1. PCB boards can be categorized into single-layer boards, double-layer boards, and multi-layer PCB boards. Various electronic components are integrated onto the PCB. In the simplest form, the single-layer PCB has components mounted on one side, while the wiring is placed on the opposite side. To allow the component pins to pass through to the other side, holes must be drilled in the board. The pins of the components are then soldered to the opposite side. For this reason, the two sides of such a PCB are referred to as the **component side** and the **solder side**.
2. A double-layer PCB can be considered as two single-layer boards stacked together. Electronic components and wiring are placed on both sides of the board. In some cases, a connection needs to be made from a wire on one side to the other side of the board, which is accomplished using a via. A via is a small hole filled or coated with metal that connects the circuits on both sides of the PCB. Many modern computer motherboards use 4-layer or even 6-layer PCBs, while graphics cards typically utilize 6-layer PCBs. High-end graphics cards, like the NVIDIA GeForce4 Ti series, often use 8-layer PCBs. This is known as a multi-layer PCB. In multi-layer PCBs, connecting the layers involves vias, which help link the different layers of the board. However, not all vias need to penetrate the entire PCB. These are referred to as **buried vias** and **blind vias**. Blind vias connect internal layers to the outer layer without going all the way through the PCB, while buried vias are only visible on the internal layers, connecting only those internal layers.
3. In multi-layer PCBs, certain layers are dedicated to the ground and power supply. These are categorized as **signal layers**, **power layers**, and **ground layers**. If a PCB has components requiring different power supplies, it will typically have more than two layers for power and signal routing. The more layers a PCB has, the higher the production cost, but additional layers help improve signal integrity and stability.
4. The professional manufacturing process of PCB boards is quite complex. For example, in the production of a 4-layer PCB, the middle two layers undergo processes such as rolling, cutting, etching, and oxidation. The four layers of the PCB are typically the **component surface**, **power layer**, **ground layer**, and **solder pressure layer**. These layers are then stacked together and pressed into a single PCB. Next, through-holes are drilled, and after cleaning, processes like printing, copper plating, etching, testing, solder masking, and silk screening are applied to the outer two layers. After passing all quality checks, the entire PCB, including motherboards, is stamped into the final form and vacuum-sealed for packaging. If the copper layer is poorly applied during the manufacturing process, it may result in weak bonding, leading to potential short circuits or unwanted capacitance effects (which could cause signal interference). Attention must also be paid to the vias. If a via is not centered but positioned too far to one side, it may cause uneven matching or contact with the power or ground layers, which can result in short circuits or inadequate grounding.
**Copper Wiring Process**
The first step is to establish the wiring between the components. We use the negative film transfer method to transfer the working film onto the metal conductor. This process involves applying a thin layer of copper foil over the entire surface and removing any excess material. There is also an alternative method called supplementary transfer, which is less commonly used. This technique involves laying copper wiring only where it is needed, but we won’t cover that method here. A positive photoresist is made with a sensitizer that dissolves upon exposure to light. There are various ways to treat the photoresist on the copper surface, but the most common approach is to heat and roll it onto the surface, which contains the photoresist. It can also be sprayed on in liquid form, though dry film types offer higher resolution and can produce thinner traces. The photoresist layer serves as a template for the PCB during manufacturing. Before the photoresist is exposed to UV light, a light shield is used to protect certain areas, preventing exposure and leaving those areas intact as the wiring pattern. After developing the photoresist, the exposed copper is etched away. Etching can be done by immersing the board in an etching solvent or spraying the solvent onto the board. Common etching solvents include ferric chloride. After etching, the remaining photoresist is removed.
**1. Wiring Width and Current**
Typically, the minimum width should not be less than 0.2mm (8mil).
For high-density, high-precision PCBs, the trace pitch and width are usually around 0.3mm (12mil).
When the copper foil thickness is approximately 50µm, the trace width should be between 1–1.5mm (60mil), supporting a current of up to 2A.
In most common cases, 80mil traces are used, with particular attention given to applications involving microprocessors.
**2. What is the Frequency for High-Speed PCB Boards?**
A signal is considered high-speed when its rise/fall time is less than 3–6 times the signal’s transmission time.
For digital circuits, the critical factor is the steepness of the signal edge, specifically the rise and fall times.
According to the classic text *High-Speed Digital Design*, a signal is considered high-speed when the time it takes to rise from 10% to 90% of its maximum amplitude is less than six times the delay of the trace itself.
In other words, even an 8kHz square wave signal can be considered high-speed if the edges are sufficiently sharp, and transmission line theory must be applied during routing.
**3. Considerations for Power and Ground Wire Layout**
The power trace should be as short and direct as possible, ideally laid out in a tree structure rather than a loop.
Ground loop issue: For digital circuits, the voltage drop caused by ground loops is often only tens of millivolts, while the anti-interference threshold for TTL circuits is 1.2V, and for CMOS circuits, it can be as high as half the supply voltage. This means that a circulating ground wire typically does not adversely affect circuit operation. In fact, leaving the ground wire unclosed can lead to even worse issues, as the pulsed currents generated by digital circuits can cause imbalances in the ground potential at different points.
Using a 2Gsps oscilloscope to measure ground currents, the pulse width of the ground current is about 7ns. Under the influence of a large pulse current, if a branch ground wire (25mil width) is used, the potential difference between ground points can reach 100mV.
When a ground loop is employed, the pulse current is distributed across various points of the ground plane, greatly reducing the chance of interference. With a closed ground wire, the measured maximum instantaneous potential difference between ground points on the PCB is typically one-half to one-fifth of that in an unclosed ground configuration. Naturally, the measured data will vary depending on the density and speed of the circuit board. The data I refer to above is based on a Z80 Demo board in Protel 99SE.
For low-frequency analog circuits, I believe the issue of frequency interference from space cannot be easily simulated or calculated. If the ground wire is left open, it will not generate eddy currents. According to Beckhamtao’s theory, “the induced power frequency voltage on the ground wire will be higher.” For example, in a project using a precision pressure gauge with a 14-bit A/D converter, the actual effective resolution was only 11 bits due to 15mVp-p of power frequency interference on the ground line.
A solution to this problem involves separating the analog ground into distinct loops on the PCB. The ground trace from the front-end sensor to the A/D converter is routed in a branch pattern, using flying leads. After producing the mass-produced PCB based on this design, there have been no further issues.
In another case, a friend encountered humming in the output of a DIY amplifier. I recommended cutting the ground loop, which resolved the issue. Afterward, this individual reviewed the PCB diagrams of several high-end “Hi-Fi” devices and found that none of them used ground loops in their analog circuits.
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