PCB Blog - Method of mechanical cutting of printed circuit boards

1. Cut
Shearing is a step in the mechanical operation of the printed circuit boards, and the rough shape and outline can be given by shearing. The basic cutting method is suitable for a wide variety of substrates, usually no more than 2mm thick. When the cut board exceeds 2mm, the cut edge will appear rough and uneven, so this method is generally not used.

The shearing of the laminate can be manual operation or electromechanical operation, no matter which method has common characteristics in operation. Shears typically have a set of adjustable shear blades, as shown in Figure 10-1. The blade is rectangular, the bottom edge has an adjustable angle of about 7°, the cutting length can reach 1000mm, the longitudinal angle between the two blades is usually selected between 1°-1. 5°, using epoxy glass substrate Able to reach 4°, the gap between the cutting edges of the two blades is less than 0.25mm.

The angle between the two blades is selected according to the thickness of the material being cut. The thicker the material, the greater the angle required. If the shearing angle is too large or the gap between the two blades is too wide, the board will crack when cutting the paper substrate. However, for the epoxy glass substrate, because the material has a certain bending strength, even if there is no crack, The board will also deform. To keep the edges of the base plate clean during shearing, the material can be heated in the range of 30 - 100°C. In order to obtain a clean cut, the board must be held down firmly by a spring mechanism to prevent other inevitable displacement of the board during the shearing process. In addition, parallax can also lead to a tolerance of 0.3 0.5rnrn, which should be reduced to 0.3 rnrn, using an angular scale to improve accuracy. The shears are capable of handling a variety of sizes, capable of delivering repeating sizes. Large machines are capable of cutting hundreds of kilograms of substrates per hour.

2. Sawing
Sawing is another method of cutting substrates. Although the dimensional tolerance of this method is similar to shearing (0.3 - 0.5 rnrn), this method is preferable because the cutting edge is very smooth and neat. In the printed circuit board manufacturing industry, circular sawing machines with movable worktables are mostly used. The speed of the saw blade is adjustable from 2000 - 6000r/rnin. However, once the cutting speed is set, it cannot be changed. It does this with heavy pulleys with more than one V-belt.

The high-speed moving steel blade has a diameter of about 3000rnrn, and it can cut paper indulgent materials at a rate of 2000-3000r/rnin with about 1.2-1.5 teeth per 1cm circumference. For epoxy glass substrates, use a tungsten carbide edged blade. The diamond wheel will cut better, although it is a big investment in the beginning, but because of its long life and the ability to improve the edge cutting effect, it is very beneficial for future work. Here are a few things to pay attention to when using a cutting machine:
1) Pay attention to the cutting force acting directly on the edge and check the firmness of the bearing. There should be no abnormal sensations when examined by hand;
2) For safety reasons, the teeth should always be covered by a protective device;
3) The mounting shaft and engine should be placed accurately;
4) There should be a gap between the saw blade and the bracket, so that the board has a good support for edge cutting;
5) The circular saw should be adjustable, and the height range between the blade and the board should be 10-15mm;
6) Blunt teeth and too rough teeth will make the cutting edge not smooth, so replace it;
7) Wrong cutting rate will cause the cutting edge to be unsmooth, it should be adjusted appropriately, thick material needs to choose slow speed, while thin material can be cut quickly;
8) It should operate at the speed given by the manufacturer;
9) If the saw blade is thin, a reinforcement pad can be added to reduce vibration.

3. Punching
When the printed circuit board design has other shapes or irregular contours in addition to the rectangle, using a die cutting die is a quicker and more economical method. Basic punching operations can be done with a punch press, which produces a clean cut edge better than using a saw or shear. Sometimes, even punching and punching can be done at the same time. However, when good edge effects or tight tolerances are required, die cutting falls short. In the printed circuit board industry, die cutting is generally used for cutting paper substrates, and rarely used for cutting epoxy glass substrates. Die cutting enables the cutting tolerance of printed circuit boards to be within ±(0. 1 - o. 2mm).

1) Punching of paper substrates
Since the paper substrate is softer than the epoxy glass substrate, it is more suitable for cutting by die cutting. When using die cutting tools to cut paper substrates, consider the springback or curvature of the material. Because paper substrates often spring back, usually the die cut section is slightly larger than the die. 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 excess size. As one has noticed, when punching, the die is larger than the size of the hole, and when punching, the die is smaller than normal. For circuit boards with complex shapes, step-by-step tools are used, such as cutting the material one by one. As the die cuts it one by one, the shape of the material gradually changes. In this way, the holes are pierced through the first step or two, and the punching of other parts is finally completed. Punching and die-cutting after heating can improve the cutting of printed circuit boards, such as heating the strip to 50 -70 'C before die-cutting. However, care must be taken not to overheat, as this will reduce flexibility after cooling. In addition, attention should be paid to the thermal expansion of the paper-based benzoin material, since it exhibits different expansion properties in the z-direction and the y-direction.

2) Punching of epoxy glass substrate
When the desired shape of the epoxy glass substrate cannot be produced by shearing or sawing, a special punching method can be used for punching, although this method is not popular, so only when the cutting edge or size requirements are not too strict can use this method. Because although functionally acceptable, the cut edges do not look neat. Since the resilience performance of the epoxy glass substrate is smaller than that of the paper substrate, the tool for punching the epoxy glass substrate must be closely matched between the die and the punch. The die-cutting of epoxy glass substrates should be carried out at room temperature. Because the epoxy glass substrate is hard and difficult to punch, it will reduce the life of the punch and will be worn out soon. The use of carbide punch can receive better cutting results.

