PCB cascading design

1. PCB

PCB has two important components: Core and PP (Prepreg, semi-solid sheet).

Both surfaces of the Core are coated with copper foil, which acts as a conductive layer. Between the two surfaces are filled with solid material, which is made of reinforced glass fiber soaked in solid resin.

There is no copper foil on the surface of PP, which is composed of semi-solid resin and glass fiber. Compared with Core, it is softer. It forms the so-called infiltrating layer, which mainly plays a filling role in PCB and is used to bond Core board Core. In factory production, the wetted layer is generally unable to exceed 3 PP (the thickness is about 20mil). Due to the semi-solid nature of PP, the thickness of the PP in each PCB manufactured may be different, so the consistency of impedance control may also be different.

Cooperate with Core and PP to realize the design and manufacture of multi-layer PCB.

2. The copper foil

Copper foil thickness is often expressed in ounces (oz), oz is the unit of mass (gold and silver are well understood), copper foil 1oz means 1oz mass (28.35g) copper foil evenly spread on 1 square foot area, the thickness is 1.37mil, about 1.4mil=35um.

Copper foil materials commonly used in circuit board are mainly divided into calendered copper foil and electrolytic copper foil. The calendered copper foil has strict molecular structure, smoother surface (conducive to the transmission of high-speed signals), good bending resistance and flexibility, and is not easy to break. Therefore, it is generally used in products with bending requirements.

3. PCB board

The most commonly used PCB sheet is FR4, which uses glass fiber cloth as reinforcement and epoxy resin as binder.

The other is the composite substrate CEM1, which is widely used in the industry of household appliances. It uses wood pulp fiber paper or cotton pulp fiber paper as core reinforcement material and glass fiber cloth as surface reinforcement material. Both of them are impregnated with flame retardant epoxy resin. The plate is mainly used in single-panel applications. Its mechanical strength is lower than FR4 sheet, but the price is relatively cheaper, but also has flame retardant properties.

PCB material main parameter is the relative dielectric constant and material loss Angle, the two together can represent complex dielectric constant, this in the signal integrity SI reading notes one of //zhuanlan.zhihu.com/p/126633951, which shows the relative dielectric constant, the greater the signal propagation in the medium speed is slower; The larger the material loss Angle, the greater the medium loss. Therefore, in the design of high-speed signal PCB, the material with small dielectric constant and material loss Angle will be chosen as far as possible, and correspondingly, the cost will be higher.

4. PCB laminated structure and impedance design

Common signal impedance types include single-ended 50 ohm, difference 100 ohm, single-ended radio frequency 50 ohm, etc. Impedance parameters can be calculated by software SI9000, which will not be detailed in this paper. Later, I will write a separate article on common impedance calculation for explanation. Need special mention is 50 euro single-ended rf line impedance control, rf line usually walk on the surface (aims to qualify directly, not via the possibility of changes to reduce the impedance discontinuity, and the surface signal speed), and to enhance the immunity ability and reduce the loss, compared to common signal, radio frequency line need to be more coarse line width, generally at least 15 mil. As we know in the previous article, when the control characteristic impedance remains unchanged, the distance between the signal line and the reference layer must be increased while the line width is increased. Therefore, the RF line in the surface layer needs to refer to the third layer between the layers, and the second layer just below the surface RF line is cut out.

The PCB layers that are commonly used in work are 12 layers, 8 layers and 6 layers, and those above 20 layers have also been encountered. In general, in larger companies, PCB Layout and schematic design are done by different people, but this does not mean that hardware engineers do not need to know PCB related knowledge. On the contrary, in many cases, Layout engineers do not know which signals on the board are the most important and which are the second. Therefore, it is better for hardware engineers to indicate the points of attention when the signal is in the Layout on the schematic diagram, and actively communicate with Layout engineers to point out the problem points so as to help them change. This requires hardware engineers must also be more clear about PCB Layout related knowledge points, in order to put forward effective guidance.

In practical work, the number of layers of multi-layer boards is generally determined by Layout engineers, and the thickness is usually 1.6mm or 2mm. If there is no special requirement, most of the boards are also FR4. There are some basic principles for PCB cascading Settings, which are of great benefit to improving signal integrity quality. We can check whether the design given by Layout engineers is reasonable according to these principles and put forward suggestions for improvement.

(1) The power plane and the ground plane are as close as possible, preferably adjacent, because two complete planes can form a large plane capacitor, which is equivalent to increasing the decoupling capacitor, reducing the impedance of the power supply to the ground, and contributing to the integrity of the power PI (PDN). In actual PCB design, it is difficult to set all the power plane and ground plane adjacent to each other. However, it is generally possible to set the power plane and ground plane in the two adjacent laminates at the most center, such as layers 4 and 5 of 8-layer plates and layers 6 and 7 of 12-layer plates. These two layers are closely adjacent and have good coupling effect. If there are many kinds of power supply in a certain power supply layer in the design, the power supply layer will be divided into relatively fragmented layers. The power supply layer can be set as the adjacent layer to obtain relatively good capacitance coupling effect. For example, the 6th layer is GND, and the 7th layer is set as the fragmented power supply layer with many kinds of power supply. The common good laminated distribution structure of 8-layer and 12-layer plates is shown in Figure 1 and Figure 2 respectively (the laminated management in Altium Designer, and the impedance can be visually given according to the laminated setting with allegro). The thickness of the laminated plates in the figure has not been optimized according to the impedance control.

Fig. 1 8-layer plates are commonly distributed in cascades

Fig. 2 12 laminates are commonly distributed in a cascade

(2) All signal layers (especially critical signals) should be adjacent to a complete plane (both power supply and ground plane) as far as possible to ensure a complete and reliable reflux plane and impedance continuity. High - speed signals pay special attention to avoid cross - segmentation. When the signal flows back, the path with the least impedance to the drive path is automatically selected as the return path (forming the transmission line structure).

(3) When the signal is changed, it is better not to change the reference layer; Second, if the reference layer is changed, it is better not to change the network attribute of the reference layer, and when the signal layer is changed, add the same pass hole as the network attribute of the reference layer near the signal pass hole, which was mentioned in the previous MIPI article about PCB pass hole. If the signal is changed, the network properties of the reference layer are also changed. It is better to make the two reference layers as close as possible. These measures are to ensure that the impedance of the return path is as continuous as possible, and the impedance between layers is as low as possible, thus reducing the signal voltage drop and signal distortion.

(4) Try to avoid two signal layers (including two power layers) adjacent to each other to reduce crosstalk. It is difficult to avoid the situation, the adjacent signal layer should be orthogonal routing, a horizontal routing, a vertical routing.