Cutting is the first step in the mechanical operation of printed circuit board, and the general shape and outline can be given by cutting. The basic cutting method is suitable for all kinds of substrates, and the thickness is usually less than 2 mm. When the cutting plate exceeds 2 mm, the cutting edge will be rough and irregular, so this method is generally not adopted.

The shearing of laminated plates can be operated manually or electrically, no matter which method has the same characteristics in operation. Shearing machines usually have a set of adjustable cutting blades, as shown in figure 10- 1. The blade is rectangular, the adjustable angle of the bottom edge is about 7, the cutting length can reach 1000mm, the longitudinal angle between the two blades is usually between 1- 1.5, and the maximum angle with epoxy glass substrate is 4, and the gap between the two blades should be less than 0.25mm.

The angle between the two blades should be selected according to the thickness of the cutting material. The thicker the material, the greater the required angle. If the shearing angle is too large or the gap between the two blades is too large, the board will crack when cutting the paper substrate. But for epoxy glass substrate, because the material has a certain bending strength, even if there is no crack, the plate will be deformed. In order to keep the bottom edge clean during shearing, the material can be heated in the range of 30- 100℃.

In order to obtain a neat cut, the cardboard must be firmly pressed down by a spring device to prevent other inevitable displacements of the cardboard during the shearing process. In addition, parallax will lead to a tolerance of 0.3±0.5 rnrn, which should be reduced as much as possible, and the accuracy can be improved by using corner marks.

Scissors can handle various sizes and provide accurate repeated sizes. Large machines can cut hundreds of kilograms of substrates per hour.

2 sawing

Sawing is another method to cut the substrate. Although the dimensional tolerance of this method is similar to that of shearing (0. 3-0.5 rnrn), this method is preferable because its cutting edge is very smooth and tidy.

In the printed circuit board manufacturing industry, circular saws with movable tables are mostly used. The speed range of saw blade is 2000-6000 rpm. But once the cutting speed is set, it cannot be changed. This is achieved by a heavy pulley with more than one V-belt.

The diameter of high-speed steel blade is about 3000rnrn, and it can cut paper materials at the speed of 2000-3000r/rnin, about 1. 2- 1. 5 teeth per 1 cm circumference. For epoxy glass substrates, a blade with a tungsten carbide blade is used. The cutting effect of diamond grinding wheel will be better. Although the initial investment is large, the trimming effect is improved due to the long service life, which is very beneficial to the future work.

The following are some problems that should be paid attention to when using the cutting machine:

1) Pay attention to the cutting force directly acting on the blade and check the firmness of the bearing. There should be no abnormal feeling when checking by hand;

2) For the sake of safety, the toothed plate should always be covered by protective devices;

3) The installation shaft and engine should be placed accurately;

4) The clearance between the saw blade and the bracket should be minimum, so that the plate can be well supported for trimming;

5) The circular saw should be adjustable, and the height range between the saw blade and the plate is10-15 mm;

6) Blunt teeth and too rough teeth will make the cutting edge not smooth, so it is better to replace it;

7) The wrong cutting speed will lead to uneven cutting edge, which should be adjusted appropriately. Thick materials need to choose a slower speed, while thin materials can be cut quickly;

8) It shall run at the speed given by the manufacturer;

9) If the sawtooth is thin, a reinforcing pad can be added to reduce the vibration.

3 punch holes

When the printed circuit board design has other shapes or irregular contours, it is faster and more economical to use stamping die. The basic punching operation can be completed with a punching machine, and the cutting edge is neat, and the effect is better than that of a sawing machine or a shearing machine. Sometimes even punching and punching can be done at the same time. However, when good edge effect or small tolerance is required, punching can not meet the requirements. In the printed circuit board industry, punching is generally used to cut paper substrates, but rarely used to cut epoxy glass substrates. Punching can make the cutting tolerance of printed circuit board within (0. 1-0.2 mm).

