In circuit board production, the term thickness refers to the weight of copper used to build the copper traces. So how thick is 1 oz copper?
The standard trace thickness in the PCB industry is usually 1 oz per square foot. So you don’t have to give this specification to your manufacturer unless you want heavier copper. Read on to learn more!
What Is 1 oz Copper Thickness?
Picture yourself having liquid or malleable copper materials, which you can pour or roll on a flat surface. The copper distribution of a single ounce of copper poured or flattened on one square foot is 1 oz copper thickness.
You don’t have to do this process manually because the thickness is already known. 1 oz copper is 1.37 mils or 34.79 microns thick.
Here’s how this copper weight compares with heavier copper layers.
|Thickness of Copper Foil|
The thickness for 0.5-oz of copper is half that of 1 oz. (0.7 mils).
Why Copper Weight (Thickness) Matters
Copper weight determines the maximum current carrying capacity of the PCB traces. The heavier or thicker it is, the more electrical current the layer can handle.
A DDR3 module with multiple electrical connections
This factor is vital to consider in impedance-controlled boards. Trace width affects impedance. So depending on the kind of control you want, you have to adjust thickness accordingly to meet your requirements.
Typical Board Stack Up With 1 Oz Copper
A multilayer PCB with the standard copper layer thickness can have this stack-up.
|Layer Number||Layer Name||Thickness|
|3||Upper Solder Resist||0.4 mils|
|4||Upper Copper Layer||1.37 mils|
|5||Dielectric 2||7.0 mils|
|6||Inner Copper Layer 1||1.37 mils|
|7||Dielectric 1||42 mils|
|8||Inner Copper Layer 2||1.37 mils|
|9||Dielectric 3||7.0 mils|
|10||Bottom Copper Layer||1.37 mils|
|11||Bottom Solder Resist||0.4 mils|
It is vital to note that you can have different copper weights in the layers. For instance, you can place 1 oz copper on the inner layers and thicker copper on the external layers.
And the best practice is to have the same copper weight on the mirror-opposite layer in the stack-up.
Using the example with four copper PCB layers above, the inner layers should have the same copper weight. The outer ones should also match.
So the upper and bottom copper layers can have a 3 oz copper weight on the upper and bottom copper layers. In the same board, the inner layers one and two can have a 1-oz thickness.
Best Trace Width for 1 Oz Copper Thickness
A trace’s cross-section ultimately determines the maximum current it can transmit. So while the thickness (trace height) is critical, consider the width. And the ideal trace width to implement with 1 oz copper thickness ranges from 7-12 mils.
Conductive tracks on a blue PCB
Factors determining the correct width include the following.
- Cross-section area
- Current capacity
- Trace termination
You can calculate the width by implementing these variables in this formula.
Trace width=AT x 1.378 mils
- T is the trace thickness (1.37 mils)
- A is the cross-section area
But we must first calculate the required cross-section area to get the maximum current carrying capacity. We can use this formula.
Area =(I(K x Trise)0.44)10.725
- I (amperes) is the current
- K is the layer constant (0.024 for inner layers and 0.048 for outer layers according to IPC-2221 standards)
- T is the temperature rise in °C
Switching the maximum current carrying capacity to be the subject of the formula gives this equation.
I=A0.725x (K x Trise)0.44
How To Calculate The Resistance of 1 Ounce of Copper Thickness
While calculating the trace width and cross-sectional area, you can compute the trace resistance to determine its electrical performance.
Use this formula.
Resistance=pLA x [1+a(T-Troom)]
- p is the resistivity parameter (1.7×10-6) ohm-cm
- L is the trace length
- A is the cross-section area (trace width x trace thickness in cm)
- a is the resistivity temperature coefficient (3.9×10-3)
- T is the trace temperature
- Troom is the room or ambient temperature (deducting from T gives the temperature rise)
Recommended Minimum Spacing Between 1 Oz Copper Traces
The thicker the PCB trace, the more spacing you must place between it and other conductive lines. Placing them too close to each other can cause crosstalk, reflections, coupling, and other forms of signal transmission interference.
A green PCB with copper traces (note the spacing)
The minimum spacing for 1 oz copper traces is 3.5 mils or 0.089mm. Here’s how this minimum spacing compares with thicker copper weights.
|Copper Foil Thickness||Minimum Trace Spacing|
|1 oz.||0.089mm (3.5 mils)|
|2 oz.||0.203mm (8 mils)|
|3 oz.||0.254mm (10 mils)|
|4 oz.||0.255mm (14 mils)|
Advantages of the Standard 1 Oz Copper Thickness
This copper weight is a reliable PCB solution due to these benefits.
Superb Electrical Conductivity
Since copper has superb electrical properties, 1 oz copper thickness is sufficient to handle the electrical connections in electrical devices. But the weight is more practical for consumer electronics and other low-power applications.
Copper traces on a green-printed circuit board
PCB manufacturers use this copper thickness as standard because it is practical for most applications and cost-effective. Thicker copper layers or using other materials like gold increases the PCB cost.
High Decomposition Temperature
This copper thickness has superb thermal resistivity because its decomposition temperature exceeds 270°C. So these traces have a high tolerance, resulting in dimensional stability at different thermal states.
Disadvantages of the Standard 1 Oz Copper Thickness
- Not ideal for flexible PCBs
- Inadequate for high power applications (will generate extreme heat)
Thermal Via Considerations for 1 Oz Thickness of Copper
When dealing with 1 oz copper boards, consider these thermal via designs to ensure efficient heat dissipation.
- Design/build small thermal vias to avoid weakening the solder joint (solder wicking)
- Balance the via size to prevent increased thermal resistance and solder wicking (should not be too small or too big)
- Adjust the thermal via to the copper
- Cover the thermal vias using a solder mask to minimize solder joint errors
Conductive copper traces connected to through-hole vias
1 Oz Copper Thickness Applications
- Automotive devices
- Military electronics
- Wireless technologies
- Medical equipment
- High-speed telecommunication PCBs
The 1 oz standard copper thickness has a wide range of applications because it strikes the perfect balance between high electrical conductivity and low manufacturing cost.
But you can blend it with heavy copper in a multilayer PCB to increase the current-carrying capacity in electronic device circuits.
That’s it for now. We hope this article has been insightful. Contact us with your PCB designs today, and we’ll advise on the best copper weight to place on your board.