The design performance of multi substrate is similar to that of single substrate or double substrate, that is, pay attention to avoid filling too many circuits with too small space, resulting in unrealistic tolerance, high inner capacity, and even endangering the safety of product quality. Therefore, the performance specification should consider the complete evaluation of the thermal shock, insulation resistance, welding resistance, etc. of the inner line. The following describes the important factors to be considered in multi substrate design.
1、 Mechanical design factors
Mechanical design includes the selection of appropriate plate size, plate thickness, plate stacking, inner copper cylinder, aspect ratio, etc.
1 plate size
The board size shall be optimized according to the application requirements, the size of the system box, the limitations of the circuit board manufacturer and the manufacturing capacity. Large circuit boards have many advantages, such as fewer substrates and shorter circuit paths between many components, so that they can have higher operation speed, and each board can have more input and output connections. Therefore, large circuit boards should be preferred in many applications. For example, in personal computers, larger motherboards are seen. However, it is difficult to design the signal line layout on the large board, which requires more signal layers or internal wiring or space, and the heat treatment is also more difficult. 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.
2 plate thickness
The thickness of 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 aperture and thickness required for high-quality drilling and electroplating, the length of component pins required for automatic insertion and the type of connection used. The thickness of the whole circuit board consists of the conductive layer, copper layer, substrate thickness and prepreg thickness on both sides of the board. It is difficult to obtain strict tolerances on synthetic multi substrates, and about 10% of the tolerance standard is considered reasonable.
3 stacking of plates
In order to minimize the probability of plate distortion and obtain a flat finished plate, the delamination of multi base plates should be symmetrical. That is, it has an even number of copper layers and ensures that the thickness of copper is symmetrical to the pattern density of copper foil of the plate layer. Generally, the radial direction of the construction material used for lamination (e.g. glass fiber cloth) should be parallel to the edge of the laminate. Because the laminate shrinks radially after bonding, the layout of the circuit board will be distorted, showing variable and low spatial stability.
However, the warpage and distortion of multiple substrates can be minimized by improving the design. Through the average distribution of copper foil on the whole layer and ensuring the structural symmetry of multiple substrates, that is, ensuring the same distribution and thickness of prepreg materials, the purpose of reducing warpage and distortion can be achieved. Copper and rolled layers shall be made from the central layer of multi substrate to the outermost two layers. The minimum distance (dielectric thickness) specified between two copper layers is 0.080mm.
It is known from experience 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, two adjacent copper layers if each layer is 30 thick μ m. The thickness of the prepreg shall be at least 2 (2 x 30 μ m） =120 μ m. This can be achieved by using two layers of prepreg (the typical value for fiberglass fabrics is 1080).
4 inner copper foil
The most commonly used copper foil is 1oz (1oz per square foot of surface area). However, for dense plates, the thickness is extremely important, and strict impedance control is required. Such plates need to be used
0.50z copper foil. For the power layer and ground layer, it is better to choose 2oz or heavier copper foil. However, etching heavy copper foil will reduce controllability and it is not easy to achieve the desired pattern of linewidth and spacing tolerance. Therefore, special processing technology is required.
According to the pin diameter or diagonal size of components, the diameter of plated through hole is usually kept between 0.028 and 0.010in, which can ensure sufficient volume for better welding.
6 aspect ratio
“Aspect ratio” is the ratio of plate thickness to borehole diameter. It is generally believed that 3:1 is the standard aspect ratio, although high aspect ratios such as 5:1 are also commonly used. The aspect ratio can be determined by factors such as drilling, slag removal or back etching and electroplating. When maintaining the aspect ratio within the producible range, the via should be as small as possible.
2、 Electrical design factors
Multi substrate is a system with high performance and high speed. For higher frequencies, the rise time of the signal is reduced, so the control of signal reflection and line length becomes very important. In the multi substrate system, the requirements for the controllable impedance performance of electronic components are very strict, and the design should meet the above requirements. The factors that determine the impedance are the dielectric constant of the substrate and prepreg, the conductor spacing on the same layer, the dielectric thickness between layers and the thickness of copper conductor. In high-speed applications, the lamination sequence of conductors in multi substrate and the connection sequence of signal network are also very important. Dielectric constant: the dielectric constant of substrate material is an important factor in determining impedance, propagation delay and capacitance. The dielectric constants of substrates and prepregs using epoxy glass can be controlled by changing the percentage of resin content.
The dielectric constant of epoxy resin is 3.45 and that 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 illustration for determining and controlling the dielectric constant.
The prepreg with relatively low dielectric constant is suitable for RF and microwave circuits. In RF and microwave frequencies, the signal delay caused by low dielectric constant is low. In the substrate, the low loss factor can minimize the electrical loss.
Prepreg ror 4403 is a new material produced by Rogers company. This material is compatible with other substrates used in a standard multi substrate (FR-4 material) structure (e.g., RO 4003 or ro 4350 used for microwave plates).