SMD vs SMT vs PTH: Ultimate Guide to PCB Assembly Terms

What is Surface Mount Technology (SMT)?

Surface Mount Technology (SMT) is a method of manufacturing electronic circuits in which the components are mounted directly onto the surface of a printed circuit board (PCB). SMT has largely replaced the through-hole technology construction method of fitting components with wire leads into holes in the circuit board.

SMT components are usually smaller than their through-hole counterparts. It’s become possible to fit very tiny components onto circuit boards, allowing manufacturers to fit more components in a smaller PCB area.

The main advantages of SMT over the older through-hole technique are:

Smaller components
Much higher number of components and many more connections per component
Fewer holes need to be drilled through multilayer circuit boards
Simpler and faster automated assembly
Solder can be applied more precisely
Lower resistance and inductance at the connection
Better mechanical performance under shake and vibration conditions
SMT parts can be placed on both sides of the circuit board

How SMT Works

The components are placed on pads or lands on the surfaces of the PCB. In most cases the components are soldered to the pads using a reflow soldering process.

The key steps in SMT are:

Solder Paste Application: A thin layer of solder paste is applied to the PCB pads where the SMT components will be placed. This is typically done using a solder paste printing process or by dispensing paste with a needle.
Component Placement: The SMT components are placed onto their respective pads with the solder paste. High-speed pick-and-place machines are used to place the components with great accuracy.
Reflow Soldering: After component placement, the PCB goes through a reflow oven. The solder paste is melted in a controlled way, creating permanent solder joints between component leads and PCB pads as it cools and solidifies.
Inspection: The soldered PCB is inspected to ensure all components are properly placed and soldered. Automated optical inspection (AOI) systems are commonly used.

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What is Surface Mount Device (SMD)?

Surface Mount Devices (SMD) are the electronic components that are designed specifically for use in Surface Mount Technology (SMT) assembly. SMDs are mounted directly onto the surface of a printed circuit board (PCB).

SMDs come in a variety of package types, but they all share one key characteristic: they have small metal tabs or end caps that can be soldered directly to the PCB surface, instead of wire leads that go through holes in the board.

Common SMD Packages

There are many types of SMD packages. Here are some of the most common:

Resistors and Capacitors: These usually come in small rectangular ceramic chip packages. Common sizes are 0201, 0402, 0603, 0805, 1206 (these numbers represent the size in inches, e.g., 0603 is 0.06″ x 0.03″).
Transistors and Diodes: These often use SOT (Small Outline Transistor) packages like SOT-23, SOT-223, etc.
Integrated Circuits: ICs use a wide variety of packages including QFP (Quad Flat Pack), QFN (Quad Flat No-leads), BGA (Ball Grid Array), and many others. The package chosen depends on the number of pins needed and the device’s power dissipation requirements.
LEDs: LEDs for SMT often come in small chip packages like 0603, 0805, etc.

Advantages of SMDs

The main advantages of SMDs over through-hole components are:

Smaller size allows for denser PCB designs
Automated assembly is faster and more efficient
Components can be placed on both sides of the PCB
Lower lead inductance and resistance improves high-frequency performance
Mechanically more robust under vibration and shock

What is Through-Hole Technology (THT)?

Through-Hole Technology (THT), also known as Plated Through-Hole (PTH), is a method of mounting electronic components on PCBs. In THT, component leads are inserted through holes drilled in the PCB and soldered to pads on the opposite side.

THT was the primary method of PCB assembly until Surface Mount Technology largely replaced it in the 1980s and 1990s. However, THT is still used for certain types of components, especially those that require high mechanical strength or are subject to high power dissipation.

How THT Works

The key steps in THT assembly are:

Drilling: Holes are drilled into the PCB where the THT component leads will be inserted.
Component Insertion: The component leads are inserted through the drilled holes. This can be done manually for small production runs or by machines for larger volumes.
Soldering: The component leads are soldered to the PCB pads on the opposite side of the board. This is typically done using a wave soldering process, where the bottom side of the PCB is passed over a wave of molten solder.
Inspection: As with SMT, the soldered PCB is inspected to ensure proper soldering and component placement.

Advantages of THT

While SMT has largely replaced THT for most PCB assembly, THT still has some advantages:

Mechanically stronger solder joints, important for components subject to physical stress
Easier to swap or replace individual components for repairs
Some components, like large power transistors or connectors, are only available in THT packages
Can handle higher power dissipation than SMT

SMD vs SMT vs PTH: Key Differences

Here’s a comparison table summarizing the key differences between SMD, SMT, and PTH:

Characteristic	SMD	SMT	PTH
Full Name	Surface Mount Device	Surface Mount Technology	Plated Through-Hole
Definition	Components designed for SMT assembly	PCB assembly method where components are mounted on the surface	PCB assembly method where component leads go through holes
Component Sizes	Very small (down to 01005 package)	N/A (uses SMD components)	Larger than SMD
Solder Application	N/A (solder paste is used)	Solder paste printing or dispensing	Wave soldering
Component Placement	N/A (pick-and-place machines are used)	Automated pick-and-place machines	Manual or automated insertion
Typical Use Case	N/A (SMDs are components, not an assembly method)	High volume, dense PCB designs	High power, mechanical strength, or when using THT components
PCB Density	N/A (enables high density)	High density possible	Lower density than SMT
Assembly Speed	N/A (enables fast assembly)	Fast automated assembly	Slower than SMT
Mechanical Strength	Good	Good	Excellent
Repair/Rework	Difficult	Difficult	Easier than SMT

FAQ

1. Can SMD components be used in THT assembly?

No, SMD components are designed specifically for SMT assembly and do not have the leads necessary for THT assembly. However, THT components can be used in SMT assembly with the use of adapter PCBs.

2. Is SMT cheaper than THT?

For high volume production, SMT is usually cheaper than THT due to the faster and more automated assembly process. However, for low volumes or designs requiring THT components, THT can be more cost-effective.

3. What is the smallest SMD component?

The smallest standard SMD package is 01005, which measures 0.4mm x 0.2mm. Even smaller packages like 008004 exist but are not widely used.

4. Do I need to use solder paste for THT assembly?

No, solder paste is used for SMT assembly. For THT, the components are typically soldered using a wave soldering process.

5. Can I mix SMT and THT components on the same PCB?

Yes, it’s common to have a mix of SMT and THT components on the same PCB. The SMT components are typically placed and soldered first, followed by the THT components.

In this evolving world of electronics, understanding the differences between SMD, SMT, and PTH is crucial for anyone involved in PCB design or assembly. While SMT has become the dominant assembly method, THT still has its place, and knowing when to use each can be the key to a successful PCB design.