High-Density Interconnect (HDI) PCBs: BGA Routing and Impedance Control Techniques

Introduction

As electronic devices continue to shrink in size while increasing in functionality, High-Density Interconnect (HDI) printed circuit boards (PCBs) have become essential components in modern electronics design. Among the most challenging aspects of HDI PCB design is the proper routing of Ball Grid Array (BGA) packages and maintaining precise impedance control throughout these dense interconnects. This article explores the critical considerations and best practices for BGA routing and impedance control in HDI PCB designs.

Understanding HDI PCBs and BGA Packages

HDI PCB Characteristics

HDI PCBs are characterized by their fine line widths and spaces, microvias, and high connection pad density. These boards typically feature:

• Trace widths and spaces of 3 mil (0.075 mm) or less
• Microvias (laser-drilled vias) with diameters under 6 mil (0.15 mm)
• Sequential lamination processes
• High layer counts (often 8+ layers) in compact form factors

BGA Package Challenges

BGA packages offer significant advantages for high-pin-count integrated circuits, but present unique challenges in HDI PCB design:

1. High I/O density: Modern BGAs can have hundreds or thousands of balls with pitches as small as 0.3 mm
2. Escape routing difficulty: Routing all signals from under the BGA requires careful planning
3. Power delivery challenges: Providing clean power to all parts of the BGA
4. Signal integrity concerns: Maintaining proper impedance and minimizing crosstalk

BGA Routing Strategies in HDI PCBs

Escape Routing Fundamentals

The primary challenge in BGA routing is “escaping” all signals from beneath the component. Key strategies include:

1. Fanout patterns: Determining optimal via placement for signal escape

• Dog-bone fanouts (via-in-pad for 0.8mm pitch and below)
• Microvia-in-pad for ultra-fine pitch BGAs
• Staggered via patterns to maximize routing channels

1. Layer allocation: Assigning signals to appropriate layers

• Critical signals on inner layers between reference planes
• Less sensitive signals on outer layers
• Dedicated layers for power and ground

1. Via selection: Choosing appropriate via types

• Through-hole vias for non-dense areas
• Blind and buried vias for layer transitions
• Microvias (laser-drilled) for high-density areas

Advanced Routing Techniques

For very high-density BGAs (0.5mm pitch and below), designers must employ advanced techniques:

1. Any-layer HDI: Using microvias that can connect any layers as needed
2. Via-in-pad: Placing vias directly in BGA pads (requires plating/filling)
3. Conductive fill vias: Filled and capped vias for flat surfaces
4. Stacked and staggered vias: Multiple microvias stacked or offset

Power Delivery Considerations

Proper power distribution is critical for BGA packages:

1. Power plane segmentation: Multiple voltage islands
2. Decoupling capacitor placement: Optimal location and quantity
3. Via stitching: Multiple vias for low-impedance connections
4. Power integrity analysis: Ensuring adequate current delivery

Impedance Control in HDI PCB Designs

Importance of Impedance Control

Maintaining consistent impedance is crucial for:

1. Signal integrity (minimizing reflections)
2. Electromagnetic compatibility (reducing emissions)
3. System reliability (ensuring proper timing margins)

Key Impedance Parameters

The primary controlled impedance types in HDI PCBs:

1. Single-ended traces: Typically 50Ω or 55Ω
2. Differential pairs: Common values include 85Ω, 90Ω, or 100Ω
3. Power impedance: Target impedance for PDN (often milliohms)

Factors Affecting Impedance

Multiple factors influence trace impedance:

1. Trace geometry: Width, thickness, and length
2. Dielectric properties: Material Dk (dielectric constant) and Df (loss tangent)
3. Reference planes: Distance to nearest reference plane
4. Adjacent traces: Coupling from neighboring signals
5. Surface finish: Impact on final conductor dimensions

Impedance Calculation Methods

Common approaches to impedance calculation:

1. 2D field solvers: For simple cross-sections
2. 3D electromagnetic simulators: For complex structures
3. Industry formulas: IPC-2141 equations for quick estimates
4. Manufacturer recommendations: Based on their process capabilities

Impedance Control Implementation

Practical methods for maintaining impedance:

1. Stackup design: Careful layer arrangement and material selection
2. Trace width compensation: Accounting for manufacturing tolerances
3. Cross-hatched planes: Managing impedance with partial reference planes
4. Pre-production testing: Coupon testing for verification

Advanced Topics in HDI Impedance Control

Mixed Dielectric Environments

Modern HDI boards often use multiple dielectric materials:

1. High-speed prepregs: Low-Dk materials for critical signals
2. Standard FR-4: For non-critical areas
3. Flexible materials: For rigid-flex designs

Impedance Matching Techniques

Strategies for maintaining impedance through transitions:

1. Via impedance control: Back-drilling, via filling, or via stitching
2. Connector transitions: Matched impedance connectors
3. Differential pair routing: Maintaining pair consistency

Frequency-Dependent Effects

At high frequencies (10GHz+), additional considerations:

1. Skin effect: Current concentration at conductor surfaces
2. Dielectric loss: Material dissipation factor impact
3. Surface roughness: Effect on conductor loss

Design for Manufacturing (DFM) Considerations

Manufacturing Tolerances

Accounting for process variations:

1. Etch factor: Copper sidewall profile impact
2. Dielectric thickness variation: Layer-to-layer consistency
3. Registration tolerance: Layer-to-layer alignment

Testing and Verification

Methods to ensure design meets requirements:

1. Impedance coupon design: Test structures on panel
2. Time-domain reflectometry (TDR): Impedance profile measurement
3. Cross-section analysis: Physical verification of dimensions

Future Trends in HDI BGA Routing and Impedance Control

Emerging technologies and challenges:

1. Sub-0.3mm pitch BGAs: New routing challenges
2. Higher frequency materials: For 56Gbps+ signaling
3. Embedded components: Passive components within PCB layers
4. Additive manufacturing: Potential for new routing paradigms

Conclusion

Effective BGA routing and impedance control in HDI PCBs requires careful consideration of multiple interdependent factors. As package densities increase and signal speeds continue to rise, designers must employ increasingly sophisticated techniques to ensure reliable operation. By understanding the fundamental principles outlined in this article and staying abreast of emerging technologies, engineers can successfully navigate the challenges of modern HDI PCB design.

Successful HDI implementations combine thoughtful stackup design, appropriate material selection, meticulous routing strategies, and close collaboration with manufacturing partners. As the electronics industry continues its relentless push toward higher performance in smaller form factors, mastery of HDI BGA routing and impedance control techniques will remain an essential skill for PCB designers.