The Ultimate Guide to Diodes on Circuit Boards

Diodes are fundamental semiconductors devices used in almost all electronic devices to achieve various functions, such as: rectification, switching, signal steering, and protection. In the gadgets you use in your daily life, such as smartphones, TVs, laptops, etc., diodes underpin the key functionalities ensuring these devices are working as designed; diode failure in these devices may mean that the gadget won’t start, or work appropriately. 

Understanding the placement details of diodes on circuit boards is crucial because diodes are unidirectional components that allow current to flow in one direction and if they are placed incorrectly on PCBs, they may fail to perform as designed, potentially leading to circuit failure. This is especially important in applications that depend on controlled current flow, such as rectifiers, voltage regulators, and protection circuits. Ensuring accurate diode placement is essential for the proper functioning and reliability of electronic circuits.

In this comprehensive guide, I will explain some of the common diode types, the functional, electrical and thermal layout principles for diodes on circuit boards, and the manufacturability choices as well as serviceability practices for PCB designers and embedded engineers.

What is a Diode?

A diode is a two terminal electrical component that allow current to flow in one direction only.

The current can flow from the anode to cathode, but not from the cathode to anode. It is analogous to check valves that allow fluid flow in one direction only.

The p-n junction diodes are formed from the junction of two dissimilar semiconductor materials i.e. the p-type and n-type semiconductors.

How a Diode Operates

Forward Biasing of a Diode

When a diode is connected to a battery as shown in the figure below, electrons from the n-side and holes from the p-side are forced toward the center (i.e. the p-n interface) by the electrical field supplied by the battery. The electrons and holes combine, and current passes through the diode. A diode connected in this way, is said to be forward biased.

Reverse Biasing of a Diode

When a diode is connected to a battery as shown in Figure 3 below, holes in the n-side are forced left, while electrons in the p-side are forced to the right. This results in an empty zone around the p-n junction that is free of charge carriers, referred to as the depletion region. This depletion region has an insulation property that prevents current from flowing through the diode. A diode connected this way, in a circuit is said to be reverse-biased.

Note that, it takes a minimal voltage to turn a diode ON when it is placed in forward biased condition. For silicon diodes, it is usually ≈0.6 – 0.7 V, if you have less than this voltage rates, the diode will not conduct.

Related: Semiconductor Diodes

Common Diode Types and Characteristics

Different types of diodes are used for wide range of applications. Let’s look at some of the common diode types, their basic characteristics and applications:

Rectifier Diodes

These are diodes that are designed for high continuous current and high reverse voltage; they have a moderate forward voltage drop (V_f) for silicon diodes.

Rectifier diodes are a vital component in power supplies and battery chargers where they are used to convert AC mains i.e. line voltage to DC.

Signal (Small-Signal) Diodes

These are low-current, low capacitance diodes, which are used in switching and detection circuits.

Signal diodes are used in processing of signals in electronic equipment; they are used to acquire the audio and video signals from transmitted radio frequency signals i.e. demodulation and can also be applied to shape and modify AC signal waveforms e.g. clipping, limiting and dc restoration.

Zener Diodes

A zener diode behaves like a two-way gate to current flow. In the forward direction, it is easy to push open; you need only 0.6 V just like a standard diode. But in the reverse direction, it is harder to push open, it requires a voltage equal to the zener breakdown voltage V_z. This breakdown voltage can range from 1.8 to 200 V, depending on the model and the power ratings can range from around 0.25 to 50 W.

Zener diodes are designed to operate in controlled breakdown (reverse) region to provide a reference or regulation voltage; specified V_Z and dynamic resistance.

Zener diodes applications include:

• Used as voltage references.
• Used as simple shunt regulators.
• Employed in waveform clipping, etc.

Transient Voltage Suppression (TVS) Diodes

TVS diodes are designed to clamp high-energy transients and electrostatic discharge (ESD) with fast response and high peak power absorption.

Typical applications of TVS diodes include: Surge protection in power rails and data/communication lines.

Related: How to Properly Install TVS Diodes in Your PCB Design

Varactor (Varicap) Diodes

A variable capacitance diode is known as a Varicap diode or a varactor.

If a diode is reverse biased, an insulating depletion region forms between the two semi-conductive layers. In most diodes the width of the depletion region may be changed by varying the reverse bias. This varies the capacitance.  This effect is heightened in Varicap diodes.

Typical applications of Varicap diodes include: tunable filters, frequency modulation circuits, VCOs, and so on.

PIN Diodes

This is a fast low capacitance switching diode. It is fabricated in similar way to a silicon switching diode but with an intrinsic region added between the p-n junction layers. This produces a thicker depletion region. As a result, there is lower capacitance than a reverse biased switching diode.

