Electronic Design - PCB multilayer impedance and four-layer PCB design

1. PCB multi-layer impedance circuit board proofing factory

What does PCB impedance mean? The so-called PCB impedance refers to the resistance, inductance, and capacitance in a circuit that hinders alternating current and is called "impedance". At a certain frequency, relative to GND&VCC in the transmission signal line of electronic devices, the resistance of high-frequency signals or electromagnetic waves in the propagation process is called characteristic impedance, which is a vector sum of electrical impedance, inductive reactance, and capacitive reactance. The unit of PCB impedance is ohms, which is usually represented by Z.

Why do PCB boards need impedance and what is the use of PCB boards as impedance? The PCB manufacturing process needs to go through processes such as copper sinking and tin electroplating. The plates used in these production links need to ensure low resistivity in order to ensure the PCB board. The overall impedance is low and meets the product quality requirements, so that the product can operate normally. Various signal transmissions will occur in the PCB board conductors. Due to the PCB board itself due to factors such as etching and stacking thickness, the impedance value will change, resulting in a decrease in the performance of the PCB. Therefore, it is necessary to ensure that the PCB impedance value is controlled within a certain range. So in the PCB manufacturing process, what are the main factors that affect the PCB impedance value!

1. When the line width and line moment are less than 25mm, the line impedance value is 1 ~ 4 ohm smaller than the middle of the board, and the impedance value is affected by the position when it is greater than 50mm The range of change is reduced. Under the premise of meeting the utilization rate of the imposition in the engineering design of production materials, it is recommended that the cutting size meets the impedance line and the distance between the board edge is greater than 25 mm; 2, the difference in the residual copper rate at different positions of the PCB imposition will lead to a difference in impedance ~3 ohm, when the uniformity of the pattern distribution is poor (the residual copper ratio is different), it is proposed to rationally lay the blocking point and the electroplating shunt point on the basis of not affecting the electrical performance, so as to reduce the difference in dielectric thickness and plating at different positions. Copper thickness difference,PCB imposition resistance, the main factor is the uniformity of the thickness at different positions, followed by the uniformity of the line width; 3. The lower the glue content of the prepreg, the better the uniformity of the thickness after lamination, and the side flow of the PCB. The large amount of glue causes the dielectric thickness to be too small and the dielectric constant to be too large, so that the impedance value of the circuit near the edge of the board is smaller than the middle area of the imposition; 4. For the outer layer circuit, the copper thickness difference has a normal influence on the impedance within 2 ohm, but the etching line width difference caused by the copper thickness difference has a greater influence on the impedance, and the outer layer needs to be upgraded Copper plating uniformity ability;

Two, four-layer PCB circuit board design considerations

As the basic component of all electronic products, PCB board is not only to combine components, but also to ensure the rationality of circuit design to avoid confusion and errors caused by manual wiring and wiring. PCB design work before printed circuit boards is important. Unreasonable circuit design not only fails to achieve the desired functional effect, but also greatly increases the manufacturing cost. So what problems should we pay attention to when designing four-layer PCB circuit boards!

1. Reasonable and compliant wiring layout, such as input/output, AC/DC, strong/weak signal, high frequency/low frequency, high voltage/low voltage, etc. The design direction should be linear and not blend with each other to prevent mutual interference When conditions permit, a straight line is the most ideal trace, and the most unfavorable trend is a loop, but it can be improved by setting isolation. For DC or small signal/low voltage circuit boards, the design requirements can be less strict. 2. The seemingly inconspicuous connection location is the object that design engineers pay attention to. Under normal circumstances, it is required to share the same location. However, due to various restrictions in the design process, it is difficult to face it. Do your best, every engineer has his own set of solutions, this problem will not be explained. 3. Reasonable and fully-regulated layout of power supply filtering and decoupling capacitors. Under normal design conditions, several power supply filtering and decoupling capacitors are drawn in the schematic, but they are not specified one by one where they should be connected. These capacitors are For switching elements or other components that require filtering/decoupling, these capacitors should be arranged as close as possible to these elements. 4. When designing the line diameter, make the line as wide as possible if possible. The high-voltage and high-frequency line should be smooth and not designed to be sharp chamfers. The ground line design is the same, as wide as possible, the most It is better to use a large area of copper, which will greatly improve the grounding point problem. 5. If the number of via holes is too much, a slight carelessness may bury hidden dangers when sinking copper. At the same time, the parallel line density is not easy to be too large, and it is easy to connect into one piece during welding. Of course, these are based on the experience of the operator. The equipment has a lot to do with it.