1. Punching of paper substrate

Because paper substrate is softer than epoxy glass substrate, it is more suitable for punching and cutting. When cutting a paper substrate with a punching tool, the elasticity or curvature of the material should be considered. Because the paper substrate often rebounds, the die-cutting part is usually slightly larger than the die-cutting part. Therefore, the size of the mold should be selected according to the tolerance and the thickness of the substrate, which is slightly smaller than the printed circuit board to compensate for the redundant size. It has been noticed that when punching holes, the mold is larger than the hole size, and when punching holes, the mold is smaller than the normal size.

For circuit boards with complex shapes, it is best to use stepping tools, such as cutting materials one by one. As the die cuts them one by one, the shape of the material gradually changes. In this way, the hole was punched in the first step or two, and finally the punching of other parts was completed. Punching holes after heating can improve the cutting effect of printed circuit boards, such as punching holes after heating the slats to 50 -70 'C C. However, care must be taken not to overheat it, because it will reduce the scalability after cooling. In addition, we should also pay attention to the thermal expansion of benzene materials for papermaking, because they show different expansion characteristics in Z direction and Y direction.

2. Punching of epoxy glass substrate

When the required shape of epoxy glass substrate cannot be produced by shearing or sawing, a special punching method can be used to punch holes. Although this method is not popular, this method can only be used when the cutting edge or size requirements are not too strict. Because although the function is acceptable, the cutting edge does not look neat. Because the rebound performance of epoxy glass substrate is smaller than that of paper substrate, the tool for stamping epoxy glass substrate should be closely matched with punching die and punching machine. The drilling of epoxy glass substrate should be carried out at room temperature.

Because the epoxy glass substrate is hard, it is difficult to die-cut, which will reduce the life of the punch and will soon wear out. The punch of cemented carbide tool bit can obtain better cutting effect.

4 milling

Milling is usually used in situations where printed circuit boards are required to be cut neatly, with smooth edges and high dimensional accuracy. Ordinary milling speed is in the range of 1000-3000r/min, and high-speed steel milling machines with straight teeth or spiral teeth are generally used. But for epoxy glass substrate, it is best to use carbonized pigeon tool because of its long service life. In order to avoid delamination, the back of the printed circuit board must have a solid pad during milling. For detailed information about milling machines, tools and other operations, please refer to the standard description of these equipment in the factory or shop.

5 grinding

In order to obtain better edge effect and higher dimensional accuracy than shearing or sawing, especially when the outline of printed circuit board is irregular, grinding method can be selected. In this way, when the dimensional tolerance is (0. 1-0.2 mm), the cost is less than that of punching. Therefore, in some cases, the size beyond the punching hole can be trimmed in the subsequent grinding process to obtain a smooth cutting edge.

The multi-spindle machine used now makes the grinding speed very fast, and the input and total cost of workers are less than that of stamping. When the wiring of the circuit board is close to the edge, grinding may be the only cutting method that can obtain satisfactory cutting quality of the circuit board.

The basic mechanical operation process of grinding is similar to mirror cutting, but its cutting speed and feed speed are much faster. The plate moves along the vertical grinding surface with reference to the grinding fixture. According to the grinding needs, the grinding fixture is fixed on the bushing concentric with the grinding tool. The position of the printed circuit board in the grinding fixture is determined by the alignment holes of the material.

There are three main grinding systems, they are:

1) needle grinding system;

2) Tracking or recording the needle grinding system;

3) CNC grinding system.

1. pin grinding

Pin grinding is most suitable for small batch production, with smooth cutting edge and high precision. The needle grinding system has an accurate template, which is made of steel or aluminum and strictly follows the outline required by the printed circuit board. The template also provides needles for positioning the board. Three or four boards are usually stacked on the protruding positioning pins on the workbench. The diameter of the used cutter and the locating pin is the same, and the grinding direction of the stacked plates is opposite to the rotation direction of the cutter. Usually, because the grinder is easy to make the plate deviate from the locating pin, it takes about two to three cycles of grinding to ensure the correct grinding trajectory.

