1. Board size
Board size should be optimized based on application requirements, system box size, board manufacturer's limitations, and manufacturing capabilities. Large circuit boards have many advantages, such as fewer substrates, shorter circuit paths between many components, which allows higher operating speeds, and each board can have more input and output connections, so In many applications, large circuit boards should be preferred. For example, in personal computers, large motherboards are all seen. However, it is more difficult to design the layout of signal lines on large boards, and more signal layers or internal connections or spaces are needed, and the heat treatment is more difficult. Therefore, the designer must consider various factors such as the size of the standard board, the size of the production equipment, and the limitations of the manufacturing process. Some guidelines for selecting the standard printed circuit board/board dimensions are given in 1PC-D-322.
2. Board thickness
The thickness of the multiple substrates is determined by a number of factors such as the number of signal layers, the number and thickness of the power planes, the aspect ratio of the apertures and thicknesses required for high-quality punching and plating, the required component lead lengths for automatic insertion and The type of connection used. The thickness of the entire circuit board consists of the conductive layer, copper layer, substrate thickness, and prepreg thickness on both sides of the board. Obtaining tight tolerances on synthetic multi-substrates is difficult, and about 10% of the tolerance standards are considered reasonable.
3. Cascade of plates
In order to minimize the chance of twisting the board and obtain a flat finished board, the delamination of the multiple boards should be kept symmetrical. That is, it has an even number of copper layers and ensures that the thickness of the copper and the density of the copper foil pattern of the board are symmetrical. The radial (eg, glass fiber cloth) of the construction material that is typically used to laminate the crucible should be parallel to the edges of the laminate. Because the laminate shrinks in the radial direction after bonding, it can distort the layout of the circuit board, exhibiting variability and low space stability.
However, warping and twisting of multiple substrates can be minimized by improving the design. Through the uniform distribution of the copper foil on the entire level and the symmetry of the structure of the multi-substrate, that is to ensure the same distribution and thickness of the prepreg material, the purpose of reducing warpage and twisting can be achieved. The copper and laminated layers should be made from the center layer of the multi-substrate up to the outermost two layers. The minimum distance (dielectric thickness) specified between the two copper layers is 0.080 mm.
It is empirically known that the minimum distance between two copper layers, that is, the minimum thickness of the prepreg after bonding must be at least twice the thickness of the embedded copper layer. In other words, for two adjacent copper layers, if each layer is 30 μm thick, the prepreg material has a thickness of at least 2 (2 x 30 μm) = 120 μm, which can be achieved by using two layers of prepreg (glass fiber weave The typical value of cloth is 1080).
4. Inner copper foil
The most commonly used copper foil is 1 oz (1 oz copper foil per square foot of surface area). However, for dense boards, the thickness is extremely important and strict impedance control is required. This board needs to be used
0.50z copper foil. For power and ground planes, copper foil of 2oz or more is preferred. However, etching heavy copper foil results in reduced controllability and it is not easy to achieve the desired pattern of line width and spacing tolerances. Therefore, special processing techniques are required.
The diameter of the plated through holes is usually maintained between 0.028 and 0.010 in, depending on the diameter of the component pins or the size of the diagonal. This ensures a sufficient volume for better soldering.
6. Aspect ratio
The "aspect ratio" is the ratio of the thickness of the plate to the borehole diameter. It is generally accepted that 3:1 is the standard aspect ratio, although high aspect ratios like 5:1 are also commonly used. The aspect ratio can be determined by drilling, removing slag, or etching back and plating. When maintaining the aspect ratio within the producible range, the vias should be as small as possible.