In modern electronic devices, printed circuit boards (PCBs) serve as both the carrier of electronic components and the core hub for electrical interconnection. PCB performance directly determines the stability and reliability of the entire electronic system. Among the key factors affecting PCB performance, inner-layer copper thickness plays a critical role in determining circuit conductivity.
A deeper understanding of the relationship between copper thickness and electrical performance is highly valuable for PCB design, manufacturing, and electronic product optimization.
Fundamental Principles: How Copper Thickness Impacts Conductivity
Relationship Between Resistance and Copper Thickness
From a physics perspective, conductor resistance is one of the key factors influencing electrical conductivity.
According to the resistance law, conductor resistance (R) is directly proportional to conductor length (L) and inversely proportional to cross-sectional area (S), while also depending on the material resistivity (ρ). The equation is:
In PCBs, copper is the primary conductive material, and its resistivity remains relatively constant. Changes in inner-layer copper thickness effectively alter the conductor’s cross-sectional area. Increasing copper thickness enlarges the cross-sectional area, thereby reducing resistance when conductor length and resistivity remain unchanged.
For example, consider a PCB trace 100 mm long and 0.2 mm wide:
· At an inner-layer copper thickness of 18 μm (0.5 oz), the calculated resistance is approximately 0.478 Ω.
· When copper thickness increases to 35 μm (1 oz), resistance drops to approximately 0.246 Ω.
This demonstrates that thicker copper significantly lowers electrical resistance.
Relationship Between Current-Carrying Capacity and Copper Thickness
Current-carrying capability is another key indicator of circuit conductivity.
According to Joule’s Law:
where:
· Q = heat generated
· I = current
· R = resistance
· t = time
As resistance decreases, a conductor can carry higher current while generating the same amount of heat. Therefore, increasing copper thickness improves current-carrying capacity by reducing resistance.
A practical example can be seen in motor controller PCB design. An initial design using 35 μm copper traces experienced excessive temperature rise during motor startup due to high current loads, repeatedly triggering thermal protection mechanisms.
After upgrading the inner-layer copper thickness to 70 μm (2 oz), trace resistance was significantly reduced. Under identical current conditions, heat generation decreased, lowering line temperature rise from 45°C to 22°C and completely eliminating thermal protection issues while improving overall current capacity.
Impact of Copper Thickness at Different Frequencies
DC and Low-Frequency Applications
In DC or low-frequency circuits (below 1 MHz), current distribution across the conductor remains relatively uniform, meaning the entire conductor cross-section participates in conduction.
Under these conditions, conductivity follows the conventional resistance relationship: thicker copper results in lower resistance, higher current capacity, lower signal attenuation, and improved voltage stability.
For example, in a 1-meter trace carrying 10A current:
· 18 μm copper thickness produces approximately 1V voltage drop.
· 70 μm copper thickness produces only around 0.25V voltage drop.
This highlights how thicker copper significantly improves efficiency in low-frequency, high-current applications.
High-Frequency Applications
When frequencies exceed 1 MHz, the skin effect becomes increasingly important.
The skin effect refers to the tendency of alternating current to concentrate near the conductor surface as frequency rises. Effective conductive thickness is therefore no longer determined by total conductor thickness, but by skin depth (δ):
where:
· ω = 2πf (angular frequency)
· μ = magnetic permeability
· σ = electrical conductivity
For copper:
· Skin depth is approximately 6.6 μm at 100 MHz.
· Skin depth decreases to about 2.1 μm at 1 GHz.
In high-frequency scenarios, excessively thick copper foil does not necessarily improve conductivity.
Because current mainly flows along the surface, only a limited portion of the copper participates in conduction. In addition, thicker copper foil typically has greater surface roughness to improve bonding strength with substrates. However, rougher surfaces increase signal transmission path length, leading to higher signal loss.
For example, at 10 GHz:
· A 0.1 mm-wide trace using 70 μm copper foil shows insertion loss of 1.2 dB/m.
· The same trace using 18 μm copper foil exhibits only 0.8 dB/m loss.
One 5G module PCB design achieved a 15% increase in 10 GHz signal transmission distance after switching from 35 μm to 18 μm ultra-thin copper foil.
