THT Mounting: The Only Guide You Need

What is THT Mounting?

THT mounting, which stands for “through-hole technology” mounting, is a method of attaching electronic components to a printed circuit board (PCB). In THT mounting, the component leads are inserted through holes drilled in the PCB and soldered to pads on the opposite side.

THT mounting was the dominant method of PCB assembly for decades until the rise of surface mount technology (SMT) in the 1980s. Despite the widespread adoption of SMT, THT mounting is still used today for certain types of components and applications.

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Advantages of THT Mounting

THT mounting offers several advantages over other PCB assembly methods:

Mechanical Strength

One of the main benefits of THT mounting is the mechanical strength it provides. Because the component leads are inserted through holes in the PCB and soldered in place, THT-mounted components are held very securely. This makes THT mounting ideal for applications where the PCB may be subject to vibration, impact, or other mechanical stresses.

Ease of Manual Assembly

Another advantage of THT mounting is that it is relatively easy to perform manually. Unlike SMT, which requires specialized equipment and a steady hand to place tiny components, THT components can be inserted and soldered with just a soldering iron and some basic tools. This makes THT mounting a good choice for low-volume production, prototyping, or hobbyist projects.

Compatibility with Large Components

THT mounting is also well-suited for large components that would be difficult or impossible to mount using SMT. Examples include:

Power transformers
Large capacitors
Connectors
Heat sinks

These components often have leads that are too large or too widely spaced to be compatible with SMT pads.

Disadvantages of THT Mounting

Despite its advantages, THT mounting also has some notable drawbacks compared to SMT:

Lower Component Density

Because THT components require holes to be drilled in the PCB, they take up more space than SMT components of equivalent functionality. This limits the component density that can be achieved with THT mounting, which can be a problem in applications where PCB space is at a premium.

Reduced Automation

THT mounting is also less amenable to automation than SMT. While THT components can be inserted and soldered by machine, the process is generally slower and less efficient than pick-and-place machines used for SMT assembly. This makes THT mounting less suitable for high-volume production.

Higher Assembly Costs

The reduced automation and lower component density of THT mounting generally result in higher assembly costs compared to SMT. This is particularly true for large production runs, where the efficiency gains of SMT can add up to significant cost savings.

THT Mounting Process

The THT mounting process consists of several steps:

PCB Preparation: Holes are drilled in the PCB to accommodate the component leads. The holes must be the correct size and spacing for the specific components being used.
Component Insertion: The component leads are inserted through the holes in the PCB. This can be done manually or with the aid of an insertion machine.
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 machine, which applies molten solder to the entire board at once.
Inspection and Testing: After soldering, the PCB is inspected for defects such as bridged or missing solder joints. Electrical tests may also be performed to verify the functionality of the assembled board.

Types of THT Components

THT components come in a wide variety of packages and sizes. Some common types include:

Type	Description	Typical Uses
Axial Lead	Cylindrical body with leads extending from each end	Resistors, capacitors, diodes
Radial Lead	Cylindrical body with both leads extending from the same end	Electrolytic capacitors, LEDs
DIP (Dual In-line Package)	Rectangular package with two parallel rows of leads	ICs, sockets, connectors
TO (Transistor Outline)	Metal can package with leads extending from the base	Power transistors, voltage regulators

THT Soldering Techniques

Proper soldering technique is critical for creating reliable THT solder joints. Some key considerations include:

Soldering Iron Temperature

The soldering iron temperature should be appropriate for the type of solder being used. For lead-based solder, a temperature of around 650-750°F (343-399°C) is typical. Lead-free solder may require higher temperatures.

Solder Type

The solder used for THT mounting should have the appropriate composition and diameter for the application. A common choice is 63/37 tin-lead solder in a diameter of 0.031-0.062 inches (0.8-1.6 mm).

Flux

Flux is a chemical agent that helps the solder flow and bond to the component leads and PCB pads. It is important to use the correct type of flux for the application and to clean any flux residue after soldering.

Joint Inspection

After soldering, each joint should be visually inspected to ensure that it is properly formed and free of defects. A good THT solder joint should have a concave fillet that extends from the component lead to the PCB pad, with no gaps or excess solder.

FAQ

What is the difference between THT and SMT mounting?

THT mounting involves inserting component leads through holes in the PCB and soldering them to pads on the opposite side. SMT mounting involves placing components directly onto pads on the surface of the PCB and soldering them in place. SMT allows for higher component density and greater automation, while THT provides stronger mechanical bonds and is easier to do manually.

Can THT and SMT components be used on the same PCB?

Yes, it is possible to use both THT and SMT components on the same PCB. This is known as a “mixed technology” or “hybrid” assembly. However, it does require careful design to ensure that the placement of THT components does not interfere with the SMT assembly process.

What is the best way to remove a THT component?

To remove a THT component, first heat up the solder joints with a soldering iron to melt the solder. Then, use a pair of pliers or a desoldering tool to gently pull the component leads out of the holes. Be careful not to apply too much force, as this can damage the PCB. Once the component is removed, use desoldering braid or a solder sucker to remove any excess solder from the holes.

Can THT components be reused?

In general, THT components can be reused if they are removed carefully and the leads are still in good condition. However, some components, such as electrolytic capacitors, may degrade over time and should not be reused. It is also important to consider the cost and time involved in removing and reinstalling components versus simply using new ones.

What are some common defects in THT soldering?

Some common defects in THT soldering include:

Bridging: Solder connecting adjacent joints or pins
Cold joints: Dull, grainy solder joints that result from insufficient heat
Insufficient wetting: Solder not adhering properly to the component lead or PCB pad
Excess solder: Too much solder on the joint, which can cause short circuits
Tombstoning: One end of a component lifting off the PCB due to uneven heating

Careful technique and visual inspection can help prevent and identify these defects.

Conclusion

THT mounting may not be as popular as it once was, but it remains an important technique for PCB assembly. Its main advantages – mechanical strength, ease of manual assembly, and compatibility with large components – make it well-suited for certain applications. By understanding the THT mounting process, component types, and soldering techniques, you can ensure reliable and high-quality THT assemblies. While SMT may be the dominant method today, THT still has a place in the world of electronics manufacturing.