What is Through Hole PCB Assembly? Process, Benefits, and Drawbacks

Through-hole PCB assembly is a durable way of mounting circuit board components. It forms robust connections and suits bulky parts and other needs, such as high-power circuits. Join us as we explore this technology below. We will examine how it works and discuss its good and bad sides. Let’s start with the through-hole meaning.

What is Through Hole PCB Assembly?

Through hole is an assembly method that involves inserting the legs of leaded parts into drilled holes on a PCB. The holes emerge on the other side, which gives the technique its name. After positioning, the leads are manually or automatically soldered on the reverse side.

Assemblers often shorten through-hole assembly to thru-hole and sometimes THT (through-hole technology). It was the only available technique when circuit boards appeared many years ago.

The thru-hole method is still prevalent but used alongside surface mount technology, a newer technique requiring no holes. THT assembly is helpful when assembling circuit boards for the following conditions.

• Harsh environments where the board is prone to vibrations
  
• Applications that subject the PCB to high voltages and currents
  
• When making test PCBs and having to use breadboards
  

Manufacturers use both through-hole and surface-mount assembly methods and components. Let’s see how the two assembly technologies differ and whether one is better.

Difference between SMT and THT

There are several differences between the two techniques. To begin with, THT assembly attaches parts using holes drilled through the board. In surface mount technology, there are no holes—instead, components bond to the upper surface using solder.

Because THT components bond to the PCB using through holes, you can only utilize one side of the PCB board when you assemble them. With SMT, you can use both sides. This option allows PCB makers to design dense circuits.

Another difference between SMT and THT is that, whereas thru-hole assembly can mount heavy parts, SMT cannot. But that also makes THT components take up space, making it unsuitable where space is limited.

SMT has grown in popularity today, especially with the emergence of high-density circuit boards. However, THT remains helpful, such as when ruggedness is paramount or if the part is too heavy for SMT.

Through Hole PCB Assembly Components

The through-hole technique mounts various components, from capacitors to diodes and large-sized parts like transformers. The parts generally fall into radial and axial groups, categorized by their leads.

Radial Lead Components

Radial THT parts have terminals that run through the body. The terminals enter from one side and exit in the other. Radial component examples include resistors, LEDs, some diode and transistor types, and capacitors.

Axial Lead Components

Axial parts have terminals that enter and exit the components from one side. Common examples include ICs, voltage regulators, rectifiers, etc.

Through Hole Technology Process

The thru-hole assembly process involves the following steps: inserting component leads into holes, soldering them, and cleaning the assembled board to remove residuals.

Inserting Components

The process begins with positioning components above the circuit board and inserting their leads into holes. The holes are either plated (PTH) or non-plated (NPTH), and part placement is manual or automated.

• Manual Placement – Manual insertion is more cost-effective in volume runs or when creating prototypes.
  
• Automated Placement – autonomous machines selectively pick and place components on the board. This placement method is faster and less prone to errors.
  

Soldering

After positioning the components and inserting their terminals, soldering can now begin. The solder will mechanically bond the parts to the board. Again, the soldering process is manually executed or automatic.

• Manual Soldering – workers skillfully solder leaded components to the board using hand tools. Because it’s slow and cumbersome, hand soldering is only preferred when prototyping.
  

Manual soldering is also cost-effective and mainly used when assembling low-volume circuit boards or reworking PCBAs.

• Automated Soldering – wave soldering is the most used technique for mass-producing PCBAs. The bottom of the PCB panel gets wetted by a wave of solder.
  

Alternatively, a robotic arm selectively applies molten solder. This soldering method is best when using SMT and THT components or if reworking boards.

Cleaning

After soldering, the circuit boards need cleaning. Cleaning removes flux residuals, solder balls, and other contaminants like dirt and grime.

Several cleaning methods are available, such as using deionized water mixed with various cleaning agents or solvents like isopropyl alcohol or acetone.

Cleaning may involve immersing the boards in solvent cleaners or aerosol sprays used to direct the cleaner to the board. Modern methods use ultrasonic waves to agitate stuck debris, making cleaning more effective.

What are the Advantages of Through Hole PCB Assembly?

Through-hole PCB assembly is preferred when installing heavy components like relays, electrolytic capacitors, and connectors. Manufacturers also use it when creating prototypes, given its ease of use. It offers several, which include those explained below.

Mechanical Strength

It establishes sturdy physical connections and can survive harsh operating conditions. This benefit makes THT PCB assembly suit military and industrial circuit boards. They resist damage, which contributes to the durability of the circuit board.

Parts Reliability and Performance

Components assembled using THT make sound electrical connections. They perform better and can handle larger voltage or current levels. They also dissipate heat more efficiently, making them best for military or aerospace applications.

Repair and Testing Ease

You can quickly test or swap THT components compared to SMD devices. This feature is crucial when reworking boards or during prototyping when you mount components on breadboards.

Through Hole PCB Assembly Drawbacks

The through-hole assembly method has a few drawbacks when used to populate printed circuit boards. Apart from requiring holes and prolonging the PCB fabrication process, it has these disadvantages.

Space Constraints

The size of THT components eats up circuit board estate, making dense circuits challenging to design. Additionally, you can only use them on one side of the board, reducing available space.

Placement Speed

Whether manual or automated, thru-hole assembly is generally slower than SMT; it also requires holes to be drilled on the board, which can increase the PCBA production time and cost. To avoid that, manufacturers only use it when SMT is not viable.

Soldering Problems

Using THT can cause soldering issues such as bridging and tombstoning. Careful control helps avoid these problems. Designers must ensure proper pad spacing. Implementing practical board inspection and testing procedures is also crucial during assembly.

Conclusion

Through-hole PCB assembly involves inserting component leads into through holes. Despite being an earlier assembly method, it remains relevant today. Manufacturers use it when mounting bulky components, producing PCBAs for harsh application conditions, or producing test versions.