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It is well known that a PCB (Printed Circuit Board) transforms a schematic design into a physical circuit. However, don’t underestimate the complexity of this process. While some elements may work perfectly in theory, translating them into real-world performance can be extremely challenging. What seems simple on paper often becomes difficult to implement, and even what others achieve may not be replicable due to various constraints. Therefore, while it’s easy to say that a PCB is just a board, creating a functional and high-quality PCB is no small task.
In the field of microelectronics, two major challenges are handling high-frequency signals and weak signals. The quality of the PCB production plays a crucial role in overcoming these challenges. Even with the same schematic and components, different PCBs made by different designers can yield very different results. So, how do you create a good PCB? Based on our past experience, I’d like to share my insights on several key aspects.
First, clearly define the design goals. When you receive a design task, the first step is to understand exactly what you’re aiming for. Is it an ordinary PCB, a high-frequency board, a low-noise signal processing board, or one that combines both high frequency and small signal handling? For an ordinary board, the main focus should be on reasonable layout, clean routing, and accurate mechanical dimensions. If there are medium-load lines or long traces, special attention must be given to reduce loading effects. Long lines should be reinforced to prevent reflections. At frequencies above 40 MHz, signal crosstalk becomes a concern, and higher frequencies impose stricter limitations on trace lengths. According to distributed parameter network theory, the interaction between high-speed circuits and their connections is a critical factor that cannot be ignored in system design. As gate speeds increase, signal line impedance also rises, leading to more crosstalk between adjacent lines. Additionally, high-speed circuits typically consume more power and generate more heat, which requires careful thermal management.
When dealing with millivolt or microvolt-level weak signals, extra care is needed. These signals are highly susceptible to interference from stronger signals. Shielding is often necessary to maintain a good signal-to-noise ratio; otherwise, the useful signal may be overwhelmed by noise and become unusable.
The testing phase should also be considered during the design stage. The placement of test points and their isolation is important, as some small or high-frequency signals cannot be directly measured with probes.
Other factors, such as the number of board layers, component packaging, and mechanical strength, should also be taken into account before finalizing the PCB design.
Second, understand the functions of the components used and their layout and routing requirements. Some components have specific layout and routing needs. For example, analog amplifiers used in certain applications require stable power supplies with minimal ripple. The analog small-signal section should be kept away from power components. On some boards, the small-signal amplification area is shielded to block stray electromagnetic interference. Chips like GLINK, which use ECL technology, consume a lot of power and generate significant heat. Their placement must be carefully planned to ensure proper cooling. If natural convection is used, the chip should be placed in an area with good airflow, and its heat dissipation should not affect other components. Similarly, if high-power devices like horns are installed, they may introduce noise into the power supply, which should be addressed during the design phase.
Third, consider component placement. One of the primary concerns during layout is electrical performance. Components that are closely connected should be placed together, especially for high-speed lines. The layout should aim to keep traces as short as possible, separating power and small-signal devices. In addition to performance, the arrangement should also be neat and easy to test. The mechanical size of the board and the position of connectors must be carefully considered.
Grounding and transmission delay times are also critical in high-speed systems. The delay on signal lines significantly affects the overall system speed, especially for ECL circuits. Although the internal speed of integrated circuits is high, the delay caused by interconnection lines (about 30 cm, resulting in 2 ns of delay) can greatly reduce the system's performance. For example, shift registers and synchronous counters are preferably placed on the same board to avoid timing mismatches caused by unequal delays on different boards. If that’s not possible, clock lines from a common source to each board must be equal in length when synchronization is essential.
Fourth, consider the wiring. With the development of OTNI and star fiber optic networks, more PCBs now feature high-speed signal lines above 100 MHz. Understanding the basics of high-speed transmission lines is essential.
A "long" signal path on a PCB can be considered a transmission line. If the line delay is much shorter than the signal rise time, reflections during the signal rise will be suppressed, and overshoot, ringing, and oscillations won't occur. For most current MOS circuits, since the rise time is much longer than the line propagation delay, traces can be up to meters long without distortion. However, for faster logic circuits, especially ultra-fast ECL, the trace length must be significantly reduced to maintain signal integrity.
