Method and Precautions For Mechanical Cutting of PCB

1. Cut

Shearing is the first step in the mechanical operation of printed circuit boards, through which the approximate shape and outline can be given. The basic cutting method is suitable for a wide variety of substrates, usually up to 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 laminates can be manual or electromechanical, no matter which method has common characteristics in operation. Shears usually have a set of adjustable cutting blades. Its blade is rectangular, and the bottom edge has an adjustable angle of about 7°, and the cutting length can reach 1000mm. The longitudinal angle between the two blades is usually best selected between 1°-1.5°, and epoxy glass is used The substrate can reach a maximum of 4°, and 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 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. In order to keep the edge of the base plate clean during shearing, 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 displacement of the board during the cutting process. In addition, parallax can also cause a tolerance of 0.3 0.5rnrn, which should be minimized, and the use of corner marks can improve accuracy.

Shears are capable of handling a wide range of sizes and are able to provide precise repeatable dimensions. Large machines are capable of cutting hundreds of kilograms of substrate per hour.

2. Sawing

Sawing is another method of cutting substrates. Although the dimensional tolerances for this method are similar to shearing (0.3 - 0.5 rnrn), this method is preferable because the cut edges are very smooth and clean.

In the printed circuit board manufacturing industry, circular sawing machines with movable worktables are mostly used. The speed adjustable range of the saw blade is 2000 - 6000r/rnin. But once the cutting speed is set, it cannot be changed. It is achieved by 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-made materials at a rate of 2000-3000r/rnin with about 1. 2- 1.5 teeth per 1cm circumference. For glass epoxy substrates, use tungsten carbide-tipped inserts. A diamond wheel will cut better and although it is a big initial investment, it will be very beneficial later on due to its longevity and ability to improve edge cutting.

The following are a few issues that need to be paid 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 not be any unusual sensations when examined by hand;

(2), for the sake of safety, the tooth piece should always be covered by the protective device;

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

(4) The gap between the saw blade and the bracket should be the smallest, so that the board can be well supported 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 it is best to replace them;

(7) The wrong cutting speed will cause the cutting edge to be rough, it should be properly adjusted, thick materials need to choose slow speed, and thin materials can be cut quickly;

(8) It should be operated according to the speed given by the manufacturer;

(9) If the tooth blade of the saw is very thin, a reinforcement pad can be added to reduce vibration.

3. Punching

When a printed circuit board design has shapes other than rectangles or irregular contours, using a die cut tool is a quick and economical method. Basic punching operations can be done with a punch press, which provides a cleaner cut edge than a saw or shear. Sometimes, even punching and die cutting can be done at the same time. However, when fine edge effects or tight tolerances are required, punching falls short. In the printed circuit board industry, die cutting is generally used to cut paper substrates, but is rarely used to cut epoxy glass material substrates. Die cutting enables cutting tolerances of printed circuit boards within ±(0. 1 - o. 2mm).

(1) Die-cutting of paper substrates

Because paper substrates are softer than epoxy glass substrates, they are more suitable for die-cutting. When cutting paper substrates with a die cutting tool, consider the springback or curvature of the material. Because paper substrates tend to spring back, usually the die cut is slightly larger than the die. Therefore, the size of the mold is 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 hole size, and when punching, the die is smaller than normal.

For circuit boards with complex shapes, it is best to use step-by-step tools, such as cutting the material one by one, and the shape of the material changes gradually as the die cuts it one by one. In this way, the hole is punched through in one or two initial steps, and the rest of the parts are finally punched. Punching and punching after heating can improve the cutting effect of printed circuit boards, for example, heating the strip to 50 -70 'C before punching. However, care must be taken not to overheat, as this will reduce the stretchability after cooling. In addition, attention should be paid to the thermal expansion of the paper-made benzene material, because it exhibits different expansion properties in the z direction and y direction.

(2) Punching of epoxy glass substrate

When shearing or sawing cannot produce the desired shape of the epoxy glass substrate, it can be punched with a special punching method, although this method is not popular, so only when the cutting edge or size requirements are not too strict only when this method is used. Because although functionally acceptable, the cut edges don't look very clean. Since the rebound performance of the epoxy glass substrate is smaller than that of the paper substrate, the tool for punching the epoxy glass substrate should have a tight fit between the die and the punch. Die-cutting of epoxy-glass substrates should be performed at room temperature.

Because the epoxy glass substrate is hard and difficult to punch, the life of the punch will be reduced and it will be worn out soon. A punch with a carbide tip can receive better cutting results.

