What Is Through-Hole PCB Assembly? A Visual Decoder

THT is how components are mounted relative to PCBs. The body of the component is typically located on or above one side of the PCB, while its leads or terminals extend through holes drilled into the PCB and connect to pads or lands.

A soldered THT connection has five visible elements:

1. The component body holds the functional part.
2. The lead or terminal comes from the component and passes through a hole in the PCB.
3. The hole cut into the PCB allows for the passage of the lead or terminal.
4. The pad or land is the conductive surface on which the lead or terminal is connected.
5. The solder connection provides electrical conductivity between the lead or terminal and the pad or land.

If the hole in the PCB is plated, a conductive barrel lines the hole and connects to the land; that barrel is not part of every THT assembly. In addition, IPC assembly specifications (IPC-A-610J; IPC-J-STD-001J) highlight the distinction between supported and unsupported THT connections, while NASA’s workmanship documents depict plated-through and non-plated-through connections as separate structures (NASA-STD-8739.3).

When tracing the lead from the assembly to the PCB, mounting type and solder-joint quality are separate; thus, the presence or absence of a solder mound alone does not indicate acceptability of the solder joint.

Through-Hole vs. PTH, Via, and Press-Fit

While THT describes component mounting rather than board material, PTH and NPTH describe hole construction.

Common Through-Hole Components

Through-hole components are often used where the package form, terminal geometry, mechanical loading applied to the board, and how they are handled all favor through-board connections. Common through-hole component forms include axial or radial type devices. Axial components have a lead on either end of the component body (for example, standard resistors and certain types of diodes). Radial components have both leads coming from the same side of the component body. For example, many electrolytic capacitors use a radial lead configuration. DIP and SIP configurations have an arrangement of multiple leads in two rows or one row.

Connectors, switches, relays, transformers, and terminal blocks may use a substantial pin, tab, or a similar configuration to attach through the board. Other types of through-board terminals such as lead frames, posts, and board-mounted hardware may also be used, even though they may not look like an IC or resistor.

These are general descriptions of the through-hole package forms, not how each of them is assembled. For example, the hole geometry, the method of inserting leads, soldering access, mechanical loading, and inspection requirements can vary significantly between a resistor, transformer, and multipin connector.

Where Through-Hole Mounting Adds Practical Value

Connectors and terminal blocks transfer insertion, extraction, cable, or screw-terminal forces into the PCB. The through-board pin can be used for retention; however, mechanical design is still affected by housing alignment, keepouts, chassis interfaces, and the number of mating cycles.

Transformers, relays, large capacitors, and power semiconductors may use leaded packages with larger terminals and greater thermal mass. Since the size and copper connections of these components can affect whether wave soldering, selective soldering, or hand soldering is practical, these characteristics also affect how to attach the component to the PCB.

Although leaded parts are usually more convenient for breadboarding, manual assembly, probing, and replacement when suitable packages are available, this advantage does not make every THT component easy to remove. Plated holes or large copper areas can make rework thermally demanding.

A typical power supply board is an example where two different methods of mounting components end up being side-by-side rather than either one replacing the other: the AC input terminal block and the main filter capacitors are usually through-hole mounted for their retention and thermal mass reasons previously discussed, while the control logic, feedback, and gate-drive ICs surrounding them are surface mount so they may occupy less space and have short lead lengths.

Advantages and Limitations of Through-Hole Assembly

When to Use THT, SMT, or Mixed Assembly

Mixed technology PCBs can use reflowed SMT with either wave, selective, hand, or intrusive reflow methods for THT components. IPC educational material treats these methods as process options selected around component and board constraints rather than as interchangeable methods from which to choose (Through-Hole Assembly Options for Mixed Technology Boards).

Through-Hole PCB Assembly Process

Typically, through-hole PCB assembly flows begin with verifying the components and PCB followed by making or forming leads (if necessary) and inserting components into the PCB without forcing leads into mismatched holes. After inserting the components into their respective holes in the PCB, create a connection between the component and PCB using an acceptable soldering process, clean up the PCB (if applicable), and inspect or test the assembly.

Wave soldering methods permit the simultaneous creation of multiple bottom-side connections. Selective soldering methods restrict the solder contact to specific locations, hand soldering methods allow for localized soldering, and intrusive reflow (pin-in-paste) uses solder paste deposited onto the PCB and reflow to solder compatible THT components (IPC Through-Hole Assembly Syllabus; Indium Corp.).

Inspection can include correct component orientation, lead position, solder wetting, hole fill levels (if applicable), bridging, continuity, cleanliness and functional test. IPC-A-610J and IPC-J-STD-001J address different parts of acceptability and soldered-assembly requirements, with the applicable product criteria providing the basis for acceptance.

Through-Hole Design Checks

You should always use component drawings and your PCB fabricator’s capabilities and limitations rather than assume there is one universal solder-fill or clearance value for all through-hole components.

Common Through-Hole Assembly Defects

The repairability will vary depending on the component package, pin count, copper area, construction of the hole, and the way the board stack-up is configured. Large copper areas and plated-through holes can make heat removal more difficult, so a THT component is not automatically easy to replace.

Conclusion

A lead or a tab that will carry insertion force or cable pull, or has significant thermal mass generally indicates a hole and land combination; conversely, a dense cluster of small, low-load terminals would typically indicate one or more surface-mount pads.