SI (Signal Integrity)

Evaluation of the soundness of signal quality.

What is evaluated:

・Whether waveforms are distorted

・Whether there is delay

・Whether malfunctions occur

PI (Power Integrity)

Evaluation of the stability of power delivery.

What is evaluated:

・Whether the voltage is stable

・Whether it can follow instantaneous current

・Whether noise is getting in

In short: SI = the quality of signals, PI = the quality of power.

The reasons are clear.

① Higher speeds → signal rise times become steeper

② Higher frequencies → expanding into the GHz band

③ Higher density → distances between wires shrink

④ Higher currents → power supply noise increases

Result: signals and power interfere with each other.

Eye Patterns

A comprehensive indicator of signal quality.

Evaluates:

・Eye width

・Noise margin

・Jitter

A closing eye = more communication errors.

Jitter

Timing fluctuation.

Causes:

・Power supply noise

・Crosstalk

・PLL fluctuation

Fatal in high-speed communication.

Crosstalk

Interference between adjacent wires.

It gets worse with denser wiring.

Reflection

Occurs due to impedance mismatch.

A cause of waveform distortion.

IR Drop

Voltage drop due to wiring resistance.

Gets worse as current increases.

Power Supply Ripple

Periodic fluctuation of voltage.

A cause of logic malfunctions.

Simultaneous Switching Noise (SSN)

Occurs when many I/Os operate simultaneously.

Induces power supply fluctuation.

PDN Impedance

The resistance and inductance of the power delivery path.

Low-impedance design is essential.

Evaluation relies on both measurement + analysis.

Measurement:

・High-speed oscilloscopes

・TDR (Time Domain Reflectometry)

・Network analyzers

Analysis:

・Electromagnetic field analysis

・Circuit simulation

・Transmission line analysis

For SI / PI, analysis is essential from the design stage.

There are many influencing factors.

Examples:

・RDL wiring width and thickness

・Bump height

・TSV density

・Substrate dielectric constant

・Ground placement

Results:

・Signal delay

・Increased noise

・Increased power consumption

In other words, the package = a signal quality component.

There are three characteristics.

① Ultra-wideband communication

HBM bandwidth: on the order of several TB/s

→ SI design is most important.

② Ultra-high current

GPUs and AI chips: on the order of hundreds of amperes

→ PI design is a lifeline.

③ 3D stacking

With TSV and interposer structures:

・Wiring length increases

・Resonance increases

Evaluation is evolving from 2D → 3D electromagnetic field analysis.

Typical examples:

Impedance-matched design

→ Reduces reflection

Ground shielding

→ Blocks noise

Placement of decoupling capacitors

→ Stabilizes power

Wiring length optimization

→ Reduces delay

Differential wiring design

→ Improves noise immunity

SI / PI is layout design itself.

SI / PI defects cannot be seen from the outside.

Main symptoms:

・Communication errors

・Reduced frequency

・Malfunctions

・Increased heat generation

Therefore: they must be eliminated at the design stage.

・SI = evaluation of signal quality

・PI = evaluation of power stability

・Their importance increases with higher speed and density

・Packaging directly affects electrical performance

・In AI chips, they are the most important design area

1.What is the difference between SI and PI?

2.What is the main cause of crosstalk?

3.What causes IR drop?

4.Why is SI design important for HBM?

 

Column Supervisor: Koji Kakumoto (President, OTIS Co., Ltd.)

After studying abroad and working in planning and development at a trading company, he joined OTIS Co., Ltd. in 2011. While primarily working in the Corporate Planning Department, he has also served concurrently in manufacturing and technical divisions, and since 2018 has served as President, working to drive business growth and strengthen the organization.

 

This article is a general technical explanation intended for educational purposes and does not refer to any specific company, product, or technology.