PIN diodes applications include:

• They are used in place of switching diodes in radio frequency (RF) applications.
• Used in attenuators and high-speed photodetectors.  

Diode Functional Placement Principles in a PCB Layout

Proximity

Place diodes as close as possible to the node they serve. Ensure that:

• Protection diodes such as transient voltage suppressors (TVS) are placed within 1 – 2 mm of connector pins or IC pins for fastest transient clamping. Key point to keep in mind, place the TVS diodes closer to the point where the electrostatic discharge (ESD) is likely to happen.
•  Place signal diodes close to the driver/receiver ICs to reduce trace parasitics.

Orientation

Where feasible, you should standardize diode polarity across the board; this will simplify inspection and assembly work.

You should mark cathode clearly on silkscreen and component outline.

Grouping

Cluster the diodes by function such as rectification group, clamp group, indicator LEDs, and so forth, this will help you simplify routing and thermal planning making your work much easier.

Loop Area

For switching applications or high di/dt currents, reduce the current loop area between the source, diode, and the return to minimize Electromagnetic Interference (EMI) and voltage overshoot. Large current loop area can make the circuit prone to electromagnetic coupling and interference, which may interfere with the operation of the device or other devices.

Diode Electrical Layout Guidelines for PCBs

Trace Sizing and Copper Area

For power diodes, use wide, short traces or copper pours sized to carry the expected current. You may consider thicker copper with the appropriate sizes where necessary.

For signal diodes, the standard signal traces are usually sufficient but ensure you keep them short.

Related: What is Trace Length Matching in PCB Design?

Parasitic Inductance and Resistance

Ensure that the traces to diodes are kept short and use broad traces to reduce series inductance.

Reduce the lead length for through-hole diodes or avoid them in high-speed paths.

Why Use Short traces?

Short traces on PCBs reduce unwanted electrical and electromagnetic effects and enhance reliability. Key reasons for their use include:

• Lower parasitic inductance and resistance – shorter conductors have less series inductance and resistance, reducing the voltage drop and energy stored in switching loops.
• Faster, cleaner signals – less trace length, reduces signal decay, rise/fall-time degradation, and reflections especially when trace length << signal strength.
• Reduced electromagnetic interference (EMI)/radiated emissions – the short loops and traces emit less radiated energy; the reduced loop area lowers magnetic coupling and interference.
• Enhanced signal integrity – the shorter routes reduce opportunities for crosstalk, ringing, and impedance discontinuities. It is easier to meet controlled-impedance requirements.
• Improved power delivery – the short paths between the supply, decoupling capacitors and IC pins lower the loop impedance, enhancing transient response and reducing ground bounce.
• Superior transient protection – protection components such as transient voltage suppressors (TVS), clamp diodes work more effectively when they are placed with minimal trace length to the protected node and return.
• Thermal and reliability benefits – shorter, wider copper for power paths lower heating and current crowding; the fewer vias and traces reduce the failure points.

Generally a reliable technique is to keep critical traces i.e. switch nodes, decoupling loops, protection paths, high-speed networks) as short as practically possible. You should place components close together and route returns directly under the trace.

Return Paths

You should provide a low-impedance, direct return from the diode cathode/anode to the supply or ground plane. For transient voltage suppressors (TVS) diodes, place the ground vias immediately next to the diode’s ground pad.

Decoupling and Filtering

If diode function relates to switching nodes, place the decoupling capacitors adjacent to the diode and the switching device and keep the interconnections short.  

Thermal Management for Diodes on PCBs

Copper Pads and Thermal Vias

Copper pads can be described as exposed areas of copper on a PCB where components are normally soldered or where vias terminate. They serve electrical, mechanical and thermal roles on a PCB while vias on the other hand, are plated holes that connect copper layers in a PCB.

In terms of thermal management as far as copper pads and thermal vias are concerned, consider the following factors:

• For power diodes, increase the copper pad area and use thermal vias to internal ground or power planes to spread heat.
• Typical practice is to consider several 0.3 – 0.5 mm vias in the pad area connected to an internal plane; follow the manufacturer thermal vias recommendations.

Heat Sinking and Airflow

When it comes to heat sinking and airflow, consider the following:

• Orient the diodes so heat-dissipating surfaces face airflow or heat sink mounting holes.
• Leave clearance for heat-sinks or thermal straps.

Thermal Isolation

Thermal isolation isolates a PCB component thermally from nearby copper, other components, or the board so it runs cooler, or to prevent heat spreading.

Key factors to consider for thermal isolation:

• Ensure hot components such as power regulators, high-current MOSFETs, LEDs, etc., are thermally kept separate.
• Keep heat-sensitive components such as precision ADCs, crystals, microcontrollers, away from high-dissipation diodes.

