PCB News - Factors to consider in multi-layer circuit board design

The design performance of multi-layer PCB boards is mostly similar to single-layer or double-layer boards. Pay attention to the reasonable layout of the circuit, and consider factors such as inner layer capacity, insulation resistance, welding resistance, and product safety. The following content mainly describes the important factors that should be considered in the design of the multi-layer PCB boards from the electrical and mechanical aspects of the design.

1. Mechanical design factors

The mechanical design includes choosing the appropriate board thickness, stacking of boards, board size, inner copper tube, aspect ratio and so on.

1. Board thickness

The thickness of the multi-substrate is determined by many factors, such as the number of signal layers, the number and thickness of power boards, the aspect ratio of the aperture and thickness required for high-quality punching and plating, the length of the component pins required for automatic insertion, and The type of connection used. The thickness of the entire circuit board consists of the conductive layer on both sides of the board, the copper layer, the thickness of the substrate and the thickness of the prepreg material. It is difficult to obtain tight tolerances on synthetic multi-substrates, and a tolerance standard of about 10% is considered reasonable.

2. The stacking of boards

In order to minimize the possibility of board distortion and obtain a flat finished board, the layering of multiple substrates should be symmetrical. That is to have an even number of copper layers, and to ensure that the thickness of the copper and the copper foil pattern density of the board layer are symmetrical. Generally, the radial direction of the construction material used for the laminate (for example, fiberglass cloth) should be parallel to the side of the laminate. Because the laminate shrinks in the radial direction after bonding, this will distort the layout of the circuit board, showing variability and low dimensional stability. However, by improving the design, the warpage and distortion of the multi-substrate can be minimized. Through the even distribution of copper foil on the entire level and ensuring the symmetry of the multi-substrate structure, that is, ensuring the same distribution and thickness of the prepreg material, the purpose of reducing warpage and distortion can be achieved. The copper and laminated layers should be made from the center layer of the multi-substrate to the two outermost layers. The minimum distance (dielectric thickness) specified between two copper layers is 0.080 mm. It is known from experience that the minimum distance between two copper layers, that is, the minimum thickness of the prepreg material after bonding, must be at least twice the thickness of the embedded copper layer. In other words, if the thickness of each of the two adjacent copper layers is 30μm, the thickness of the prepreg material is at least 2 (2 x 30μm) = 120μm, which can be achieved by using two layers of prepreg material (glass fiber) .

3. Board size

The board size should be optimized according to the application requirements, the size of the system box, the limitations of the PCB board manufacturer and the PCB manufacturing capacity. Large circuit boards have many advantages, such as fewer substrates, shorter circuit paths between many components, so that they can have a higher operating speed, and each board can have more input and output connections, so in Large circuit boards should be the first choice in many applications. For example, in personal computers, you see larger motherboards. However, it is more difficult to design the signal line layout on a large board, requiring more signal layers or internal wiring or space, and the difficulty of heat treatment is also greater. Therefore, the designer must consider various factors, such as the size of the standard board, the size of the manufacturing equipment, and the limitations of the manufacturing process. Some guidelines for selecting standard printed circuit board sizes are given in 1PC-D-322.

4. Inner copper foil

The most commonly used copper foil is 1oz (1oz of copper foil per square foot of surface area). However, for dense boards, the thickness is extremely important, and strict impedance control is required. This kind of board needs to be used

0.50z copper foil. For the power plane and ground plane, it is best to choose 2oz or heavier copper foil. However, etching heavy copper foil will reduce the controllability, and it is not easy to achieve the desired pattern of line width and pitch tolerance. Therefore, special processing techniques are required.

5. Hole

According to the component pin diameter or diagonal size, the diameter of the plated through hole is usually kept between 0.028 and 0.010in, which can ensure enough volume for better welding.

6. Aspect ratio

The "aspect ratio" is the ratio of the thickness of the plate to the diameter of the hole. It is generally believed that 3: 1 is the standard aspect ratio, although a high aspect ratio like 5:1 is also commonly used. The aspect ratio can be determined by factors such as drilling, slag removal or etch back and electroplating. When maintaining the aspect ratio within the range that can be produced, the vias should be as small as possible.

2. Electrical design factors

Multi-substrate is a high-performance, high-speed system. For higher frequencies, the rise time of the signal is reduced, so signal reflection and line length control become critical. In a multi-substrate system, the requirements for the controllable impedance performance of electronic components are very strict, and the design must meet the above requirements. The factors that determine the impedance are the dielectric constant of the substrate and the prepreg material, the spacing of the wires on the same layer, the thickness of the interlayer dielectric and the thickness of the copper conductor. In high-speed applications, the lamination sequence of the conductors in the multi-substrate and the connection sequence of the signal net are also crucial. Dielectric constant: The dielectric constant of the substrate material is an important factor in determining impedance, propagation delay, and capacitance. The dielectric constant of the glass epoxy substrate and the prepreg material can be controlled by changing the percentage of the resin content.

The prepreg material with relatively low dielectric constant is suitable for use in radio frequency and microwave circuits. In radio frequency and microwave frequencies, the signal delay caused by the lower dielectric constant is lower. In the substrate, the low loss factor can minimize the electrical loss.

The dielectric constant of epoxy resin is 3.45, and the dielectric constant of glass is 6.2. By controlling the percentage of these materials, the dielectric constant of epoxy glass may reach 4.2-5.3. The thickness of the substrate is a good indicator for determining and controlling the dielectric constant.