Introduction

Copper thickness is a critical parameter in the design and performance of printed circuit boards (PCBs). The most common copper weight referred to is 1 oz, but what does that translate to in terms of actual copper thickness?

This article provides a detailed overview of 1 oz copper, including its equivalent thickness in metric units, key properties, usage considerations, and how it compares to other common copper weights seen in PCB fabrication. We’ll also look at calculating finished copper thickness after plating and etched trace widths.

Understanding copper thickness selection, processing effects, and manufacturing capabilities allows optimizing PCBs for functionality, cost, and manufacturability.

What is 1 Ounce Copper?

1 oz copper refers to 1 ounce of copper per square foot of surface area on a PCB layer. This is a weight density measured in ounces per square foot (oz/ft2).

The key to converting to a linear copper thickness is knowing the density of pure copper:

Copper Density

• Density of copper = 8.96 g/cm3
• 1 oz = 28.35 g
• 1 ft2 = 929 cm2

Calculating 1 oz Thickness

• 1 oz / 929 cm2 = 0.0305 g/cm2
• With density 8.96 g/cm3, thickness = 0.0305 / 8.96 = 0.0034 cm
• Therefore, 1 oz copper = 0.0034 cm = 34 μm (microns) thick

So while PCB copper thickness is referred to by weight (ounces), it can be converted to an actual linear metric distance in microns or mils.

Key Properties of 1 oz Copper

Some of the key material properties of 1 oz or 34 micron thick copper in PCBs:

• Conductivity – High electrical conductivity, around 59 MS/m for rolled copper foil
• Solderability – Excellent solder wetting and bonding performance
• Adhesion – Copper-clad laminate materials promote reliable copper-substrate bonding
• Etchability – Responds well to PCB etchants and processes for pattern definition
• Flexibility – Adequate ductility and malleability for bending and flexing stress
• Thermal relief – Copper expands/contracts to relieve thermal strain on PCBs

1 oz strikes an ideal balance of capabilities across electrical, mechanical, and manufacturing needs in PCB fabrication.

PCB Copper Weight Designations

Several standard copper ounce weights are used in PCB manufacturing:

As seen above, 1 oz sits between the common half and 2 ounce weights in terms of copper thickness. The trend is increasing density designs requiring heavier copper like 2-4 oz.

Why Use 1 Ounce Copper?

Here are some of the main reasons 1 ounce copper became the standard for PCB fabrication:

Electrical Performance

• Thick enough for low resistance power distribution on supply layers
• Thin enough for controlled impedance on signals below 100 MHz

Physical Strength

• Provides adequate structural integrity minimizing risk of cracking
• Durable enough for multiple via holes, component holes, and cutouts

Etching Capability

• Thin traces/spaces can be reliably etched compared to higher copper weights

Cost

• Lower material cost compared to thicker copper foils
• Reduces fabrications steps like sequential lamination

Soldermask Process

• Compatible mask thicknesses exist that can cover 1 oz copper
• Thicker copper requires screen printing process

The 1 oz weight occupies the sweet spot balancing electrical, mechanical, and fabrication requirements while avoiding the cost and complexity of thicker copper PCBs.

PCB Trace Widths With 1 oz Copper

One important design consideration with 1 oz copper is the resulting trace width after etching.

The table below shows typical minimum trace widths when using 1 oz starting foil:

This assumes approximately 50% copper etchback during patterning of external layers and ~30% on inner layers.

Narrower traces are possible but require advanced etching capability, finer line photolithography, and electrical test validation.

For reference, 2 oz copper allows roughly twice the remaining trace thickness after etch. This enables heavier current carrying capability.

Plated Copper Thickness

In addition to the base copper foil, PCB fabrication includes plating steps that further increase copper thickness:

• Electroless Plating – An initial chemical plating process prepares conductors for electrolytic plating by depositing 1-2 μm of copper.
• Electrolytic Plating – Electroplating builds up 15-30 μm of copper thickness depending on the plating profile.
• Solder, Tin, or Gold Plating – Final metal platings deposit 1-10 μm over the nickel for soldering or corrosion protection.

