Mechanical cutting method of printed circuit boards
1 Shearing
Shearing is the first step in the mechanical operation of printed circuit boards. It can give a rough shape and outline. The basic cutting method is suitable for a variety of substrates, usually with a thickness of no more than 2mm. When the cut board exceeds 2mm, the cut edge will appear rough and uneven, so this method is generally not used.
Laminates can be cut manually or by electric machinery. Regardless of the method, there are common characteristics in operation. The shearing machine usually has a set of adjustable shear blades, as shown in Figure 10-1. The blade is rectangular, and the bottom blade has an adjustable angle of about 7°. The cutting length can reach 1000mm. The longitudinal angle between the two blades is usually preferably selected between 1°-1.5°. The maximum angle can reach 4° when using epoxy glass substrate. The gap between the cutting edges of 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 larger 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 epoxy glass substrate, due to the material’s certain bending strength, the board will deform even if no cracks appear. In order to keep the edge of the bottom plate neat during the shearing process, the material can be heated in the range of 30 – 100 ° C.
In order to obtain a neat cut, the board must be firmly pressed down by a spring device to prevent other inevitable displacements of the board during the shearing process. In addition, parallax can also cause tolerances of 0.3 0.5rnrn, which should be minimized. The use of corner marks can improve accuracy.
Shearing machines can handle a variety of sizes and can provide accurate repeat sizes. Large machines can cut hundreds of kilograms of substrates per hour.
2 Sawing
Sawing is another way to cut substrates. Although the dimensional tolerance of this method is similar to shearing (0. 3 – 0.5rnrn), this method is more desirable because its cutting edges are 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 to 6000r/rnin. However, once the cutting speed is set, it cannot be changed. It is achieved by a heavy pulley with more than one V-belt.
The diameter of the high-speed steel blade is about 3000rnrn, and it can cut paper and other materials at a rate of 2000-3000r/rnin with about 1.2-1.5 teeth per 1cm circumference. For epoxy glass substrates, a blade with a tungsten carbide blade is used. The cutting effect of the diamond wheel will be better. Although it is a large investment at the beginning, it is very beneficial for future work due to its long service life and ability to improve edge cutting effect.
The following are a few issues that need to be paid attention to when using the cutting machine:
1) Pay attention to the cutting force directly acting on the edge and check the firmness of the bearing. There should not be any abnormal feeling when checking 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) The gap between the saw teeth and the bracket should be minimal, so that the board has 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 rough, it is best to replace them;
7) The wrong cutting speed will cause the cutting edge to be rough, and it should be properly adjusted. Thick materials need to choose a slow speed, while thin materials can be cut quickly;
8) The speed given by the manufacturer should be followed;
9) If the saw teeth are very thin, a reinforcement pad can be added to reduce vibration.
3 Punching
When the printed circuit board design has other shapes or irregular contours besides rectangles, using a punching die is a faster and more economical method. Basic punching can be done with a punch press, which gives a clean cut edge and is better than with a saw or shear. Sometimes, punching and punching can even be done simultaneously. However, punching does not meet the requirements when good edge effects or small tolerances are required. In the printed circuit board industry, punching is generally used to cut paper substrates, and rarely used to cut epoxy glass substrates. Punching can make the printed circuit board cut with a tolerance within ± (0.1 – o.2mm).
（1）Punching of paper substrates
Since paper substrates are softer than epoxy glass substrates, they are more suitable for cutting with punching methods. When using a punching tool to cut paper substrates, the rebound or bending of the material should be considered. Because paper substrates often rebound, the punching part is usually slightly larger than the die. Therefore, the size of the die should be selected according to the tolerance and the thickness of the substrate, slightly smaller than the printed circuit board to compensate for the excess size. As people have noticed, when punching, the die is larger than the hole size, and when punching, the die is smaller than the normal size.
For boards with complex shapes, it is best to use a stepping tool, for example, cutting the material strip by strip, and the shape of the material gradually changes as the die punches it strip by strip. In this way, the holes are punched through in the first one or two steps, and the rest of the punching is completed. The cutting effect of printed circuit boards can be improved by punching and punching after heating, for example, heating the strips to 50-70’C before punching. However, care must be taken not to overheat them, as this will reduce the elasticity after cooling. In addition, attention should be paid to the thermal expansion of paper phenolic materials, which have different expansion properties in the z-direction and y-direction.
（2）Punching of epoxy glass substrates
When the required shape of epoxy glass substrates cannot be produced by shearing or sawing, a special punching method can be used for punching, although this method is unpopular and should only be used when the cutting edge or size requirements are not too strict. Because although it is acceptable in function, the cutting edge does not look very neat. Since the rebound performance of epoxy glass substrates is smaller than that of paper substrates, the tools for punching epoxy glass substrates must have a close fit between the die and the punch press. Punching of epoxy glass substrates should be carried out at room temperature.
Since epoxy glass substrates are hard and difficult to punch, the life of the punch press will be reduced and it will soon be worn out. Using a punch press with a carbide tip can achieve better cutting results.
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 speeds are in the range of 1000-3000r/min, usually using linear or spiral tooth high-speed steel milling machines. However, for epoxy glass substrates, it is best to use carbide pigeon tools because they have a longer life. To avoid delamination, the back of the printed circuit board must have a solid backing plate when milling. For detailed information on milling machines, tools and other operating aspects, refer to the factory or store’s standard instructions for these equipment.
