Skip to main content

Signal Electromigration (Signal EM): Violations, Examples, and Practical Fixes

  Hello Dear Readers,   Today in this post, I will provide some deep insight into the Signal Electromigration (Signal EM): Violations, Examples, and Practical Fixes. 1. Introduction: As technology nodes shrink into the deep‑submicron and nanometer regime (7nm, 5nm, 3nm and beyond), electromigration (EM) has become a first‑order reliability concern—not only for power/ground (PG) networks but also for signal nets. Signal EM failures are often underestimated because signal currents are transient and bidirectional. However, with higher switching activity, tighter metal pitches, thinner wires, and aggressive timing closure, signal EM can cause latent or early‑life failures if not addressed properly. This article explains: What Signal EM is and how it differs from PG EM Typical Signal EM violation scenarios Detailed, practical examples Root causes behind each violation Proven solutions and best practices to fix and prevent Signal EM issues 2. What is Signal Electromigration: El...
Post a Comment

Signal Electromigration (Signal EM): Violations, Examples, and Practical Fixes

 Hello Dear Readers, 

Today in this post, I will provide some deep insight into the Signal Electromigration (Signal EM): Violations, Examples, and Practical Fixes.


1. Introduction:

As technology nodes shrink into the deep‑submicron and nanometer regime (7nm, 5nm, 3nm and beyond), electromigration (EM) has become a first‑order reliability concern—not only for power/ground (PG) networks but also for signal nets. Signal EM failures are often underestimated because signal currents are transient and bidirectional. However, with higher switching activity, tighter metal pitches, thinner wires, and aggressive timing closure, signal EM can cause latent or early‑life failures if not addressed properly.

This article explains:

  • What Signal EM is and how it differs from PG EM

  • Typical Signal EM violation scenarios

  • Detailed, practical examples

  • Root causes behind each violation

  • Proven solutions and best practices to fix and prevent Signal EM issues

2. What is Signal Electromigration:

Electromigration is the transport of metal atoms caused by momentum transfer from conducting electrons. Over time, this atomic movement leads to:

  • Voids → increased resistance → open circuits

  • Hillocks → shorts to neighboring wires

Signal EM vs Power EM

AspectPower EMSignal EM
Current natureMostly DC / unidirectionalTransient, bidirectional
Analysis modeAverage / RMS currentPeak, RMS, duty‑cycle based
Affected netsVDD/VSS straps, railsClocks, resets, buses, high‑toggle nets
Common misconceptionAlways criticalOften ignored (but dangerous)

Even though signal current reverses direction, current density peaks during switching edges can exceed EM limits, especially on clocks and high‑fanout nets.

3. Why Signal EM is Critical in Advanced Nodes:

Signal EM risk increases due to:

  • Narrower metal widths → higher current density

  • Higher operating frequencies → more switching events

  • Large capacitive loads (CTS buffers, long routes)

  • Aggressive cell downsizing

  • Multi‑bit flop and high‑drive clock buffers

A signal EM failure may pass functional tests but fail after weeks or months in the field, making it a serious reliability risk.

4. Key Parameters in Signal EM Analysis:

Signal EM tools (Voltus, Voltus Fi, RedHawk‑SC, Totem, etc.) typically evaluate:

  1. Peak Current Density (Jpeak)

  2. RMS Current Density (Jrms)

  3. Effective Duty Cycle

  4. Metal width and thickness

  5. Temperature

  6. Via count and enclosure

Violation condition (simplified):

J_actual > J_allowed(process, metal, temp, lifetime)

5. Common Signal EM Violation Scenarios (with Examples):

Example 1: Clock Net EM Violation

Scenario

  • High‑frequency clock (2–4 GHz)

  • Driven by strong CTS buffers

  • Routed using minimum‑width M2/M3

Observed Violation

  • EM report flags multiple segments with RMS current density > limit

Root Cause

  • Very high toggle rate (≈100%)

  • Large cumulative sink capacitance

  • Narrow metal width

Fix / Solution

  • Promote clock routing to higher metal layers (M5/M6)

  • Increase wire width (NDR or clock rule)

  • Insert additional clock buffers to reduce segment current

  • Use shielded clock routes (improves both SI and EM)

Example 2: Reset Net with Unexpected EM Violation

Scenario

  • Global reset net

  • Considered low‑activity logically

  • Physically very long with many sinks

Observed Violation

  • EM violation during stress or test modes

Root Cause

  • Reset toggles heavily during scan/test

  • Large fanout causes high instantaneous current

  • Tool considers worst‑case activity, not functional assumption

Fix / Solution

  • Break reset into regional reset trees

  • Add intermediate buffers

  • Route reset on wider metal

  • Apply correct activity annotation (if justified and sign‑off approved)

Example 3: Data Bus EM Violation

Scenario

  • 128‑bit data bus

  • High switching probability

  • Minimum‑width horizontal routing

Observed Violation

  • Multiple EM hotspots on middle segments

Root Cause

  • Simultaneous switching of many bits

  • Long parallel segments increasing current per wire

Fix / Solution

  • Increase wire width or spacing

  • Use higher metal layers for long bus routes

  • Segment the bus using repeaters

  • Consider bus encoding (architecture‑level fix)