There are two main methods to ensure high-speed circuits perform well on longer lines without severe waveform distortion. TTL uses a Schottky diode clamp to control fast falling edges, clamping overshoot below ground level. This reduces back-oscillation, and the slower rising edge allows for overshoot, which is attenuated by the relatively high output impedance (50–80 Ω) at the high level. Additionally, the high immunity of the high level state makes the back-oscillation issue less prominent. For HCT series devices, combining the Schottky diode clamp with a series resistor termination method yields better results.
When there is a fan-out along the signal line, the TTL shaping method becomes inadequate at higher bit rates and faster edge rates. Reflected waves tend to accumulate, causing severe distortion and reduced interference resistance. To solve this, ECL systems usually use line impedance matching to control reflections and ensure signal integrity.
Strictly speaking, for conventional TTL and CMOS devices with slower edge speeds, transmission lines are not always desirable. For high-speed ECL devices, transmission lines are not always required either. However, when used, they offer the advantage of predictable connection delay and controlled reflection and oscillation through impedance matching.
Five basic factors determine whether to use a transmission line: (1) the edge rate of the system signal, (2) the connection distance, (3) the capacitive load (how many fans out), (4) the resistive load (the line termination), and (5) the allowed percentage of overshoot and rebound (the degree of reduction in AC immunity).
There are several types of transmission lines:
1. Coaxial cable and twisted pair: commonly used between systems. Coaxial cables typically have characteristic impedances of 50Ω and 75Ω, while twisted pairs are usually 110Ω.
2. Microstrip line on a printed board: a strip conductor isolated from the ground plane by a dielectric. If thickness, width, and distance from the ground plane are controllable, the characteristic impedance is also controllable. The characteristic impedance Z0 of a microstrip line is calculated using the formula:
$$
Z_0 = \frac{120}{\sqrt{\varepsilon_r}} \cdot \ln\left(\frac{2h}{w} + \sqrt{\left(\frac{2h}{w}\right)^2 + 1}\right)
$$
3. Stripline in a printed board: a copper strip placed between dielectric layers between two conductive planes. Its characteristic impedance is also controllable based on line thickness, width, dielectric constant, and distance between conductive planes.
4. Termination of transmission lines: at the receiving end, a resistor equal to the line’s characteristic impedance is connected, forming a parallel termination. This is primarily used for optimal electrical performance, including driving distributed loads. Sometimes, a 104 capacitor is connected in series with the terminated resistor to form an AC termination, reducing DC loss. A resistor can also be placed in series between the driver and the transmission line, known as serial termination. This helps control overshoot and ringing on longer lines.
5. Non-terminated transmission lines: if the line delay is much shorter than the signal rise time, the transmission line can be used without series or parallel termination. The maximum open route length is approximately:
$$
L_{max} < \frac{t_r}{2t_{pd}}
$$
Where $ t_r $ is the rise time and $ t_{pd} $ is the transmission delay per unit length.
When choosing between parallel and series termination, it depends on the designer’s preference and system requirements. Parallel termination offers faster system speed and intact signal transmission, while series termination has lower crosstalk but longer delay.
The choice between double-layer or multi-layer boards depends on the highest operating frequency, circuit complexity, and assembly density. Multi-layer boards are preferred when the clock frequency exceeds 200 MHz. For frequencies above 350 MHz, Teflon-based boards are recommended due to lower high-frequency attenuation, smaller parasitic capacitance, faster transmission speed, and better impedance characteristics.
Key principles for PCB trace design include:
1. Leave sufficient space between parallel signal lines to reduce crosstalk. If two lines are close, place a ground line between them for shielding.
2. Avoid sharp turns in signal lines to prevent impedance reflection. Use smooth circular arcs instead.
3. Calculate trace width based on the characteristic impedance formula for microstrip and stripline. Typical values range from 50 to 120 Ω. A 68 Ω impedance is often chosen for a balance between delay and power consumption.
4. On double-layer or multi-layer boards, arrange traces on opposite sides perpendicularly to minimize crosstalk.
5. Separate ground lines for high-current devices like relays and indicators to reduce noise. Connect them to a separate ground bus and the system’s main ground point.
6. Keep weak signal lines away from strong signal lines, and make them as short as possible. If possible, shield them with a ground line.
Designing a high-performance PCB requires careful planning, attention to detail, and a deep understanding of both electrical and mechanical considerations. By following these guidelines, you can significantly improve the reliability and performance of your PCB.
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