Trace Resistance and Voltage Drop: Power Integrity Basics
Ensuring your components get the power they need.
Every PCB trace is a resistor. While we like to think of them as perfect conductors, they have finite resistance that causes voltage to drop as current flows through them. In high-power designs or precision analog circuits, even a few millivolts of drop can cause system failure. Understanding and calculating trace resistance is the first step in power integrity design.
The Resistance Formula for PCB Traces
The DC resistance of a trace is calculated as R = ρ * (L / A), where ρ is the resistivity of copper, L is the length, and A is the cross-sectional area (Width * Thickness). At 20°C, the resistivity of annealed copper is approx 1.72 x 10^-8 Ω·m. Our tool automates this calculation, allowing you to quickly see how width and length affect your power rails.
Temperature Coefficient of Resistance
Copper's resistance is not constant; it increases as it gets hotter. The thermal coefficient for copper is approx 0.393% per °C. This means a trace at 100°C has about 30% more resistance than at room temperature. If you don't account for this in your power budget, your voltage drop will be significantly higher than expected during full-load operation.
Power Loss and Heat Generation
Power loss (P = I²R) is energy wasted as heat. This heat contributes to the temperature rise of the board. In high-current paths, this loss can be substantial, leading to thermal stress on the PCB substrate. Reducing resistance by using wider traces or heavier copper is the primary way to manage these losses and improve system efficiency.
Optimizing for Power Delivery (PDN)
A good Power Delivery Network (PDN) design aims to minimize resistance from the power source to the IC. This often involves using 'power planes' (entire layers of copper) instead of thin traces. For traces, keep them as short and wide as possible. Our calculator helps you verify that your chosen trace width keeps the voltage drop within the typical 1-5% tolerance of your components.
FAQ
Why is voltage drop a problem for microchips?
Most ICs have a minimum operating voltage. If the trace resistance causes the voltage to drop below this threshold (e.g., a 3.3V rail dropping to 3.0V), the chip may reset, perform logic errors, or enter a brownout state.
Does switching to silver-plated traces help?
Silver is more conductive than copper, but the benefit is small compared to simply making the copper trace wider or thicker. Plating is usually done for corrosion resistance or skin-effect management at RF, not for DC power delivery.
How do I calculate resistance for a 2 oz copper board?
Simply use our trace resistance calculator and select 2 oz (70µm) as the copper thickness. You'll see the resistance drop by half compared to a standard 1 oz board.