Taking these finishing processes into account, the total finished copper thickness with 1 oz base foil is typically:

• External layers – 50-65 μm
• Internal layers – 25-35 μm

This demonstrates why you cannot assume 1 oz foil alone when designing current carrying traces or considering etchback. The full plating thickness must be incorporated.

Comparing 1 oz to Other Copper Weights

It is helpful to compare the characteristics of 1 oz foil to the capabilities of other common copper weights:

In creasing copper weight improves current carrying capacity and fine line capability but negatively impacts flexibility, etchability, and cost. 1 oz offers a good balance across these tradeoffs.

Manufacturing With 1 oz Copper

Fabricating PCBs with 1 oz foil is well suited to standard processes:

• Photolithography – Widely available dry films and liquid photo resists can adequately cover 1 oz foils. 2 oz and above requires screen printing resists.
• Etching – Most etching processes including cupric chloride and ammonium persulfate can reliably etch 1 oz. Thicker foils often need aggressive etch cycles.
• Soldermask – Standard liquid photoimageable (LPI) soldermask materials at 25-75 μm thickness are formulated for 1 oz copper coverage.
• Plating – Common plating chemistries have sufficient throwing power to cover 1 oz copper on high-aspect ratio PTHs and holes.
• Lamination – 1 oz foil works with typical heat and pressure lamination processes without special preparation.

1 oz copper is generally the highest weight that standard PCB fabrication processes can accommodate without requiring specialized plating chemistries, custom resist or mask materials, and additional process steps. This allows more competitive pricing.

When Higher Copper Weights Are Required

While 1 oz copper can meet requirements for many board designs, increased copper thickness is necessary in certain situations:

• Very high current loads (>50A) requiring thick power traces
• High density interconnects with fine lines and spaces (<0.1mm)
• Complex RF circuits demanding thicker copper for lower loss
• Multilayer boards (>12 layers) that can accommodate thicker copper
• Silver immersion plating that can play on thick copper foils
• Flex-rigid boards combining rigid sections with high flex layer counts

In these cases, the cost and manufacturing considerations may be justified to gain higher conductance, finer resolution, or improved RF performance.

Conclusion

One ounce or 34 microns is the most common copper thickness used in PCB design and manufacturing. This weight provides an ideal balance of capabilities fitting standard fabrication processes while meeting electrical and mechanical requirements.

Understanding the relationships between copper weight, finished plated thickness, photolithography limits, and etchback allows optimizing PCB stackups. Increases in copper thickness beyond 1-2 oz are only warranted when specific design constraints around high power, fine features, or RF response necessitate the cost premium.

This overview of 1 oz copper thickness in metrics, key properties, design impact, manufacturability, and comparisons to other weights provides PCB designers deeper insight into selecting the appropriate copper to achieve functional, affordable boards.

Frequently Asked Questions

How does copper thickness affect current carrying capacity?

Thicker copper increases allowable current for a given trace width. As a rough guideline, a 25 μm thick trace can carry 1 A/mm, while 140 μm thick trace can handle 5 A/mm.

What are the downsides of using thinner copper foils?

Thinner copper below 1 oz is more prone to fracturing, has higher resistance resulting in losses, and limits soldermask thickness. But it enables tighter resolution and lower cost.

How does copper weight impact flex PCBs?

Thinner copper improves flex life by reducing stresses. 1/2 oz is common for flexible circuits instead of 1 oz. But conductors may need to be wider to compensate for lower current capacity.

Why is plated copper thickness important?

The plating deposits can often double the total copper thickness. This must be considered when designing traces for target impedance or current load, not just the base foil weight.

What determines the minimum trace width that can be etched?

The minimum chemically etched trace width is dependent on copper thickness, photolithography capabilities, etching process parameters, and quality validation. Thinner copper enables narrower traces.