What is PCB Copper Foil? A Complete Guide

A PCB consists of conductive and insulating layers. Today, we will talk about the PCB copper foil, the conductive part that bonds to the dielectrics’ surface. Let’s define it before delving into its types, features, and applications.

What is PCB Copper Foil?

PCB copper foil is the thin, conductive sheet circuit board manufacturers apply to substrate surfaces, including the inner layers. The foil, being a metal with exceptional qualities, serves various purposes, such as:

• Forming paths that convey power and information from one component to another
  
• Producing ground connections to prevent electromagnetic interference and signal loss
  
• Acting as thermal conduction parts and removing heat from high-temperature components
  
• Making attachment pads that mount components to the PCB board, connecting them to the circuitry
  

Why Copper is Used in PCB

Many printed circuit board types use it for the traces and ground or power planes. Other material options include silver, gold, and aluminum. Several reasons explain why PCB makers prefer copper.

• High thermal conductivity
  
• High electrical conductivity
  
• Cu is less costly than comparable options like silver, Ag
  
• The material is malleable and easy to produce into thin layers
  
• It has a high solderability level, a significant requirement in PCB production
  
• It exhibits good adhesion properties, making it easy to bond to various materials
  
• It’s generally corrosion-resistant and usable in different application environments
  

PCB Copper Foil Types

There are typically two main types. We name them by their manufacturing method, which can be mechanical or electrical. In light of that, the foil can be electro-deposited or rolled annealed.

Electro-deposited Foil

ED foil production involves progressively depositing Cu on a rotating drum. The drum’s speed and deposition time determine its thickness. Thicker foils take longer, and the drum speed is lower.

Overall,ED foil is less commonly used in high-speed circuits due to its surface roughness. On the upside, it has a low surface oxygen rate, allowing it to bond with metals and non-metals. The following is a list of its key qualities.

• It features vertical grains
  
• High profile and reduced flexibility
  
• Relatively rougher, with a roughness average of 0.35 to 0.40 microns
  
• The smooth side is the one that faces the drum
  
• Mainly used to fabricate rigid printed circuit boards
  
• A low surface oxygen rate
  

Rolled Annealed Foil

RA foil manufacturing differs from the ED type. Cu ingots pass through rollers, which press them to a desired thickness. This method produces thinner and smoother foils with better properties than the ED technique.

Because it’s a low-profile copper foil, the RA type is best for flexible boards. Additionally, the reduced roughness makes it suit high-speed PCBs where signal quality is paramount. Here is a summary of its characteristics.

• Horizontal grain structure
  
• A thin profile that offers higher flexibility
  
• Its surface is relatively smooth compared to the electro-deposited foil
  

Key PCB Copper Foil Features

Copper features (and which manufacturers use to determine suitability) include the following: purity, thickness, surface adhesion, and thermal performance.

Purity Level

Purity means the amount of Cu (in percentage) present. Copper used in PCBs typically has a purity of 99.70% or higher. The high purity ensures it performs as expected, enabling reliability and other requirements, such as durability.

Surface Roughness

Roughness measures its surface characteristics. It impacts bonding and peel strength, with rougher foils performing better than smooth types. On the downside, roughness distorts signals, making the foil less efficient for high-speed or high-frequency applications.

Current Carrying Capacity

Thickness determines its current carrying capacity. Thicker types offer reduced resistance and low losses. This property makes them best for power electronics. Thin ones, on the other hand, cannot handle large currents.

Thermal Characteristics

Cu has a high heat conduction rate. This property helps cool the board during operation. PCB manufacturers usually place solid Cu layers near high-heat parts. This design strategy improves thermal performance, saving the board from overheating.

Adhesion Properties

It must strongly and permanently bond to the dielectric substrate. Cu offers this property. The surface roughness determines how well it adheres, with rougher types performing better. Good adherence prevents peeling or delamination, making the PCB last longer.

PCB Copper Foil Thickness and Weight

PCB Cu thickness and weight go hand in hand, with one representing the other. This property determines how well the material performs. We typically measure weight in ounces per square foot or Oz/F².

In other words, we express it based on one ounce Cu flattened to cover a square foot. Below are different Cu weights and their corresponding thickness in mils.

• 1 oz = 1.4 mils
  
• 2 oz = 2.8 mils
  
• 3 oz = 4.2 mils
  
• 4 oz = 5.6 mils
  

PCB Cu weight typically varies from 0.25 oz-5 oz. This range translates to 0.3 mils-7mils. Exact values differ depending on the application, with requirements ranging from flexibility to signal quality and current.

For instance, thicker copper foil types flex less, making them suitable for general-purpose rigid boards. They also handle higher currents and dissipate heat better. However, they distort signals, exhibiting high insertion losses.

Low-profile versions offer opposite qualities and performance characteristics. They have their best uses, such as ultra-thin, flexible boards for compact electronics. The following section explains these uses in more detail.

Typical PCB Copper Foil Applications

As mentioned, the foil forms the traces, component pads, and other layers, such as ground and power planes. It also creates various PCB types, from single and multilayered to high current and flexible.

Single vs. Multilayer PCBs

Single-layer boards have conductive material on one side. This layer produces the traces, connecting components to make the assembly usable. In multilayer circuit boards, the Cu appears on the upper surface, bottom side, and inner layers.

High Current PCBs

Power electronics PCBs have unique requirements. The Cu in these boards is thicker and has a higher current carrying capacity. The thickness allows it to safely withstand large currents, dissipate heat, and other benefits.

Flex PCBs

Flex circuit boards use thin Cu layers to bend without breaking the conductive layers and creating opens. The foil in these applications is an advanced type and usually rolled annealed.

Conclusion

The PCB copper foil has many benefits. It has excellent electrical conductivity, exceptional heat dissipation properties, and other qualities. You can also make it into thin profiles or bond it to various materials. These characteristics make it invaluable in circuit board production.