Although the needle grinding system has high labor intensity and requires high skills of operators, it has high precision and smooth cutting edge, and is most suitable for grinding small batches and irregular plates.

2. Tracking grinding

Tracking grinding system uses template to cut, just like pin grinding system. Here, the stylus tracks the cardboard outline on the template. The stylus can control the movement of the drill shaft on the fixed table, or it can control the movement of the table if the drill shaft is fixed. The latter is usually used for multi-spindle machine tools. The template is made according to the outline of the cutting board, and its outer edge has a recording needle to track the outline. The first step of cutting is to trace the outer edge with a recording needle. In the second step, the recording needle tracks the inner edge, which can unload most of the load on the grinder and better control the cutting size. The recording needle grinding system has higher accuracy than the needle grinding system. The tolerance of mass production products can reach 0. 0 10 inch (0. 25mm) by using common operating techniques. Using a multi-spindle machine tool, 20 plates can be ground simultaneously.

3. CNC grinding system

Computer numerical control (CNC) technology of multi-drill shaft is the first choice for grinding in PCB manufacturing industry. When the production output of products is large and the appearance of printed circuit board is complex, it is generally selected.

Select CNC grinding system. In these equipments, the movements of the worktable, the drill shaft and the cutting machine are all controlled by computers, while the machine operator is only responsible for loading and unloading. Especially in mass production, the cutting tolerance of complex shapes is very small.

In CNC grinding system, the program (a series of commands) for controlling the Z-direction drilling shaft movement of rolling mill is easy to write. These programs can make the machine grind according to a certain path, and the commands of grinding speed and feed speed are also written into the program, so the design can be changed conveniently by rewriting the software program. The cutting profile information is directly input into the computer through the program.

The number of revolutions of the carbon CNC grinder can usually reach 12000-24000r/min, which requires the engine to have enough driving ability to ensure that the number of revolutions of the grinder is not too low.

Machining holes or locating holes are usually outside the circuit board. Although grinding can obtain a right-angled external structure, the internal structure needs to be cut with a tool with the same radius in the first grinding step, and then cut through a 45 angle in the second operation, so as to obtain a right-angled internal structure.

In CNC grinding machine, the parameters of cutting speed and feed speed are mainly determined by the type and thickness of the substrate. The cutting speed is 24000 r/min and the feed speed is 150 in/min, which can be effectively applied to various substrates. However, for soft materials such as PTFE, the adhesive of the substrate will flow out at low temperature, so a low rotation speed of 12000 r/min and a relative AG cutter speed of 200in/min are needed to reduce heat generation.

The commonly used cutting machine is solid tungsten carbide. Because the CNC machine tool can accurately control the movement of the workbench and ensure that the bit of the cutting machine is not affected by vibration, the cutting effect of the small diameter cutting machine is also very good.

In CNC grinding, the geometric shape of cutter gear plays an important role. Because of the high feed speed, the open gear cutting machine should be selected, which can discharge the debris quickly and conveniently. Usually, when the life of diamond-cut gears reaches 15000 linear inches, wear begins. If you need a smooth cutting edge, you should use a grooved cutter. In order to speed up loading and unloading, the machine itself should have an effective system for loading, unloading and discharging debris.

The circuit board can be loaded on the workbench of the machine in different ways and positioned correctly for grinding. The most common method is to use a workbench that can move back and forth, so that the machine can finish loading and unloading while cutting.

4. Laser grinding

Now, lasers are also used for grinding. Free programming and flexible operation mode make ultraviolet laser especially suitable for high-precision HOI cutting. The cutting speed that can be achieved is related to the material, and the typical range is 50 -500mm per second. After cutting, the edges are very neat and do not need any treatment, and the effect is the same as that required when commonly used mechanical grinding or drilling or cutting with CO2 laser (Meier and Schmidt, 2002).