Essential IPC standards for controlled impedance PCB design and manufacturing.
Design Guide for High-Speed Controlled Impedance Circuit Boards
Primary reference for controlled impedance design, including transmission line theory, impedance calculations, and test methods.
This document provides guidance for the design of high-speed controlled impedance circuit boards. It covers transmission line theory, impedance calculation methodologies, and verification procedures.
| Parameter | Class 2 | Class 3 |
|---|---|---|
| Minimum Trace Width | 0.1mm (4mil) | 0.075mm (3mil) |
| Minimum Spacing | 0.1mm (4mil) | 0.075mm (3mil) |
| Min Annular Ring | 0.05mm (2mil) | 0.05mm (2mil) |
| Max Aspect Ratio | 10:1 | 10:1 |
| Impedance Tolerance | ±10% | ±5% |
| Hole Position Accuracy | ±0.08mm | ±0.05mm |
IPC standards provide guidelines, not absolute rules. Always work with your PCB fabricator to understand their specific capabilities and tolerances. Manufacturing processes vary, and what's achievable at one fab house may not be possible at another. For controlled impedance designs, request the fabricator's stackup capabilities before finalizing your design.
| Standard | Primary Focus | Key Topics | When to Use |
|---|---|---|---|
| IPC-2141A | Controlled Impedance | Impedance formulas, test coupons, TDR | High-Speed Design |
| IPC-2221B | Generic PCB Design | Conductor width, spacing, vias, thermal | All PCB Designs |
| IPC-2152 | Current Capacity | Trace ampacity, temperature rise | Power Electronics |
| IPC-4101E | Base Materials | Dk, Df, Tg, Td, CTI specifications | Material Selection |
General Electronic Products
Consumer electronics, toys, disposable products
Dedicated Service Products
Industrial, telecom, servers, networking equipment
High Reliability Products
Medical, aerospace, military, safety-critical
| Voltage (DC or AC Peak) | External (Coated) | External (Uncoated) | Internal Layers |
|---|---|---|---|
| 0 - 15V | 0.05mm | 0.1mm | 0.05mm |
| 16 - 30V | 0.05mm | 0.1mm | 0.05mm |
| 31 - 50V | 0.1mm | 0.6mm | 0.1mm |
| 51 - 100V | 0.1mm | 0.6mm | 0.1mm |
| 101 - 170V | 0.25mm | 1.25mm | 0.25mm |
| 171 - 250V | 0.4mm | 1.25mm | 0.4mm |
Class 2 is appropriate for most commercial/industrial products. Specify Class 3 only when required by regulatory bodies (medical, aerospace, military) or when failure would cause safety hazards. Class 3 increases cost significantly due to stricter inspection and lower yield.
Include a table with target impedance values, tolerances, layer references, and trace widths. Example: "Layer 1 to Layer 2: 50Ω ±10%, 5mil trace on 4mil prepreg, Dk=4.0". Request fabricator stackup review and TDR report.
IPC-2152 supersedes the current capacity charts in IPC-2221 with more accurate values based on extensive testing. IPC-2152 accounts for thermal properties more accurately, especially for internal layers and heavy copper. Use IPC-2152 for new designs.
IPC standards are industry consensus documents, not regulations. However, they're often referenced in contracts and specifications. Military (MIL-PRF-31032) and some medical device standards reference IPC. Following IPC provides a common language between designers and fabricators.
Include IPC class, impedance targets, material requirements, and finish specifications on your fab drawing. Clear documentation prevents misunderstandings and ensures consistent builds.
For Class 2/3 boards, request FAI with TDR report, cross-sections, and impedance measurements before full production. This catches issues early.
Tight tolerances (Class 3) increase cost and reduce yield. Work with your fabricator to understand their capabilities and design to their process sweet spots when possible.
IPC standards are regularly updated. Ensure you're referencing current revisions and that your fabricator's certifications are up to date for the class you're specifying.