What is through-hole PCB assembly?

How Through-Hole PCB Assembly Works

The through-hole PCB assembly process involves several key steps:

  1. PCB Design and Fabrication
  2. Component Placement
  3. Soldering
  4. Inspection and Testing

PCB Design and Fabrication

The first step in through-hole PCB assembly is designing the circuit board layout using specialized software. The design includes determining the location and size of the through-holes for each component, as well as the placement of copper traces that connect the components electrically.

Once the design is finalized, the PCB is fabricated using a multi-step process that typically includes:

  1. Creating a copper-clad board
  2. Applying a photoresist layer
  3. Exposing the photoresist to UV light through a mask containing the PCB layout
  4. Developing the photoresist to remove the exposed areas
  5. Etching away the unwanted copper
  6. Drilling the through-holes
  7. Applying a protective solder mask and silkscreen layer

Component Placement

After the PCB is fabricated, the electronic components are inserted into the appropriate through-holes. This process can be done manually for low-volume production or prototyping, using a pick-and-place machine for high-volume production, or a combination of both methods.

Components used in through-hole PCB assembly come in various package types, such as:

  • Axial-lead components (e.g., resistors, capacitors, diodes)
  • Radial-lead components (e.g., electrolytic capacitors)
  • DIP (Dual Inline Package) components (e.g., integrated circuits)
  • SIP (Single Inline Package) components (e.g., resistor networks)

Soldering

Once the components are placed, the PCB is ready for soldering. In through-hole PCB assembly, soldering is typically performed using one of two methods:

  1. Wave Soldering
  2. Hand Soldering

Wave Soldering

Wave soldering is an automated process that involves passing the PCB over a molten solder wave. The solder adheres to the component leads and pads, creating electrical connections. This method is efficient for high-volume production and ensures consistent solder joint quality.

Hand Soldering

Hand soldering is a manual process that involves using a soldering iron to apply solder to individual component leads and pads. This method is more time-consuming and requires skilled operators, but it is suitable for low-volume production, prototyping, or rework.

Inspection and Testing

After soldering, the assembled PCBs undergo inspection and testing to ensure the quality and functionality of the final product. Common inspection and testing methods include:

  • Visual inspection
  • Automated optical inspection (AOI)
  • X-ray inspection
  • In-circuit testing (ICT)
  • Functional testing

These steps help identify and correct any defects or issues before the PCBs are shipped to the customer.

Advantages and Disadvantages of Through-Hole PCB Assembly

Through-hole PCB assembly offers several advantages and disadvantages compared to other assembly methods, such as surface-mount technology (SMT).

Advantages

  1. Mechanical Strength: Through-hole components provide stronger mechanical bonds to the PCB, making them more resistant to vibration and physical stress.
  2. Ease of Manual Assembly: Through-hole components are easier to handle and place manually, making this method suitable for low-volume production or prototyping.
  3. Simplified Rework: Replacing or repairing through-hole components is generally easier than SMT components, as they can be desoldered and removed without specialized equipment.
  4. Lower Setup Costs: Through-hole PCB assembly requires less sophisticated equipment and processes compared to SMT, resulting in lower initial setup costs.

Disadvantages

  1. Lower Component Density: Through-hole components occupy more space on the PCB than SMT components, limiting the achievable component density and resulting in larger board sizes.
  2. Increased Drilling Costs: The need for drilled holes in the PCB adds an extra fabrication step and increases the overall manufacturing cost.
  3. Limited High-Frequency Performance: Through-hole components have longer leads, which can introduce unwanted inductance and limit high-frequency performance compared to SMT components.
  4. Slower Assembly Speed: The through-hole assembly process is generally slower than SMT, particularly for high-volume production, due to the need for manual placement and soldering.

Applications of Through-Hole PCB Assembly

Despite the growing popularity of SMT, through-hole PCB assembly remains a viable option for various applications, such as:

  1. High-power electronics: Through-hole components can handle higher power levels and dissipate heat more effectively than SMT components.
  2. Harsh environments: The stronger mechanical bonds provided by through-hole components make them more suitable for applications subject to vibration, shock, or extreme temperatures.
  3. Prototyping and low-volume production: Through-hole assembly is often more cost-effective and efficient for small-scale projects or prototypes.
  4. Legacy designs: Many older electronic designs rely on through-hole components and may not be easily converted to SMT without significant redesign efforts.

Combining Through-Hole and SMT Assembly

In some cases, a combination of through-hole and SMT assembly techniques may be used on a single PCB. This approach, known as mixed-technology assembly or hybrid assembly, allows designers to take advantage of the benefits of both methods while minimizing their drawbacks.

For example, a PCB may use SMT for the majority of its components to achieve high density and performance, while using through-hole assembly for components that require stronger mechanical bonds or are not available in SMT packages.

When combining through-hole and SMT assembly, it is essential to carefully plan the assembly process to ensure compatibility and optimal results. Factors to consider include:

  • Solder paste and solder wave compatibility
  • Reflow and wave soldering temperature profiles
  • Component placement sequence
  • Potential shadowing effects during reflow soldering

Future of Through-Hole PCB Assembly

As electronic devices continue to become smaller, more complex, and more high-performance, the use of through-hole PCB assembly is likely to decline in favor of SMT. However, through-hole assembly will likely remain a valuable technique for specific applications and niche markets where its advantages outweigh its limitations.

Advancements in materials, processes, and equipment may also help to mitigate some of the disadvantages of through-hole assembly, such as:

  • Developing more compact through-hole component packages
  • Improving the efficiency and automation of through-hole assembly processes
  • Enhancing the high-frequency performance of through-hole components

As the electronics industry evolves, it is essential for designers and manufacturers to stay informed about the latest developments in PCB assembly technologies and to select the most appropriate methods for their specific applications and requirements.

Frequently Asked Questions (FAQ)

  1. What is the difference between through-hole and surface-mount PCB assembly?
    Through-hole PCB assembly involves inserting component leads through holes drilled in the board and soldering them to pads on the opposite side, while surface-mount assembly involves placing components directly onto pads on the surface of the board and soldering them in place.

  2. When is through-hole PCB assembly preferred over surface-mount assembly?
    Through-hole assembly is often preferred for applications that require stronger mechanical bonds, higher power handling, or operation in harsh environments. It is also suitable for low-volume production, prototyping, or legacy designs that have not been converted to SMT.

  3. Can through-hole and surface-mount components be used together on the same PCB?
    Yes, a combination of through-hole and surface-mount components can be used on a single PCB, known as mixed-technology or hybrid assembly. This approach allows designers to take advantage of the benefits of both methods while minimizing their drawbacks.

  4. Is through-hole PCB assembly becoming obsolete?
    While the use of through-hole assembly is declining in favor of surface-mount technology, it is not becoming obsolete. Through-hole assembly remains a valuable technique for specific applications and niche markets where its advantages outweigh its limitations.

  5. What are the key steps in the through-hole PCB assembly process?
    The key steps in through-hole PCB assembly include PCB design and fabrication, component placement, soldering (wave or hand soldering), and inspection and testing. Each step plays a crucial role in ensuring the quality and functionality of the final assembled PCB.