Designing an ASIC (Application Specific Integrated Circuit) is like building a complex city where billions of tiny “buildings” (transistors and standard cells) must be arranged wisely, roads (wires) must connect them efficiently, and traffic (signals) must flow at the right times. In VLSI terminology, this whole physical layout process is called Place and Route (P&R), one of the most crucial stages in ASIC physical design.

As chips incorporate advanced features like AI accelerators, heterogeneous computing, multi-power domains, and chiplets, the Place and Route process has evolved dramatically. This blog simplifies the basics of Place and Route for beginners, explains why it matters, and highlights the latest industry practices shaping how modern ASICs are built.

What Is Place and Route?

Place and Route (P&R) refers to the physical implementation stages of an ASIC, where:

1. Placement: Standard cells, macros, memories, and I/O elements are placed on the silicon die with legal positioning.
2. Routing: Interconnects (the wires) are created to connect these placed elements according to the netlist and timing goals.
   
   

Together, Place and Route transforms the logical representation of the design (netlist from synthesis) into a physical layout (GDSII/OASIS) that can be fabricated.

In short:

Placement = Where do components go?
Routing = How are they connected?

Why Place and Route (P&R) Matters

ASIC designs are no longer simple two-domain layouts. Modern challenges include:

Multi-Domain Heterogeneous SoCs

AI accelerators, RISC-V cores, CPU clusters, and specialized DSP blocks on the same die require careful physical partitioning.

Power & Clock Complexity

Multiple power rails, clock domains, and DVFS (Dynamic Voltage and Frequency Scaling) constraints make physical implementation harder.

Routing Congestion & Signal Integrity

On advanced nodes like 3nm–2nm, interconnect delay and crosstalk dominate performance, making routing optimizations critical.

AI-Assisted P&R Tools

Next-gen tools use machine learning to optimize placement and routing decisions dynamically.

Understanding these dynamics will make you a more effective ASIC designer or physical design engineer.

The Place and Route Flow (Step by Step)

Let’s walk through the main stages of P&R in a clear and up-to-date way.

1. Physical Synthesis (Optional Pre-P&R)

Some flows apply physical synthesis to optimize logic with physical constraints in mind, adjusting gate sizes, restructuring logic, and creating better nets for timing and area.

This helps the upcoming placement stage by giving a better initial physical view of design.

2. Floorplanning

Floorplanning defines:

• Core area
• Block boundaries
• Power and clock regions
• Macro placements (memory, hard IPs, SERDES)

Floorplanning increasingly relies on AI-assisted proposals that balance timing, congestion, and power.

Key Outputs of Floorplan:

• Block sizes and shapes
• Power grid plan (UPF / CPF integration)
• Initial placement constraints

3. Placement

Placement is the step where:

• Standard cells and macros are positioned
• Congestion and proximity to timing critical paths are considered
  
  

Modern placement uses:

• Clustering to group related cells
• Timing-driven strategies to optimize critical nets
• Power awareness to reduce hot spots
  
  

Tools from Synopsys, Cadence, and Siemens EDA use physical analysis engines to iteratively refine placement based on congestion, timing, and power.

Placement Objectives:

• Minimize global and local wirelength
• Meet setup and hold timing constraints
• Balance power distribution and thermal impact

4. Clock Tree Synthesis (CTS)

While sometimes treated as a separate step, CTS is tightly coupled with placement. It builds the clock distribution network to:

• Minimize skew
• Balance latency across domains
• Support clock gating and low-power modes

CTS quality determines whether timing closure is possible in later stages.

5. Routing

Once placement is acceptable, routing begins.

Global Routing
Routes signals at a macro level, deciding which regions the wires will travel.

Detailed Routing
Physically routes wires with exact geometries while obeying DRCs (Design Rule Checks) and avoiding congestion.

Modern routers support:

• Crosstalk mitigation
• Shielding for high-speed nets
• Multi-layer optimization
• Multi-corner awareness
  
  

Good routing minimizes delay and ensures signal integrity.

6. Optimization, ECO & Timing Closure

After routing, final adjustments (ECOs — Engineering Change Orders) are done:

• Fix timing violations
• Adjust buffer insertion
• Optimize nets for power or area
  
  

Tools now provide auto-ECO features that use data analysis and AI to suggest changes that improve timing.

7. DRC/LVS and Signoff Checks

The final layout must be checked against:

• DRC (Design Rule Check) – Manufacturability rules
• LVS (Layout vs Schematic) – Ensures the layout matches the logical intent
• Antenna violations
• Power integrity checks (IR drop, EM)
  
  

Signoff engines are integrated with cloud-based regression to handle multiple PVT corners.

Key Concepts in Place and Route

Congestion

Regions of the chip where too many wires cross, making routing hard.

Solutions:

• Adjust placement
• Add buffering
• Use advanced routing layers intelligently

Timing Closure

Making setup and hold times meet constraints across all corners and modes.

Strategies:

• Buffer tuning
• Gate resizing
• Path restructuring
  
  

Power Awareness

Modern P&R tools integrate power constraints from formats like UPF (Unified Power Format).

This ensures:

• Low-power modes don’t violate design constraints
• Power rail distribution is valid
  
  

Multi-Corner Multi-Mode (MCMM)

Verification across multiple conditions:

• Voltages
• Temperatures
• Frequencies
• Power states
  
  

Routing and CTS must satisfy timing across all these modes.

Tools Commonly Used

Here’s an updated list of tools that dominate the P&R landscape:

These tools often integrate Python APIs for automation and data analysis.

Practical Tips for Freshers

If you’re just getting started, here’s how you can build intuition and skill quickly:

Learn Timing Constraints Early

Knowing how to write and interpret SDC (Synopsys Design Constraints) is crucial.

Understand Power Formats (UPF)

Low power is everywhere; UPF is no longer optional. Learn how to integrate power intent early.

Analyze Reports Carefully

Tools produce multiple reports:

• Timing (setup/hold)
• Congestion
• Power
• DRC/LVS
  
  

Learning how to read them accelerates debugging.

Use Visualization Tools

Visual feedback (heatmaps, clock trees, routing density) helps you see problems before they become failures.

Practice with Small Designs

Start with small modules: FIFOs, ALUs, simple controllers. Gradually scale up to block-level designs.

Common Challenges and Solutions

Why Place and Route Skills Matter

Companies are looking for engineers who:

• Understand P&R flows end-to-end
• Can analyze and interpret complex reports
• Automate flows using Python/TCL
• Balance timing, power, and area in physically constrained designs

These skills open doors to roles in physical design, STA (Static Timing Analysis), DFT, and CAD automation.

Conclusion

Place and Route is where logic becomes silicon. It’s a complex, data-driven stage that balances timing, power, congestion, and manufacturability. By understanding the basics—from placement and clocking to routing and optimization—you’ll build a strong foundation for a career in ASIC physical design.

Remember:

• Start with strong constraint knowledge
• Practice interpreting metrics
• Learn to use modern tools effectively
• Embrace automation and scripting
  
  

With these skills, you’ll be well-prepared for the challenges of tomorrow’s semiconductor landscapes.