4. Milling

Milling is often used where clean cuts, smooth edges and high dimensional accuracy are required for printed circuit boards. Common milling speeds are in the range of 1000 - 3000r/min, and straight or helical toothed high speed steel milling machines are usually used. However, for epoxy glass substrates, it is better to use carbide pigeon tools because of their longer life. To avoid delamination, the backside of the PCB must have a solid backing when milling. For milling machines, tools, and other operating details, refer to the factory or shop's standard instructions for such equipment.

5. Grinding

In order to obtain a better edge effect than shearing or sawing and to achieve higher dimensional accuracy, especially when the printed circuit board has irregular outlines, the grinding method can be selected. With this method, it costs less than punching when the size tolerance is ± (0.1-0.2mm). Therefore, in some cases, the excess size of the punching can be trimmed in the subsequent grinding process to obtain a smooth cutting edge.

The multi-spindle machines in use today allow for very rapid grinding with less labor input and less overall cost than with punching. When the traces of the board are close to the edge, grinding may be the only trimming method that can obtain a satisfactory board cut quality.

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

There are mainly three grinding systems, which are: a, needle grinding system; b, tracking or recording needle grinding system; c, numerical control (NC) grinding system.

(1), needle grinding

Needle grinding is best suited for small batch production, smooth cutting edges, and high precision grinding. The Pin Grinding System has a precise template in steel or aluminum that is fabricated to the exact contours of the printed circuit board and also provides pins for board alignment. There are usually three or four boards stacked on the alignment pins protruding from the bench. The cutter and positioning pins used are the same diameter, and the stacked boards are ground in the opposite direction to the rotation of the cutter. Usually, it takes about two or three cycles of grinding to ensure the correct grinding trajectory, since the grinder tends to cause the board to deviate from the alignment pins.

Although the needle grinding system requires high labor intensity and requires highly skilled operators, it has high precision and smooth cutting edges, and is most suitable for grinding small batches and irregularly shaped boards.

(2), tracking grinding

Tracking grinding systems use templates for cutting just like pin grinding systems. Here, the stylus traces the outline of the board on the stencil. The stylus can control the movement of the drill shaft on a fixed table, or it can control the movement of the table if the drill shaft is fixed. The latter are often used on multi-spindle machines. The template is manufactured to the contour of the cut board, with a registration pin on its outer edge that traces the contour. The first step in cutting is to trace the outer edge with a stylus. In the second step, the stylus traces the inside edge, which takes most of the load off the grinder for better control over cut size. Record pin grinding systems are more precise than pin grinding systems. Using general operating techniques, the tolerance of mass-produced products can reach ±0. 0l0in (0. 25mm). Up to 20 boards can be ground simultaneously with the multi-spindle machine.

(3), NC grinding system

Computer Numerical Control (CNC) technology with multiple spindles is the method of choice for 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, the CNC grinding system is generally selected. In these devices, the movement of the table, drill shaft and cutting machine is controlled by computer, and the operator of the machine is only responsible for loading and unloading. Especially for high-volume production, complex shapes can be cut with very tight tolerances.

In the CNC grinding system, the program (a series of commands) that controls the movement of the drilling shaft in the z direction of the 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. , the design can be easily changed by rewriting the software program. The information of the cutting contour is input directly into the computer through the program.

The rotation speed of the carbonaceous CNC grinding machine can usually reach 12000 - 24000r/min, which requires the engine to have sufficient driving capacity to ensure that the rotation number of the grinding machine will not be too low.

Machining or pilot holes are usually on the outer portion of the board. Although grinding can achieve a right-angled outer structure, the inner structure needs to be cut with a knife of equal radius in the first grinding step, and then cut at a 45° angle in the second operation, so that a right-angled inner structure can be obtained .

In CNC grinding machines, the cutting speed and feed speed parameters are mainly determined by the substrate type and thickness. 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 PTFE and other similar materials, the adhesive of the substrate will flow out at low temperature, Therefore, a low speed of 12000r/min and a high feed speed of 200in/min are required to reduce heat generation.

Commonly used cutters are of the solid tungsten carbide type. Since the CNC machine can precisely control the movement of the worktable 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.

In CNC grinding, the geometry of the cutter gear plays an important role. Due to the high feed rate, a cutter with open gears should be selected so that the debris can be discharged quickly and easily. Typically, diamond cut gears begin to wear out at 15,000 linear inches in life. If a very smooth cut edge is desired, use a grooved cutter. In order to speed up loading and unloading, the machine itself must have a system for efficiently loading, unloading and discharging debris.

Boards can be loaded onto the machine's table by different methods while being properly positioned for grinding. The most common method is to use a table that can move back and forth so that loading and unloading can be done while the machine is cutting.

(4), laser grinding

Now, lasers are also used for grinding, free programming and flexible operation modes make UV lasers 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. The cut edges are very clean and do not require any treatment, as is often required for mechanical grinding or punching or CO2 laser cutting (Meier and Schmidt, 2002).