Copper Thickness and Conductivity Stability
Temperature Stability
Copper resistivity varies with temperature. In general:
· Higher temperatures increase resistivity.
· Lower temperatures reduce resistivity.
However, different copper thicknesses respond differently to thermal changes.
Thicker copper foil has a more stable crystal structure and is less sensitive to temperature fluctuations.
At -40°C:
· 18 μm copper foil exhibits resistance fluctuation of ±3%.
· 70 μm copper foil fluctuates only ±1.5%.
At elevated temperatures:
· 18 μm copper foil resistance rises 16% at 125°C.
· 70 μm copper foil increases only 12%.
In one automotive PCB application operating in a 120°C engine compartment environment, 18 μm copper traces experienced signal transmission error rates twice as high as those using 70 μm copper. Switching to thicker copper foil resolved the issue completely.
This demonstrates that increased copper thickness improves conductivity stability under extreme temperatures.
Mechanical Stability and Conductivity
Inner-layer copper thickness is also closely linked to PCB mechanical stability, which indirectly affects conductivity.
Adequate copper thickness provides sufficient mechanical strength to prevent PCB bending, cracking, or solder joint failure during operation.
When subjected to mechanical stress, thin copper layers are more prone to cracking, resulting in open circuits or poor electrical contact. Thicker copper layers improve board toughness and resistance to wear, shock, and mechanical stress, maintaining stable electrical connections.
During vibration testing (10–2000 Hz):
· 18 μm copper solder joints increased contact resistance from 5 mΩ to 20 mΩ after 1,000 vibration cycles.
· 70 μm copper solder joints increased only from 5 mΩ to 8 mΩ.
Increasing thick-copper pad size by 0.3 mm beyond component lead dimensions further reduced vibration-induced resistance fluctuations.
Humidity and Thermal Reliability
High-temperature, high-humidity environments accelerate copper oxidation, increasing resistance.
After 1,000 hours in an 85°C / 85% RH environment:
· 18 μm copper resistance increased by 20%.
· 70 μm copper resistance increased by only 10%.
Because the oxide layer represents a smaller percentage of total thickness in thick copper foil, conductivity degradation is less severe.
One outdoor power supply PCB doubled its service life in humid environments after switching from 18 μm to 70 μm copper foil.
Guidelines for Selecting Inner-Layer Copper Thickness
Based on Current Requirements
Current demand should be the primary consideration when selecting PCB copper thickness.
· Standard consumer electronics circuits generally perform well with 1 oz (35 μm) copper.
· High-current applications such as industrial inverters and EV charging systems often require 2 oz (70 μm) or thicker copper.
For example, EV battery management systems experience frequent high-current flow, making thicker copper essential for thermal stability and long-term reliability.
Considering Frequency
For high-frequency applications such as 5G base stations and millimeter-wave radar systems, engineers must balance skin effect and signal loss considerations.
In such applications, 18–35 μm copper thickness is often preferable to minimize high-frequency attenuation while maintaining impedance control and signal integrity.
Environmental Conditions
In harsh environments involving high temperature, humidity, or vibration, thicker copper foil above 70 μm is generally recommended.
Thicker copper provides superior oxidation resistance and mechanical reliability, making it suitable for aerospace and other mission-critical electronics operating under extreme conditions.
Balancing Cost and Performance
Although thicker copper improves conductivity and reliability, it also increases manufacturing cost.
Copper is one of the primary PCB raw materials, and greater thickness significantly raises material consumption. Thick copper processing also requires more complex manufacturing equipment and longer production cycles.
Therefore, PCB designers must carefully balance performance requirements and cost constraints.
For cost-sensitive consumer electronics, thinner copper may be selected provided basic electrical performance requirements are met.
Conclusion
Inner-layer copper thickness has a profound and multifaceted impact on PCB conductivity.
From resistance and current-carrying capability to high-frequency signal transmission and long-term environmental stability, copper thickness remains a decisive factor in PCB electrical performance.
When designing PCBs, engineers should carefully evaluate current demands, operating frequency, environmental conditions, and manufacturing cost to determine the optimal copper thickness and ensure the reliability and performance of electronic products.
Source：NetEase