rinse the solution with municipal water.
3) Brush plate
The mesh number of the brush is divided into 300 mesh and 500 mesh. 300 rough brushes shall be applied. The pressure of the brush roller for rough brushing shall be controlled below 2.2 to prevent unacceptable scratches on the surface of the substrate and copper layer from affecting the appearance of the PCB surface. Then use 500 meshes for fine brushing. At this time, the brush roller pressure should be controlled between 2.4 and 2.5. Fine brush coating can improve the brush mark density on the steel plate surface, thus enhancing the mechanical effect between resistance welding and the steel plate surface. The function of the brush plate is to mechanically scrape away the dirt on the surface of the plate and make the copper surface rough. Before operation, open the water source to spray water, and regularly check the nozzle to ensure the spray pressure, which can reduce the pollution of nylon on the plate surface and holes, and extend the service life of nylon brush roller.
4) High pressure water flushing
High pressure water flushing pressure shall be kept above 12.5bar. It can be seen from the pressure gauge that this is not enough in actual operation. Whether the nozzle is unblocked directly affects the actual pressure acting on the plate surface. In addition, the water for high-pressure water flushing is not recycle water, and the recycle water is not effective. High pressure water washing is to wash off the copper powder after brushing the board, so as not to cause insufficient insulation resistance on the board surface and affect the conductivity of the board surface. When connecting the plate, the operator should check whether there is copper powder on the surface of the plate, so as to solve the problem as soon as possible.
5) Squeeze roll
Three groups of water squeezing rollers are generally used for the hair brushing machine. The function of the water pressing roller is to absorb the water on the surface of the printed board after high-pressure water washing. The PCB after water extrusion shall not have any trace, otherwise, invisible oxidation will be formed in the drying section, and a layer of light black will appear after printing the solder barrier, which will also cause the solder barrier to fall off in serious cases. The water squeezing roll should always be kept in a semi dry state (there is water in the hands, but it cannot squeeze water) to achieve good water squeezing effect and extend the service life of the water squeezing roll.
6) Cold air drying
The main function of cold air drying is to blow the water out of the hole to avoid the formation of new oxidation points on the board surface of the hot air drying section. When drying the plates with cold air, the wind from the windward knife should be blown to the plates first to prevent water drops in the holes from forming oxidation points on the surface of the plates, prevent the plates from being blocked, and avoid affecting the production progress.
7) Hot air drying
The temperature of the drying section shall be controlled between 75 and 85 ℃, and hot air drying shall be used to dry the water vapor on the board surface and in the holes to avoid insufficient adhesion of ink after wire mesh resistance welding.
5. Analysis and discussion of common problems in pretreatment
1) There are oxidation spots on the copper surface
There are oxidation spots on the copper surface, which will seriously lead to the falling off of the solder resist during the hot air leveling process. Generally, after the PCB board is delivered for use, the circuit with oxidation point is easy to break. The oxidation point can be seen when connecting the plate. Check the extrusion roll and the cold air drying section when problems are found.
2) Large areas of copper darken after printing
After printing, a large area of copper is dark, and the ink on the circuit board looks inconsistent. At the same time, it will also affect the adhesion between the ink and the copper layer, which should be reworked immediately. Before rework, check the temperature of the extrusion roller and the hot air drying section of the brush to see whether the extrusion effect is good and whether the displayed temperature of the drying section is consistent with the actual temperature.
3) There is no paint in some places
There are two kinds of non brushing places: the left side of the same PCB has obvious brush marks, but the right side does not; Whether there is an electric brush in the middle of the same PCB.
The first reason is that the brush rollers are not parallel, which is mainly because the brush rollers are usually placed on the right side when placing small plates. As time goes by, the brush roller forms a circular platform shape from the cylinder and appears when it encounters large plates. The second reason is that the operator places the circuit board unevenly. Always check the brush roller. The inspection method is as follows: place the substrate directly below (above) the brush roller, do not open the driver, start the brush roller, and the substrate will leave wear marks. When the wear marks in Figure 2 appear, the first phenomenon will appear during the brushing process; When the wear marks in Figure 3 appear, two phenomena will occur. Under normal conditions, the edge lines of wear marks are required to be two basically parallel lines.
6. Equipment maintenance
In order to reduce the new production cost of pretreatment rework, the pretreatment equipment should be maintained regularly. The maintenance of the equipment is not to wipe the external surface of the machine. The function of maintenance is to keep the machine in normal working condition. Always check whether the brush roller is in the working state as shown in Figure 4. Instead, a sand plate with the same length as the brush roller should be used for repair. Check whether the nozzle is blocked at ordinary times to avoid affecting the pretreatment effect. The maintenance of the water pressure roll is very important. When the squeezing roller works in a dry and hard state, it cannot achieve the effect of pretreatment. Needless to say, it also shortens the service life of the squeeze roll, which is also the reason for protecting the semi dry state of the squeeze roll.

The above is the explanation given by the editor of pcb circuit board company.
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