5 Grinding
In order to obtain better edge effects and higher dimensional accuracy than shearing or sawing, especially when the printed circuit board has irregular contours, grinding can be selected. With this method, the cost is less than punching when the dimensional tolerance is ± (0.1-0.2mm). Therefore, in some cases, the excess size in punching can be trimmed in the subsequent grinding process to obtain a smooth cut edge.
The multi-axis machines used today make grinding very fast, and the labor input and total cost are less than when punching. When the board traces are close to the edge, grinding may be the only cutting method that can obtain satisfactory board cutting quality.
The basic mechanical operation process of grinding is similar to mirror grinding, but its cutting speed and feed speed are much faster. The board moves along the vertical grinding surface based on the grinding fixture. The grinding fixture is fixed on a shaft bushing concentric with the grinder according to the grinding needs. The position of the printed circuit board in the grinding fixture is determined by the alignment hole of the material.
There are three main grinding systems, which are:
1) Pin grinding system;
2) Tracking or recording pin grinding system;
3) Numerical control (NC) grinding system.
1.Pin grinding
Pin grinding is best suited for small batch production, smooth cutting edges, and high precision grinding. The pin grinding system has a precision steel or aluminum template that is made strictly according to the required outline of the printed circuit board, and the template also provides pins for locating the board. Usually three or four boards are stacked on the protruding locating pins on the workbench. The tool and the locating pins used are of the same diameter, and the stacked boards are ground in the opposite direction of the tool’s rotation. Usually, because the grinder tends to deviate from the locating pins, it takes about two or three cycles of grinding to ensure the correct grinding trajectory.
Although the pin grinding system requires high labor intensity and requires high operator skills, it has high precision and smooth cutting edges, and is most suitable for grinding small batches and irregularly shaped boards.
2.Tracking grinding
The tracking grinding system uses a template for cutting like the pin grinding system. Here, a stylus traces the outline of the board on the template. The stylus can control the movement of the drill spindle on a fixed table, or it can control the movement of the table if the drill spindle is fixed. The latter is often used on multi-spindle machines. The template is made to the outline of the cut board and has a stylus on its outer edge that traces the outline. The first step of cutting is for the stylus to trace the outer edge. In the second step, the stylus traces the inner edge, which can unload most of the load on the grinder to better control the cut size. Stylus grinding systems are more accurate than pin grinding systems. With normal operating techniques, mass-produced products can be produced with tolerances of ±0.010in (0.25mm). Multi-spindle machines can grind 20 boards at the same time.
3.NC grinding system
Computer numerical control (CNC) technology with multiple drill spindles is the preferred method of grinding in the printed circuit board manufacturing industry today. When the output of the product is large and the outline of the printed circuit board is complex, CNC grinding systems are generally selected. In these machines, the movement of the table, the drill spindle and the cutter are all controlled by a computer, while the machine operator is only responsible for loading and unloading. Especially for large-scale production, the cutting tolerance of complex shapes is very small.
In CNC grinding systems, the program (a series of commands) that controls the movement of the drill spindle in the z direction of the mill is easily written. These programs enable the machine to grind along a certain path. The commands for grinding speed and feed speed are also written into the program, and the design can be easily changed by rewriting the software program. The information of the cutting contour is directly input into the computer through the program.
Carbon CNC grinders usually have a speed of 12,000-24,000 r/min, which requires sufficient engine drive capacity to ensure that the speed of the grinder is not too low.
The processing or positioning holes are usually on the outer part of the circuit board. Although grinding can achieve right-angled external structures, the internal structures need to be cut with a tool of equal radius in the first grinding operation, and then cut at a 45° angle in the second operation, so that the right-angle internal structures can be obtained.
In CNC grinding machines, the cutting speed and feed rate parameters are mainly determined by the substrate type and thickness. A cutting speed of 24,000 r/min and a feed rate of 150 in/min can be effectively applied to many substrates, but for soft materials such as PTFE and other similar materials, the substrate adhesive will flow out at low temperatures, so a low speed of 12,000 r/min and a higher feed rate of 200 in/min are required to reduce heat generation.
The cutters commonly used are solid tungsten carbide types. Small diameter cutters also cut well because CNC machines can accurately control the movement of the worktable to ensure that the cutter drill is not affected by vibration.
In CNC grinding, the geometry of the cutter gear plays an important role. Due to the high feed rate, the cutter should be selected with open gears so that the debris can be discharged quickly and easily. Generally, diamond cutting gears begin to wear after the life reaches 15,000 linear inches. If a very smooth cutting edge is required, a grooved cutter should be used. To speed up loading and unloading, the machine itself should have an effective loading and unloading and debris discharge system.
There are different ways to load the board onto the machine’s workbench and position it correctly for grinding. The most common method is to use a workbench that can move back and forth, so that loading and unloading can be completed while the machine is cutting.
4.Laser grinding
Nowadays, lasers are also used for grinding. Free programming and flexible operation modes make UV lasers particularly suitable for high-precision HOI cutting. The achievable cutting speed depends on the material, and the typical range is 50-500mm per second. The cut edges are very neat and do not require any treatment. The effect is as required by commonly used mechanical grinding or punching or CO2 laser cutting (Meier and Schmidt, 2002).
