PCB Trace Width and Copper Weight

Understanding how trace width and copper weight affect current carrying capacity, and why USB-C connector pins are sized the way they are.

Overview

When designing PCBs for power applications, two key parameters determine how much current a trace can safely carry:

1. Trace width - The physical width of the copper trace

2. Copper weight - The thickness of the copper layer (measured in oz/ft²)

This document explains the relationship between these parameters and current capacity, based on IPC-2221 standards.

Copper Weight Basics

What is Copper Weight?

Copper weight is measured in ounces per square foot (oz/ft²):

1oz vs 2oz Comparison

Cross-section view:

1oz copper (35µm):
═══════════════════  ← 35µm thick
       0.7mm wide

2oz copper (70µm):
███████████████████  ← 70µm thick (2× more copper)
       0.7mm wide

Key point: 2oz copper has twice the cross-sectional area, so it can carry approximately 1.4× more current at the same temperature rise (not 2×, due to thermal dynamics).

IPC-2221 Current Capacity

The Standard

IPC-2221 provides guidelines for trace current capacity based on:

• Trace width

• Copper thickness

• Acceptable temperature rise

• External vs internal layer

Temperature Rise Explained

Temperature rise is how much hotter the trace gets compared to ambient:

Temperature
    ↑
    │            ┌─────────────── Steady state (equilibrium)
    │           /
    │          /
    │         /  ← Takes 5-15 minutes to stabilize
    │        /
    │       /
    │──────┘
    └────────────────────────────→ Time
         Current ON

Important: This is steady-state temperature after thermal equilibrium, not a rate of increase. Once equilibrium is reached, temperature stops rising.

Current Capacity Table (External Layer)

1oz Copper (35µm):

2oz Copper (70µm):

Why USB-C Connector Pins Are This Size

The Connection

Looking at our USB-C connector (6-pin power-only):

USB Type-C 6-Pin Connector

┌──────────────────┐
│   1    2    3    │  Top Row
│  GND  VBUS  CC1  │
└──────────────────┘
┌──────────────────┐
│  CC2  VBUS  GND  │  Bottom Row
│   4    5    6    │
└──────────────────┘

The VBUS pins have a specific pad size (approximately 0.7mm width traces on PCB) because:

1. USB-PD 3A rating requires traces that can handle 3A

2. 0.7mm @ 1oz copper handles ~2.5-3.5A (depending on temp rise)

3. Two VBUS pins in parallel = 1.5A per pin = very comfortable margin

Current Distribution

Pin 2 (VBUS) ──┬─→ ~1.5A
               │
               ├─→ Total: 3A to circuit
               │
Pin 5 (VBUS) ──┘─→ ~1.5A

This is why USB-C connectors use multiple power pins - to distribute current and reduce individual pin stress.

This Project's Design Decision

Our Requirements

Trace Width Selection: 0.7mm

We chose 0.7mm traces for power lines because:

USB-C VBUS pin pad ≈ 0.7mm
    ↓
Matching our power traces = 0.7mm
    ↓
At 1oz copper: handles ~2.5-3.5A
    ↓
Our max current: 3A (input only)
    ↓
Result: Adequate with ~15-20°C rise ✓

Copper Weight Decision: 1oz

We chose 1oz copper because:

Calculation for 0.7mm trace @ 3A with 1oz copper:

• Temperature rise: ~15-20°C above ambient

• If ambient is 25°C, trace reaches ~40-45°C

• Well below any damage threshold

• Components rated for 85°C+ operation

Current Limits and Safety Margins

What Happens at Higher Currents?

For 0.7mm trace, 1oz copper:

Failure Points

Our Safety Margin

Design point:     3A @ 0.7mm, 1oz
Trace limit:      ~5-6A before getting hot
Connector limit:  3-5A (USB-C rating)
                  ↓
Limiting factor:  USB-C connector (not trace)
Safety margin:    ~1.7-2× on traces ✓

Via Behavior (Important!)

Vias Are NOT Solid Copper

A common misconception is that larger vias = more copper = better thermal/electrical conductivity.

Reality:

What you might imagine:          What actually happens:

    ┌──────────┐                     ┌──────────┐
    │██████████│                     │┌────────┐│
    │██SOLID██ │                     ││  AIR   ││
    │██COPPER██│  ← NOT this         ││ (empty)││  ← THIS
    │██████████│                     │└────────┘│
    └──────────┘                     └──────────┘
                                      ↑        ↑
                                   Thin copper plating
                                   (25-35 microns)

Manufacturing process:

1. Drill hole through PCB

2. Electroplate thin copper layer on hole walls

3. Center stays hollow (air)

Multiple Small Vias Beat One Large Via

For thermal connections (like IC thermal pads):

One 0.5mm via:              Nine 0.3mm vias:

    ┌─────┐                    ┌─┐ ┌─┐ ┌─┐
    │     │                    └─┘ └─┘ └─┘
    │     │  1× barrel         ┌─┐ ┌─┐ ┌─┐
    │     │  surface           └─┘ └─┘ └─┘
    └─────┘                    ┌─┐ ┌─┐ ┌─┐
                               └─┘ └─┘ └─┘

Copper wall area:            Copper wall area:
π × 0.5mm × 1.6mm           9 × π × 0.3mm × 1.6mm
≈ 2.5 mm²                   ≈ 13.6 mm²  ← 5× more!

Practical Guidelines

Trace Width Quick Reference

When to Use 2oz Copper

Consider 2oz copper when:

• Continuous current >5A per trace

• Limited board space (can't make traces wider)

• High ambient temperature environment

• Automotive/industrial applications

• Thermal management is critical

For This Project

1oz copper with 0.7mm traces is the right choice:

• Cost effective

• Sufficient for 3A with acceptable temperature rise

• Standard JLCPCB option

• Traces won't be the weak point (USB-C connector limits current first)

Related Documentation

• PCB Layout Guidelines - Complete layout rules for power circuits

• USB Type-C Pinout - Why USB-C has multiple power pins

• Two-Stage Architecture - Our power conversion strategy

Document created: 2026-01-11 Applies to: zudo-pd PCB design Design decision: 1oz copper, 0.7mm power traces for 3A USB-PD input