PCB Fabrication

Precautions and process skills for double-sided wiring

by:A-TECH      2021-03-26
In today's fiercely competitive battery-powered market, designers often use double panels due to cost constraints. Although the multi-layer board (4-layer, 6-layer and 8-layer) solution has obvious advantages in terms of size, noise and performance, cost pressure has prompted engineers to reconsider their wiring strategy and adopt double-sided boards. In this article, we will discuss the correct and incorrect use of the automatic wiring function, the design strategy of the current loop when there is no ground plane, and the suggestions for the layout of the dual-panel components. Advantages and disadvantages of automatic routing and precautions for analog circuit routing When designing a PCB, you often want to use automatic routing. Usually, pure digital circuit boards (especially the signal level is relatively low, the circuit density is relatively small) using automatic wiring is no problem. However, when designing analog, mixed-signal or high-speed circuit boards, if the automatic routing tools of the routing software are used, some problems may occur, and it may even cause serious circuit performance problems. For example, Figure 1 shows the top layer of a double-sided board with an automatic wiring design. The bottom layer of the double panel is shown in Figure 2, and the circuit schematics of these wiring layers are shown in Figures 3a and 3b. When designing this mixed-signal circuit board, after careful consideration, the components were manually placed on the board to separate the digital and analog components. When adopting this kind of wiring scheme, there are several aspects that need attention, but the most troublesome is grounding. If the ground wire is routed on the top layer, the devices on the top layer are all grounded through traces. The device is also grounded on the bottom layer, and the ground wires on the top and bottom layers are connected through the vias on the far right side of the circuit board. When checking this kind of wiring strategy, the first drawback is that there are multiple ground loops. In addition, you will find that the ground return path of the bottom layer is interrupted by the horizontal signal line. The desirability of this grounding scheme is that the analog components (12-bit A/D converter MCP3202 and 2.5V reference voltage source MCP4125) are placed on the far right side of the circuit board. This layout ensures that there will be no underneath these analog chips. The digital ground signal passes through. Both of these two types of double panels have a ground plane on the bottom layer. This approach is to facilitate engineers to solve problems, so that they can quickly understand the wiring of the circuit board. Manufacturer's demonstration boards and evaluation boards usually use this wiring strategy. However, it is more common to place the ground plane on the top layer of the circuit board to reduce electromagnetic interference. Figure 1 The top layer of the circuit board designed with automatic wiring for the schematic circuit diagram shown in Figure 3 Figure 2 The bottom layer of the circuit board designed with automatic wiring for the schematic circuit diagram shown in Figure 3 Figure 3a Figure 1, Figure 2, and the wiring circuit Schematic diagram Figure 3b Figure 1, Figure 2, the analog part of the circuit schematic diagram of the wiring in the current loop design with or without a ground plane For the current loop, you need to pay attention to the following basic matters: 1. If you use wiring, you should make it as thick as possible . If wiring is to be considered for the ground connection on the PCB, the design should make the wiring as thick as possible. This is a good rule of thumb, but you should know that the minimum width of the ground wire is the effective width from this point to the end, where the 'end' refers to the point farthest from the power connection. 2. Ground loops should be avoided. 3. If the ground plane cannot be used, a star connection strategy should be used. 4. Digital currents should not flow through analog devices. When the digital device is switched, the digital current in the loop is quite large, but it is only instantaneous. This phenomenon is caused by the effective inductance and impedance of the ground wire. For the inductance part of the ground plane or ground trace, the calculation formula is V u003d Ldi/dt, where V is the voltage generated, L is the inductance of the ground plane or ground trace, di is the current change of the digital device, and dt is duration. The calculation formula for the influence on the ground wire impedance is Vu003d RI, where V is the voltage generated, R is the impedance of the ground plane or ground trace, and I is the current change caused by the digital device. These voltage changes on the ground plane or ground trace of the analog device will change the relationship between the signal and the ground in the signal chain (that is, the signal's voltage to ground). 5. High-speed current should not flow through low-speed devices. Conclusion After the digital and analog ranges are determined, careful wiring is essential to a successful PCB. Especially when the active digital trace is close to the high-impedance analog trace, it will cause serious coupling noise, which can only be avoided by increasing the distance between the traces.
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