Created:2026/05/07
Updated:2026/05/07
Author:
Takeshi Takatsudo

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:

Trace width - The physical width of the copper trace
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):

Copper Weight	Thickness	Common Use
0.5 oz	17.5 µm	Fine-pitch, low current
1 oz	35 µm	Standard (most PCBs)
2 oz	70 µm	High current, power electronics
3 oz	105 µm	Heavy power, automotive

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):

Trace Width	10°C Rise	20°C Rise	30°C Rise	45°C Rise
0.25mm	~0.8A	~1.2A	~1.5A	~1.8A
0.5mm	~1.5A	~2.2A	~2.7A	~3.3A
0.7mm	~2.5A	~3.5A	~4.5A	~5.5A
1.0mm	~3.2A	~4.5A	~5.5A	~6.5A
2.0mm	~5.5A	~7.5A	~9.0A	~11A

2oz Copper (70µm):

Trace Width	10°C Rise	20°C Rise	30°C Rise	45°C Rise
0.25mm	~1.2A	~1.7A	~2.1A	~2.6A
0.5mm	~2.2A	~3.1A	~3.8A	~4.7A
0.7mm	~3.5A	~5.0A	~6.0A	~7.5A
1.0mm	~4.5A	~6.3A	~7.7A	~9.5A
2.0mm	~7.7A	~10.5A	~12.5A	~15A

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:

USB-PD 3A rating requires traces that can handle 3A
0.7mm @ 1oz copper handles ~2.5-3.5A (depending on temp rise)
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

Rail	Voltage	Current	Power
Input (USB-PD)	15V	3A max	45W
+12V output	12V	1.5A	18W
-12V output	-12V	1A	12W
+5V output	5V	1.5A	7.5W

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:

Factor	1oz	2oz
Cost	Standard	+$5-15 extra
0.7mm @ 3A	~15-20°C rise	~10°C rise
Availability	Always in stock	Usually available
Our need	Sufficient	Overkill

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:

Current	Temp Rise	Status
2.5A	+10°C	Very safe
3.0A	+15°C	Safe ✓ (our design point)
4.0A	+25°C	OK
5.0A	+40°C	Warm but acceptable
6.0A	+60°C	Hot, near limit
7-8A	+80-100°C	Problem zone
10A+	-	Destruction

Failure Points

Temperature	What Happens
+45°C rise	IPC-2221 max recommended
+80-100°C rise	Solder joints weaken
~105°C rise	FR4 glass transition (~130°C absolute)
~150°C+	Trace delamination, board damage

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:

Drill hole through PCB
Electroplate thin copper layer on hole walls
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

Current	Min Width (1oz)	Recommended
0.5A	0.2mm	0.3mm
1A	0.4mm	0.5mm
2A	0.6mm	0.8mm
3A	0.7mm	1.0mm
5A	1.5mm	2.0mm
10A+	Use polygon/plane	GND pour

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)

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

Overview​

Copper Weight Basics​

What is Copper Weight?​

1oz vs 2oz Comparison​

IPC-2221 Current Capacity​

The Standard​

Temperature Rise Explained​

Current Capacity Table (External Layer)​

Why USB-C Connector Pins Are This Size​

The Connection​

Current Distribution​

This Project's Design Decision​

Our Requirements​

Trace Width Selection: 0.7mm​

Copper Weight Decision: 1oz​

Current Limits and Safety Margins​

What Happens at Higher Currents?​

Failure Points​

Our Safety Margin​

Via Behavior (Important!)​

Vias Are NOT Solid Copper​

Multiple Small Vias Beat One Large Via​

Practical Guidelines​

Trace Width Quick Reference​

When to Use 2oz Copper​

For This Project​

Related Documentation​