Copper is one of the most important elements in a PCB. Copper foils serve as conductive layers. If you look closer, you can see the copper tracks etched on circuit boards through which signals flow.
Copper-clad laminates, also known as cores, are made of prepregs and copper foils that have been laminated and cured. Plies of prepregs bonded together form a core. This core is bounded with copper foil on either side to form a copper-clad laminate.
Copper is well known for its ductility and electrical conductivity. The metal also offers great tensile strength and better thermal conductivity/expansion properties when compared to aluminum. Below are a few reasons to choose copper as a conductor over aluminum in circuit boards.
- Copper offers significantly lower specific electric resistivity and coefficient of thermal expansion over aluminum.
- It is more resistant to corrosion as it reacts less with water when compared to other metals.
- It is one of the easiest metals to solder.
- It is available in lower cross-sections such as 0.5 mils.
The thickness of the copper on circuits is expressed in terms of ounces. A question arises, why use the unit of weight to specify a thickness? If 1oz (28.35 grams) of copper is flattened to cover 1 square foot of surface area (0.093 square meters), the resulting thickness will be 1.4 mils. Copper weight is measured in ounces per square foot (oz/ft²). The below table shows copper weight versus copper thickness in mils.
|Copper weight||Approximate thickness in mils|
|1 oz||1.4 mils|
|2 oz||2.8 mils|
|3 oz||4.2 mils|
|4 oz||5.6 mils|
According to the IPC-2221 generic standard, circuit board trace current limits can be classified further into internal and external layers/conductors. The graphs given below show the relationship between different variables related to trace width. The variables are trace cross-sectional area, temperature rise, and maximum current carrying capacity for external conductors and internal conductors.
Below is the formula to calculate current carrying capacity of a copper trace.
I = K (ΔT)(0.44) A (0.75)
K = 0.024 for internal conductors and 0.048 for external conductors
ΔT = Maximum temperature difference in °C
A = Cross-sectional area of copper trace in mil²
I = Current carrying capacity in amperes
Electro-deposited copper: This type of copper has a vertical grain structure and a rougher surface. Electro-deposited copper is typically used in rigid PCBs.
Rolled copper: A type of copper, made very thin by processing between heavy rollers, extensively used to produce flexible and high-speed boards. Rolled copper has a horizontal grain structure and a smoother surface which makes them ideal for rigid-flex and flex circuit boards.
Copper foil selection typically depends on required copper thickness, copper purity, and copper-dielectric interface profile.
Copper thickness: Typical thickness varies from 0.25 oz (0.3 mils) to 5 oz (7 mils). The copper thickness varies as per the application. For example, a higher copper thickness is more suitable for high-power applications.
Copper purity: It is the percentage of copper found in a copper foil. Electronic grade copper foil has a purity of around 99.7%.
Copper-dielectric interface profile: Low profile copper has lower signal losses at high frequencies. Hence, it is recommended to choose low profile copper for high-frequency applications.