1. Introduction

In the world of electronics and PCBA, understanding what is resistance is essential. Electrical resistance—often referred to simply as resistance—is the fundamental property that opposes the flow of electric current in circuits. Whether designing for current delivery, signal integrity, or heat management, grasping resistance’s role helps engineers ensure reliability and performance. This article will explain what is resistance, explore its units and measurement, and show how resistance impacts PCB trace design, troubleshooting, and assembly.

2. What Is Electrical Resistance?

2.1 Definition and Basic Concept

Resistance, or electrical resistance, measures the opposition to current flow in a conductor. Analogous to friction, resistance resists movement—in this case, electron flow—and slows current through the circuit.

3. Units and Ohm’s Law

3.1 Units of Electrical Resistance

The SI unit for resistance is the ohm (Ω), named after Georg Simon Ohm.The reciprocal of resistance is conductance, measured in siemens (S).

3.2 Ohm’s Law Fundamentals

Ohm’s Law defines the direct relationship between voltage (V), current (I), and resistance (R):
V = I × R, or equivalently, R = V / I. This equation explains how resistance quantifies voltage needed for a given current or vice versa.

4. Factors Affecting Resistance

4.1 Material and Geometry

Resistance depends on the conductor’s material, length (L), and cross-sectional area (A), expressed by:
R = ρ × (L / A), where ρ (rho) is resistivity.Conductors like copper and silver have low resistivity, while insulators like rubber or glass have high resistivity.

4.2 Temperature Effects

Increasing temperature often raises resistance in metals due to increased atomic vibrations. This factor is critical for trace performance under load or in high-temperature environments.

5. How to Measure Resistance

Resistance measurement is usually done using a multimeter in resistance mode. For circuit diagnosis, measure in de-energized conditions or use voltage and current measurements with Ohm’s Law.Accurate probes and considerations for lead resistance are essential.

6. Resistance in Circuit Theory

6.1 Series vs. Parallel Resistance

    Series circuits: Resistances add (R_total = R₁ + R₂ + …).

    Parallel circuits: Reciprocity formula: 1 / R_total = 1 / R₁ + 1 / R₂ + …

This impacts current sharing and voltage distribution across components.

6.2 Behavior in PCBA

Resistance in PCB traces, vias, connectors, and components can cause voltage drops, power losses, and heat accumulation—especially relevant in high-current or high-density designs.

7. Practical Impact of Resistance in PCB Layout

7.1 Trace Resistance and Thermal Load

PCB traces, acting like resistors, exhibit resistance proportional to length and inversely to cross-sectional area. This influences power dissipation and potential heating.Designers use IPC trace calculators or design software to ensure suitable trace width and copper thickness.

7.2 Signal Integrity and High-Speed Design

While DC resistance is critical for power paths, AC traces require impedance control. Trace resistance can still contribute to signal attenuation, especially in fine or long traces.

8. Examples of Resistance in PCBA Context

    Thickened power traces reduce resistance for current carrying.

    Insulated resistance wire utilized in heating and sensing applications.

    Precision resistors manage voltage and current at component level and are designed to dissipate specific power loads.

9. How Engineers Use Resistance in PCB Design

    Designing for Efficiency: Calculating trace widths and copper weight using resistance formulas and tools ensures minimal unwanted voltage drop.

    Fault Diagnosis: Elevated resistance measurements often indicate poor solder joints, corrosion, or partial open circuits – tools help isolate such issues.

    Material Selection: Opting for materials and geometries with lower resistivity improves energy efficiency and thermal performance.

10. Summary Table