Example 4: Via‑Related Signal EM Violation

Scenario

  • Signal drops from M6 to M2 using a single via

  • High drive strength cell

Observed Violation

  • Via EM failure reported

Root Cause

  • Via current density exceeds limit

  • Single via cannot handle peak current

Fix / Solution

  • Add multiple vias (via array)

  • Improve via enclosure

  • Re‑route to avoid sharp layer drops

Example 5: Tool Mismatch (Voltus vs RedHawk‑SC)

Scenario

  • Same design analyzed in two tools

  • Different EM numbers reported

Root Cause

  • Different activity assumptions

  • Different waveform modeling

  • Temperature or metal tech file mismatch

Fix / Solution

  • Ensure same:

    • Liberty files

    • Switching activity (VCD/SAIF)

    • Temperature corners

    • Tech EM limits

  • Compare segment‑level current, not only violation count

6. Step‑by‑Step Methodology to Fix Signal EM Violations

Step 1: Classify the Net

  • Clock / Reset / Data / Control

  • High toggle or low toggle

Step 2: Identify the Failing Segment

  • Long wire?

  • Narrow metal?

  • Single via?

Step 3: Apply Physical Fixes (Preferred)

  • Increase wire width

  • Move to higher metal

  • Add shielding

  • Add vias

Step 4: Logical Fixes (If Needed)

  • Add buffers

  • Reduce fanout

  • Net segmentation

Step 5: Activity‑Based Fixes (Last Resort)

  • Refine switching activity

  • Mode‑based EM analysis

  • Apply waivers only with sign‑off approval

7. Best Practices to Prevent Signal EM Issues

  • Use non‑default routing rules (NDRs) for clocks and global nets

  • Avoid minimum‑width routing for long, high‑fanout signals

  • Prefer higher metals early in floorplanning

  • Add via arrays by default for high‑drive pins

  • Run early EM checks (pre‑CTS and post‑CTS)

  • Treat signal EM as a design requirement, not ECO cleanup

8. Conclusion:

Signal electromigration is no longer a corner‑case issue—it is a core reliability challenge in advanced nodes. Clocks, resets, and high‑activity data paths are especially vulnerable due to high current density and aggressive routing.

A robust Signal EM strategy combines:

  • Early awareness

  • Correct activity modeling

  • Strong physical design practices

  • Minimal reliance on waivers

By addressing Signal EM proactively, designers can avoid costly late‑stage ECOs and ensure long‑term silicon reliability.



Connect with me 

4.WhatsApp 


Comments

Post a Comment

Popular posts from this blog

Exploring the Role of LEF Files in VLSI Chip Design: A Beginner's Guide

Hello Dear Readers,   Today in this post, I will provide some deep insight into the LEF file role during the VLSI Chip Design process. In VLSI (Very Large Scale Integration) design, a LEF file is a file that contains information about the physical geometry of the standard cells used in a circuit. LEF stands for Library Exchange Format. A standard cell is a pre-designed logic cell that contains a specific function, such as a flip-flop or an AND gate. Standard cells are designed to be easily combinable and scalable to create more complex circuits. The physical geometry of each standard cell is defined in the LEF file. The LEF file contains information such as the width, height, and position of the pins and metal layers of each standard cell. It also contains information about the physical design rules that govern the placement of these cells on the chip. LEF files are important in VLSI design because they enable the interoperability of different design tools from different vend...
4 comments
Read more

Best Book for Designing Microarchitecture of Microprocessor Using Verilog HDL

  Hello Dear Readers, Currently, after succeeding in many topics now I starting to provide technical book reviews which were I have completed and still read books always. So let us start today's book review. Book Name:   Computer Principles and Design in Verilog  HDL Description:  Uses Verilog HDL to illustrate computer architecture and microprocessor design, allowing readers to readily simulate and adjust the operation of each design, and thus build industrially relevant skills Introduces the computer principles, computer design, and how to use Verilog HDL (Hardware Description Language) to implement the design Provides the skills for designing processor/arithmetic/cpu chips, including the unique application of Verilog HDL material for CPU (central processing unit) implementation Despite the many books on Verilog and computer architecture and microprocessor design, few, if any, use Verilog as a key tool in helping a student to understand these design techniques...
3 comments
Read more

Physical Design Methodology Engineer at Texas Instruments

  Hello Dear Readers, Texas Instruments Bangalore has a vacancy for the Physical Design Engineer role. We need an Physical Design Methodology Engineer to join our ATD team. The candidate should have a strong background in back-end design of ASIC/SoC chips. The ideal candidate will have a bachelor’s or master’s degree in Electrical Engineering or a related field. Requirements: 1 - 2 Years of experience in physical design Bachelor’s or master’s degree in Electrical/Electronics Engineering or a related field Strong understanding of physical design principles Must know the basics of floorplan, placement, CTS, routing, ECO, Physical Verification Proficiency in back-end design tools, such as Cadence Genus/Innovus/Tempus/Voltus Excellent problem-solving skills and attention to detail Effective communication and collaboration skills Responsibilities: Synthesis to GDSII Perform full Physical design flow and its verification Work closely with Digital Design and DFT engineers Ensure...
Post a Comment
Read more