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Signal crosstalk in chiplet packaging - how to reduce its impact

Signal crosstalk means that the electromagnetic energy in one signal path is captured by another signal path, resulting in signal distortion and interference. The core of chiplet is to realize high speed interconnection between chips and rewiring after multi-chip interconnection. Signal crosstalk is an important issue in chiplet packaging.

Optimizing layout, using shielding technology, adopting differential signal transmission, optimizing impedance matching, etc., who can better reduce the mutual interference between electromagnetic fields and improve signal integrity and performance?

Table of Contents

What is chiplets packaging?

What is chiplets packaging?
chiplets packaging

Chiplet means a small chip, which refers to dismantling the original “big” chip (Die) into several “small” chips (Die) at the wafer end, because the function of a single disassembled “small” chip is If it is incomplete, it needs to recombine each “small” chip through packaging, and restore the function of the original “big” chip functionally.

5D / 3D advanced packaging is the mainstream packaging technology of Chiplet

Advanced packaging technology enables the development of Chiplets. Advanced packaging technologies are currently dominated by leading companies such as TSMC, AMD, and Intel, including 2D MCM (Multi-Chip Module) to 2.5D CoWoS, EMIB, and 3D Hybrid Bonding, and these advanced packaging technologies are mainstream packaging for chiplets. technology.

What is signal crosstalk?

Crosstalk is unwanted noise (electromagnetic coupling) between the lines of a PCB. Crosstalk is one of the most insidious and difficult problems PCBs can encounter. Crosstalk can adversely affect clock signals, period and control signals, data transmission lines, and I/O. Generally speaking, crosstalk cannot be completely eliminated, but can only be minimized. In chiplet packaging, signal crosstalk is an important issue.

What is signal crosstalk?
signal crosstalk

In high-density, high-speed and high-performance chiplet systems, signal crosstalk will bring a series of negative effects on system performance. Here are some problems that signal crosstalk can cause:

1. Signal distortion: Signal crosstalk can lead to distortion of the signal waveform, which will reduce the edge rate of the signal, and lengthen the rise and fall time of the signal. This can make it difficult for the receiving end to correctly identify the signal, reducing the accuracy of the data transmission.

2. Signal amplitude attenuation: Due to the existence of signal crosstalk, the signal may experience amplitude attenuation during transmission, resulting in a lower signal level. In severe cases, this may cause the receiving end to fail to detect the signal correctly, thereby affecting the communication quality.

3. Increased bit error rate: Signal crosstalk can lead to increased Bit Error Rate (BER). A higher bit error rate means that there are more errors in the data transmission process, which affects the communication quality.

4. Degraded system performance: Due to the impact of signal crosstalk on signal quality, overall system performance may be affected. This includes reductions in data transfer rates, processing power, and response times.

5. Electromagnetic compatibility Issues: Signal crosstalk can lead to increased electromagnetic radiation, which can lead to electromagnetic compatibility issues. This may cause the system to interfere with, or receive interference from, other electronic devices.

6. Clock jitter: Signal crosstalk can affect the stability of the clock signal, resulting in clock jitter. This can affect synchronization and data transfer, reducing overall system performance.

What is the main cause of crosstalk?

What is the main cause of crosstalk in chiplet package?
Main cause of crosstalk

The three most important reasons for signal crosstalk in chiplet packaging are:

  • Electromagnetic coupling: Since the signal lines and components are close together in the package, the electromagnetic fields tend to interfere with each other. The coupling of electromagnetic fields causes electromagnetic energy on one signal path to be captured by another signal path, causing signal distortion and interference.
  • High-density layout: With the improvement of integration, signal lines and devices are becoming denser. In a limited space, the distance between signal lines is reduced, making crosstalk more likely to occur. In addition, the distance between the signal loop and the power loop may also be reduced, further increasing the risk of signal crosstalk.
  • High-speed signal:  With the improvement of chip performance, the signal transmission rate and operating frequency continue to increase. High-speed signals have higher electromagnetic radiation capabilities, which increases the risk of signal crosstalk. In high-speed signal transmission, the edge rate of the signal is faster, which is more prone to electromagnetic interference and crosstalk.

What is the method to reduce crosstalk?

Reduce chiplet packaging signal crosstalk
Reduce chiplet packaging signal crosstalk

In order to reduce signal crosstalk in chiplet packaging, corresponding strategies need to be adopted in the design, manufacturing and testing stages to help reduce mutual interference between electromagnetic fields and improve signal integrity and performance.

High-order shielding: Special shielding techniques such as multi-layer shielding, broadband absorbing materials and directional shielding techniques are used to provide effective shielding effects for signals in different frequency ranges.

Layout optimization: For high-speed signals and sensitive signals, use a three-dimensional layout strategy to optimize signal lines. Separate high-speed signal lines from sensitive signal lines, and avoid placing signal lines too long or too close together. In addition, stacked structures and vertical interconnection techniques can be considered to improve signal integrity.

Signal Integrity Analysis: During the design phase, evaluate system performance using signal integrity analysis tools and simulation software to identify possible signal crosstalk issues and take targeted action.

High-performance interconnect technology: Use high-performance interconnect technologies, such as ultra-wideband, low-latency, and high-density interconnects, to improve signal transmission performance and reduce signal crosstalk.

RF front-end optimization: For RF front-end devices, special RF optimization techniques, such as impedance matching, detuning compensation and adaptive filtering, are used to improve signal quality and anti-interference performance.

High-performance ground plane design: Optimize ground plane design to improve signal reference and suppress electromagnetic interference. Use high-performance ground plane design techniques such as multilayer ground planes, partitioned ground planes, and embedded ground planes.

Power integrity management: ensure that the power system has good stability and anti-interference ability. High-performance power supply filtering, decoupling and voltage regulation techniques are used to reduce the impact of power supply noise on the signal path.

Strict testing and verification process: Adopt advanced testing and verification methods, such as high-speed signal analysis, electromagnetic compatibility testing and radio frequency performance testing, to ensure that the design meets performance requirements and effectively reduces signal crosstalk.

Conclusion

Chiplet technology can lead to more efficient and economical semiconductor packaging, greater flexibility in the design and production of packaged products, and more compact and thermally / electrically efficient packaging. Therefore, the analysis of signal integrity is crucial in PCB layout and chiplet packaging.

FAQ
Signal crosstalk is the coupling between two signal lines, the mutual inductance and mutual capacitance between signal lines cause noise on the line. Signal crosstalk is perhaps the single most important effect affecting data transmission at high speeds.

The three most important reasons for signal crosstalk in chiplet packaging are:
1. Electromagnetic coupling
2. High-density layout
3. High-speed signal

1. Increase the spacing between signal paths;
2. Use the plane as the return path;
3. Make the coupling length as short as possible;
4. Wiring at the stripline layer;
5. Reduce the characteristic impedance of the signal path;
6. Use a stack with a lower dielectric constant;
7. Do not share return pins in packages and connectors;
8. Use guard wiring with short-circuit vias at both ends and along the entire line.

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