4. Milling
Milling is typically used where neatly cut printed circuit boards, smooth edges, and high dimensional accuracy are required. The common milling speed is in the range of 1000 - 3000r/min, usually using straight or helical tooth HSS milling machines. However, for epoxy glass substrates, carbonized pigeon tools are used because of their longer life. To avoid delamination, the backside of the printed circuit board must have a solid backing when milling. For detailed information on milling machines, tools and other operational aspects, refer to the standard factory or shop instructions for these equipment.

5. Grinding
In order to obtain a better edge effect than shearing or sawing and achieve higher dimensional accuracy, especially when the printed circuit board has irregular contour lines, the grinding method can be selected. Using this method, when the dimensional tolerance is ± (0.1-0.2mm), it costs less than punching. Therefore, in some cases, the excess dimension in the die cut can be trimmed in the subsequent grinding process to obtain a smooth cut edge. The multi-spindle machines in use today make grinding very fast, with less labor and less overall cost than with punching. When the board's traces are close to the edge, grinding may be the only way to achieve satisfactory board cut quality. The basic mechanical operation of grinding is similar to mirror grinding, but it cuts and feeds much faster. The plate is moved along the vertical grinding surface with reference to the grinding jig. The grinding fixture is fixed on a bushing that is concentric with the grinding tool according to grinding needs. The position of the printed circuit board in the grinding fixture is determined by the alignment holes in the material.
There are three main grinding systems, they are:
1) Needle grinding system;
2) Track or record the needle grinding system;
3) Numerically controlled (NC) grinding system.

5.1 Needle grinding
Pin grinding is suitable for small batch production, smooth cutting edges and high precision grinding. Pin grinding systems have a steel or aluminum template made to the exact contours of the printed circuit board that also provides pins for board positioning. There are usually three or four boards stacked on top of the locating pins protruding from the workbench. The knives and positioning pins used were the same diameter, and the stacked boards were ground in the opposite direction of the knives' rotation. Usually, because the grinder tends to deviate the board from the positioning pins, it takes about two or three grinding cycles to ensure the correct grinding trajectory. Although the needle grinding system requires high labor intensity and requires highly skilled operators, its high precision and smooth cutting edge make it suitable for grinding small batches and irregular-shaped boards.

5.2 Track grinding
The tracking grinding system uses a template for cutting just like the needle grinding system. Here, the stylus traces the outline of the board on the template. The recording needle 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 often used in multi-drilling machines. The stencil is manufactured to the contours of the cutting board and has a stylus on its outer edge that traces the contour. The cutting step is traced by the stylus to the outer edge. In the second step, the stylus traces the inner edge, which takes most of the load off the grinder for better control over the cut size. The recording needle grinding system is more accurate than the needle grinding system. 0l0in(0. 25mm) . Using general operating techniques, can make mass-produced product tolerances up to ± 0. 0l0in (0. 25mm). Up to 20 boards can be milled at the same time using a multi-axis machine.

5.3 NC grinding system
Computer Numerical Control (CNC) technology with multiple drill spindles is the method of grinding in today's printed circuit board manufacturing industry. When the output of the production product is large and the outline of the printed circuit board is complex, the CNC grinding system is generally selected. In these devices, the movement of the table, drill shaft, and cutting machine is controlled by a computer, while the operator of the machine is only responsible for loading and unloading. Especially for mass production, the cutting tolerances for complex shapes are very small.

In the CNC grinding system, the programs (a series of commands) that control the movement of the drill shaft in the z direction of the rolling mill are 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. , the design can be easily changed by rewriting the software program. The information of the cutting contour is entered directly into the computer through the program. The number of revolutions of carbon CNC grinding machines can usually reach 12000-24000r/min, which requires the engine to have enough driving capacity to ensure that the number of revolutions of the grinding machine is not too low. Machined or locating holes are usually on the outer portion of the circuit board. Although grinding can achieve a right-angled outer structure, the inner structure needs to be cut with a knife of equal radius in one step grinding, and then cut through a 45° angle in the second operation, so that a right-angled inner structure can be obtained.

In CNC grinding machines, the parameters of cutting speed and feed speed are mainly determined by the type and thickness of the substrate. The cutting speed is 24000r/min and the feed speed is 150in/min, which can be effectively applied to many substrates, but for soft materials like teflon and other similar materials, the adhesive of the substrate will flow out at low temperature, Therefore, a low speed of 12000r/min and a higher feed rate of 200in/min are required to reduce heat generation. The commonly used cutters are of the solid tungsten carbide type. Since the CNC machine can control the movement of the table to ensure that the drill bit of the cutting machine is not affected by vibration, the cutting effect of the small diameter cutting machine is also very good.

5.4 Laser grinding
Now, lasers are also used for grinding, and the free programming and flexible operating modes make UV lasers particularly suitable for high-precision HOI cutting. The achievable cutting speed is material dependent and typically ranges from 50-500mm per second. The cut edge is very neat and does not require any treatment, the effect is the same as the usual mechanical grinding or punching or cutting with CO2 laser on printed circuit boards.