A close up of an electronic component on a board

Exploring Electronic Components: THT and SMT on PCBs by Rush PCB UK

Rush PCB UK makes all types of Printed Circuit Boards for hobbyists and various types of industries. Once assembled, these PCBs have numerous components mounted on them. Typically, there are two major types of electronic components on the boards—THT or through-hole components, and SMT or surface mount components.

Through-hole components have long leads that protrude through holes in the board. Soldering their leads on the underside of the board anchors the THT components to the PCB. The excess leads have to be cut off.

Surface mount components do not require holes in the board to mount them. SMT components have short leads that sit on pads on the top surface of the board. A layer of solder paste on these pads helps to anchor the components to the board.

Circuit Board Components

Whether it is THT or SMT, circuit designers use various types of electronic components on PCBs. Major among them are:

Passive Components

  • Resistors
  • Capacitors
    • Electrolytic
    • Non-Electrolytic
  • Inductors
  • Jumpers
  • Connectors

Active Components

  • Integrated Circuits
  • Transistors
  • Diodes
    • Light Emitting Diodes
  • Rectifier blocks
  • Crystal Oscillators

Passive components do not alter the nature of the electrical signal that passes through them. Active components can alter the signal based on control signals passed on to them from other components.


Resistors are passive components presenting a known resistance in the path of the electrical signal. THT resistors usually have a radial or axial body with two leads. The body typically has multiple colors to indicate the value and tolerance of the resistance. SMD resistors are usually small rectangular blocks with a code number written on them indicating their resistance. The physical size of a resistor depends on the power it can safely handle. Higher power resistors are typically larger.


Capacitors are passive components for storing charge. Chiefly, there are two types of capacitors on a board—electrolytic and non-electrolytic. Electrolytic capacitors typically have two leads, with one of the leads marked. Non-electrolytic capacitors do not have any marked leads. There are various types of non-electrolytic capacitors. Both THT and SMT electrolytic capacitors have a metal can with leads on the underside. THT non-electrolytic capacitors have different shapes. SMT non-electrolytic capacitors are box like structures of different sizes.


Inductors are passive components for introducing a known inductance in the path of the electronic signal. Inductors typically have a core with a wire coiled on them. The core may be air or ferrite, depending on the type of the inductor. THT inductors look very much like THT resistors. They can also have color coding on them to indicate their value. SMT inductors are of cubical shapes with two leads.


Jumpers are short pieces of insulated wire for bridging two tracks. Jumpers only provide an electrical continuity from one track to another.


Connectors are passive components for terminating multiple signals to a single point on the board. Connectors can also help to carry power from an external source to the PCB. Typically, connectors have multiple pins and their design enables a mating part to couple with them mechanically. Connectors can be of various shapes and sizes.

Integrated Circuits

Integrated Circuits are active devices that control the flow of signals on the board. Designers use various types of integrated circuits, large and small. Large protruding pins identify THT integrated circuits. In contrast, SMT integrated circuits may be physically large, but their pins are tightly packed close to the body. In some SMT integrated circuits, the pins are hidden below the body.


Transistors are active devices with three or more leads. SMT transistors are physically much smaller than THT transistors are.


Diodes are active devices that primarily allow current flow in one direction while blocking it in the other. Therefore, their leads have polarity markings to denote the direction of current flow. Some THT diodes look like THT resistors, but with a black body. Some diodes are glass encapsulated. SMT diodes are smaller, and they look very much like SMT resistors or capacitors. Various types of diodes are available.

Light Emitting Diodes

Light Emitting Diodes or LEDs are active components that generate light when current passes through them in one direction. THT LEDs have a plastic body with two protruding leads. SMT LEDs have a cubical body with a lens through which the light beams out.

Rectifier Blocks

Rectifier blocks are typically four diodes arranged in a special sequence that helps convert AC waveforms to DC. Both THT and SMT rectifier blocks are cubical structures, with the SMT rectifier blocks being physically much smaller. The body of the rectifier blocks will typically have markings showing the AC and DC connections.

Crystal Oscillators

Crystal oscillators typically generate known waveforms when powered up. Both THT and SMT crystal oscillators typically have a metallic body.


This is a very brief introduction from Rush PCB UK on various types of electronic components that circuit designers use on a printed circuit board and how to identify them.