PCB Tech - How to ensure the reliability of PCB milling

The milling technology of PCB circuit board NC milling machine includes the selection of cutting direction, compensation method, positioning method, frame structure and cutting point. Are important aspects to ensure milling accuracy. Based on the importance of these aspects, Wang Gaogong, an engineer of Shenzhen jieduobang Technology Co., Ltd., summarized these important aspects.

Cutting direction and compensation method:

When the milling cutter cuts into the plate, one side is always facing the cutting edge of the milling cutter, and the other side is always against the cutting edge of the milling cutter. The former has smooth machined surface and high dimensional accuracy. The spindle always rotates clockwise. Therefore, when milling the external contour of the printed board, the NC milling machine with fixed spindle and fixed worktable should adopt counterclockwise cutting. This is commonly referred to as reverse milling. When milling the frame or slot inside the PCB, the forward milling method is adopted. Milling plate compensation is that when milling plate, the machine tool automatically installs the set value to make the milling cutter automatically offset half of the set milling cutter diameter with the center of the milling line, that is, the radius distance, so as to keep the milling shape consistent with the program setting. At the same time, if the machine tool has the function of compensation, attention must be paid to the direction of compensation and the command of using the program. If the error of using the compensation command will make the shape of the circuit board more or less equal to the length and width of the milling cutter diameter.

Positioning method and cutting point:

There are two positioning methods; One is internal positioning, the other is external positioning. Positioning is also very important for process developers. Generally, the positioning scheme should be determined during the pre production of PCB.

Internal positioning is a general method. The so-called internal positioning is to select the mounting hole, plug-in hole or other non-metallic hole in the PCB as the positioning hole. The relative position of the hole shall be on the diagonal and the hole with large diameter shall be selected as far as possible. Metallized holes shall not be used. Because the difference of plating thickness in the hole will affect the consistency of the positioning hole you selected. At the same time, it is easy to damage the plating in the hole and the edge of the hole surface when taking the board. Under the condition of ensuring the positioning of PCB printed board, the fewer the number of pins, the better. Generally, two pins are used for small plates and three pins are used for large plates, which has the advantages of accurate positioning, small plate shape deformation, good shape and fast milling speed. Its disadvantage is that there are many kinds of holes in the plate, and pins of various diameters need to be prepared. If there is no available positioning hole in the plate, it is necessary to discuss with customers to add positioning holes in the plate during early production, which is more cumbersome. At the same time, the milling template of each kind of plate is different, the management is more troublesome and the cost is higher.

External positioning is another positioning method, which is to add a positioning hole on the outside of the plate as the positioning hole of the milling plate. Its advantage is that it is easy to manage. If the production is standardized in advance, there are generally about 15 kinds of milling plate templates. Due to the use of external positioning, the board cannot be milled off, otherwise the circuit board is very easy to be damaged, especially the panel. Because the milling cutter and dust suction device will bring out the board, the PCB circuit board will be damaged and the milling cutter will be broken. The method of segmented milling is adopted to leave the joint point. First mill the plate. When the milling is finished, the program is suspended, then fix the plate with adhesive tape, execute the second section of the program, and drill out the joint point with a 3mm-4mm drill bit. It has the advantages of less template, low cost and easy management. It can mill all circuit boards without installation holes and positioning holes in the board. It is convenient for small process personnel to manage. In particular, the production of cam and other early production personnel can be simplified, and the utilization rate of substrate can be optimized at the same time. The disadvantage is that due to the use of drill bits, at least 2-3 convex points are left in the shape of the circuit board, which is not beautiful, which may not meet the customer's requirements, long milling time and slightly high labor intensity.

Frame and PCB cutting point:

The fabrication of frame belongs to the early fabrication of PCB circuit board. The frame design has an impact not only on the uniformity of electroplating, but also on the milling board. For example, if the design is not good, the frame is easy to deform or some small waste blocks are generated during milling, which will block the dust suction pipe or break the high-speed milling cutter, The deformation of the frame, especially when positioning the milling plate externally, causes the deformation of the finished plate. In addition, the good selection of the cutting point and processing sequence can keep the strength of the frame fast. Poor selection, the frame is easy to deform and scrap the printed board.

Process parameters of milling:

The profile of PCB printed board is milled with carbide milling cutter, and the cutting speed of milling cutter is generally 180 ~ 270m / min. The calculation formula is as follows (for reference only):

S=pdn/1000(m/min)

Where: P: PI (3.1415927)

d: Milling cutter diameter, mm

n; Milling cutter speed, R / min