To meet customer needs, the original PCB is cut into the required sizes; this process is called PCB milling. This article will provide an in-depth introduction to several key concepts and principles related to milling: cutting direction, board compensation, and positioning methods.
I. Cutting Direction
When milling printed circuit boards, the cutting direction is a critical factor to consider. When the milling cutter engages the board, one surface is always facing the cutting edge of the cutter, while the other is always facing away from it. The former is typically machined to a smoother finish and achieves higher dimensional accuracy. This is because the spindle always rotates clockwise. Therefore, when milling the outer contour of a PCB, we use a counterclockwise feed direction, which is commonly referred to as “up-milling.” When milling frames or slots inside the PCB, we use the “down-milling” method.
II. Milling Plate Compensation
Milling plate compensation is a key concept in milling. It allows the machine tool to automatically offset the milling cutter by half its diameter—that is, the radius—based on preset values, to ensure that the milled contour matches the program settings. At the same time, if the machine tool supports compensation, it is important to pay attention to the direction of compensation and the use of program commands. If compensation commands are used improperly, it may result in the PCB’s dimensions being either larger or smaller by an amount equivalent to the length and width of the milling cutter’s diameter.
III. Positioning Methods
Alignment is one of the key steps in PCB milling and is crucial for engineering and technical personnel. There are typically two alignment methods:
Internal positioning
Internal alignment is a common method, typically involving the use of mounting holes, connector holes, or other non-plated-through holes on the PCB as alignment holes. These holes are usually positioned diagonally relative to one another, and holes with larger diameters are preferred whenever possible. Plated through-holes are generally not used because variations in plating thickness between different holes may affect the consistency of the alignment holes. When determining PCB alignment, using fewer pins is the preferred option. Generally, small boards use two定位 pins, while large boards use three. The advantages of this method include accurate positioning, minimal board deformation, high precision, a clean appearance, and fast milling speeds. The disadvantage is the need to stock定位 pins of various diameters.
External Positioning
If the customer cannot use the internal alignment method, the only option is the external alignment method. However, this method is less efficient because it requires splitting the milling strip, and boards can only be stacked one at a time. In addition, the external alignment method may pose some quality risks, resulting in higher production costs. The external positioning method involves adding positioning holes to the exterior of the board, which are used to guide the milling process. Because external positioning is used, the entire board cannot be cut in a single pass; otherwise, the PCB is prone to damage. Therefore, the milling program must be divided into multiple segments and milled sequentially. Ultimately, a raised point remains on the board, which must be manually sanded down, affecting the PCB’s appearance.
Milling is an essential step in the manufacture of high-quality PCBs; by using the correct cutting direction, board compensation, and positioning methods, the quality and performance of the final product can be ensured.
Frequently Asked Questions
1. What is PCB milling?
PCB milling is the process of cutting the raw PCB to the dimensions specified by the customer; it is typically a critical step in the production and assembly process.
2. Why is the cutting direction important?
The choice of cutting direction affects the surface quality and dimensional accuracy of the milled surface. The correct cutting direction ensures a smooth machined surface and precise dimensions.
3. What is milling plate compensation?
Milling plate compensation is the process of automatically offsetting the milling cutter by half its specified diameter to ensure that the milled contour matches the program settings.
4. What are the different positioning methods?
Alignment methods include internal alignment and external alignment. Internal alignment uses holes within the PCB as alignment holes, while external alignment adds alignment holes to the outside of the board.
5. What are the advantages and disadvantages of the external positioning method?
External positioning is less efficient and poses quality risks, but in some cases, it is the only option because internal positioning cannot be used.