General Design Rules and Trade-offs for Thermal Isolation

• Wider copper clearances increase thermal isolation but may harm electromagnetic compatibility (EMC) and return paths; ensure you balance isolation with signal integrity.
• Reducing vias enhances isolation however it may reduce mechanical strength and plane connectivity.
• Narrow thermal necks increase electrical resistance, ensure power delivery remains the same.
• Removing copper may create hot spots elsewhere; employ thermal simulation for your critical designs.
• Manufacturing limits – the fabricator may have the minimum copper area or spacing constraints on leaving large cutouts; check DFM.

Derating and Placement

Allow additional copper near the diode for higher currents and test with worst-case thermal simulations or measurements.

Protection and Transient Suppression

Protection and transient suppression refers to safeguarding sensitive electronic components from sudden voltage spikes (transients) like electrostatic discharge (ESD), lighting surges, or switching noise, using specialized components such as transient voltage suppressor (TVS), diodes, varistors, and proper grounding techniques. These measures help prevent circuit damage, enhance reliability and extend the device lifespan.

Design considerations for Diodes used as Protection devices:

TVS/ESD Diode Placement

• Generally protection devices must be placed close to connectors or entry points where transient occur.
• Place directly at the connector or component pin with the shortest possible trace to the protected node; ground return must be immediate and short.
• Consider unidirectional TVS for dc circuits, and bidirectional TVS for AC or differential signals, offering protection in both polarities.

Clamp Diode Strategies

For inductive loads, put flyback/freewheeling diode across coils as near to the coil driver as possible; for power rails, use Schottky diodes for fast conduction and lower V_F.

Energy and Capacitance Tradeoffs

Note that higher-energy TVS diodes, more often than not, have larger capacitance; avoid them on high-speed data lines, unless specified low-capacitance parts are used.

Connector Protection Patterns

For multi-pin connectors, consider having per-pin diodes plus a common reference clamp to centralize surge energy into a robust ground/plane.

Mechanical, Assembly and Manufacturability

SMD vs. Through-Hole Diodes

Choose SMD for automated assembly and for lower parasitics. Through-hole diodes may be used for very high-current or ruggedized designs where mechanical strength is necessary.

Generally SMD diodes are best for miniaturized circuits (phones, laptops, IoT devices). They enable high-density PCB layouts.

Through-hole diodes are typically preferred in prototyping and DIY projects because they are easier to handle and require manual soldering. They are common in high-power circuits where heat dissipation and mechanical strength are critical. They are suitable for harsh environments. Applications include automotive, industrial equipment, military, aerospace, etc.

Trade-offs and Risks

SMD limitations

SMD diodes are fragile under mechanical stress, and are harder to replace manually, additionally, they have less heat dissipation.

Through-Hole Limitations

Have larger footprint, slower assembly, and higher cost in mass production.

Design Risk

Choosing the wrong package can lead to overheating, poor reliability, or inefficient assembly. Designers and engineers must balance thermal performance, mechanical strength, and board density when selecting the diode types.

Electromagnetic Interference (EMI)/Electromagnetic Compatibility (EMC) Considerations

Loop Area Reduction

Route the diode input and return paths adjacent to each other or over the same plane to minimize loop antenna area.

Snubbers and RC Networks

Place the snubber networks as close as possible to the switching device and diode to minimize dv/dt and ringing.

Grounding Strategy

Utilize solid ground planes for high-frequency return currents; ensure TVS clamps the route surge energy to a robust chassis or a ground plane.

Shielding and Separation

Keep the noisy switching diode circuits away from sensitive analog traces; you may consider guarded traces or ground pours between the domains.

Testability and Serviceability for Diodes on Circuit Boards

Probe Access

Place test points or expose pads near diodes that are likely to be the measured when debugging.

Replaceability

The serviceable diodes (such as reverse-protection diodes, indicator LEDs, etc.), should be placed near board edges or connectors for easier replacement.

In-Circuit Testing

Ensure the diode orientation and labeling support automated test fixtures; provide the test points for applying test currents and voltages.

Boundary Scan and Diagnostics                

For interface protection diodes, ensure the diagnostic paths exist so that clamps don’t obscure failure modes during testing.

Conclusion

To sum up, an appropriate diode placement balances the electrical performance, thermal management, manufacturability, and serviceability. Prioritize proximity to the protected node, minimize the loop area for switching currents, and provide adequate copper and vias for heat dissipation on power parts, and mark the polarity clearly for reliable assembly and testing. When you consider these factors, you will avoid common errors in assembly and ensure you build circuits that are reliable and can function well in the field without issues.

You may also read:

• The Manufacturing Process Behind 6 Layers PCB Board
• What are Printed circuit boards (PCBs)?
• What Is the Aluminium Layer of a PCB?