PCB Blog - Factors Influencing the Appearance of PCB Board

With the rapid development of PCB board manufacturing technology, customers are not only interested in the internal quality of PCB board, but also the hole resistance Thickness of copper foil On off test The requirements for solderability are higher and higher, and higher requirements are put forward for the appearance of PCB board, such as uniform ink color and 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 influence of silk screen pretreatment on the appearance of PCB board mainly includes: the copper under the ink has oxidation phenomenon, there is hard damage (copper layer or substrate has obvious scratch) after pretreatment, and it is found that the ink color on a large area of copper foil is uneven after printing resistance welding. The quality of pretreatment directly affects the appearance quality of PCB board, slightly causes rework, affects the production progress, delays the delivery date, and 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.

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

1. Purpose and method of pretreatment

The ultimate purpose of pretreatment is to make the copper surface free of oxide Grease and impurities, and also have a certain roughness. The board impurities after pretreatment have a certain roughness. The cleanliness of the pretreated Board shall be tested with zero ion 100A ion concentration tester to see the concentration of impurity particles in the treated board. The ion concentration shall be lower than 1.5 μ G / cm2, and the surface state of the copper layer is shown in Figure 1. Figure 1 is the surface state of the copper layer magnified 200 times, with 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 do not affect the conductivity of the PCB board.

There are many ways of pretreatment. Now there are mainly the following kinds in industry: nylon brush grinding brush Chemical cleaning treatment Alumina / pumice powder spray Alumina / pumice powder plus nylon brush Chemical cleaning plus nylon brush.

2. Process flow and precautions of pretreatment

According to the company's own conditions, the pretreatment process flow Parameters and matters needing attention in operation, a large number of experiments are carried out, and the process flow and parameters are optimized, which has achieved good results.

3. Process flow of pretreatment

Since the treatment method of chemical cleaning and nylon brush is adopted, the specific process flow is as follows:

Water pre immersion - acid pickling - water cleaning - plate brushing - high pressure water washing - municipal water washing - water squeezing by water squeezing roller - cold air drying - hot air drying

4. Specific operation and precautions of pretreatment

1) Water pre leaching

The purpose of water pre immersion is to wash the impurities on the copper surface and wet the copper layer, so as to accelerate the speed of pickling, so as to 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 is to spray 5 ~ 10% sulfuric acid solution within 1 minute. Pickling mainly removes 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 effect of pickling. After pickling, rinse off the solution with municipal water.

3) Brush plate

The mesh number of the plate brushing machine is divided into 300 mesh and 500 mesh. Rough brushing shall be carried out for 300 purposes. The brush roller pressure of rough brushing shall be controlled below 2.2 to prevent unacceptable scratches on the surface of substrate and copper layer and affect the appearance of PCB surface. Then use 500 mesh for fine brushing. At this time, the brush roller pressure should be controlled between 2.4 ~ 2.5. Fine brushing can improve the brush mark density of the plate surface, so as to enhance the mechanical effect between resistance welding and the plate surface. The function of the brush plate is to mechanically scrape off the dirt on the plate surface and coarsen the copper surface at the same time. 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 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. It can be seen from the pressure gauge that this is not enough in actual operation. Whether the nozzle is unblocked or not directly affects the actual pressure acting on the plate surface. Also, the water for high-pressure water 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 board, so as not to cause insufficient insulation resistance of the board surface and affect the conductivity of the board surface. The operator should check whether there is copper powder on the board surface when connecting the board, so as to solve it as soon as possible.

5) Water squeezing roll

Three groups of water squeezing rollers are generally used in the brushing machine. The function of the water squeezing roller is to suck up the water on the surface of the printed board after high-pressure water washing. The PCB board after water squeezing shall have no trace, otherwise the oxidation invisible to the naked eye will be formed in the drying section, and there will be a layer of light black after the solder resist is printed, which can also cause the solder resist to fall off in serious cases. The water squeezing roller shall always be kept in a semi dry state (there is moisture by hand, but it can not squeeze water), so as to have a good water squeezing effect and prolong the service life of the water squeezing roller.

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 droplets in the hole and prevent board jamming, so as not to affect the production progress.

7) Hot air drying

The temperature of the drying section shall be controlled between 75 ~ 85 degree Celsius, and hot air drying shall be used to dry the water vapor on the plate surface and in the hole, so as to avoid insufficient adhesion of the ink after silk screen welding resistance.

5. Analysis and discussion of common problems in pretreatment

1) There are spots of oxidation on the copper surface

There are oxidation spots on the copper surface, which will seriously cause the solder resist to fall off during hot air leveling. Generally, the lines with oxidation spots are easy to break after the PCB board is delivered for use. This oxidation point can be seen when connecting the plate. After finding the problem, check the squeeze roller and cold air drying section.

2) After printing, a large area of copper darkens

After printing, a large area of copper is dark, and the ink on the board looks inconsistent. At the same time, it 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 were not painted

There are two kinds of places that are not brushed: there are obvious brush marks on the left of the same PCB board, but not on the right; Is there any brush in the middle of the same PCB 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 placing small plates. Over time, the brush rollers form a round platform shape from a cylinder and appear when meeting large plates. The second is caused by the uneven placement of the board by the operator. The brush roller shall be inspected frequently at ordinary times. The inspection method is as follows: put a base plate directly below (above) the brush roller, do not turn on the drive, start the brush roller, and 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 will be two phenomena. 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 production cost increased by pretreatment rework, the pretreatment equipment shall be maintained regularly. The maintenance of equipment is not to wipe off 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 shown in Figure 4. Instead, a 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 pretreatment 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 pretreatment. Needless to say, it also shortens the service life of the water squeezing roll, which is also the reason for protecting the semi dry state of the water squeezing roll.