Why THT Assembly Still Matters in an SMT World

Everyone in electronics knows the big story: assemblies are getting smaller, denser, faster, and more automated. Surface-mount technology made that possible. IPC assembly guidance points to SMT’s major advantages, including higher circuit density, improved electrical performance, lower process cost, and strong manufacturing efficiency for modern products.¹ For most logic-heavy electronics, SMT is the default choice for very good reasons.

But that does not mean through-hole technology disappeared. It means the job changed.

In 2026, THT still matters anywhere the assembly has to survive more than a clean reflow profile and a shipping box. Connector-heavy boards, cable-loaded interfaces, high-current power entry, operator-mated modules, industrial controls, defense electronics, and long-life service hardware still put different stresses on a board than fine-pitch digital logic does. That is why mixed-technology boards remain common in real products even though SMT dominates the conversation.²

SMT Won for Good Reasons

Surface-mount technology changed electronics manufacturing because it solved real problems. It allowed manufacturers to place components on both sides of a board, reduce lead length, automate placement at scale, and build far denser assemblies than traditional leaded component methods could support.

That matters for modern electronics. Smartphones, compact control modules, high-speed computing systems, RF products, and dense digital boards depend heavily on SMT. In those applications, the physical size of the component, the electrical path length, and the economics of high-volume automated placement are all major design drivers.

The point is not that SMT is weak or less reliable. In the right application, SMT is highly reliable and often the most practical assembly method. The point is that SMT is not the answer to every mechanical, electrical, and lifecycle requirement on a board.

A Brief History: THT Is Older, But Both Technologies Are Mature

Through-hole assembly grew out of the early printed circuit board era. By the early 1950s, printed circuit production was already being described as a way to replace traditional wiring with copper circuit paths and automated soldering. A 1963 patent for direct electroplating of through holes further shows how plated-through-hole construction became a key part of PCB manufacturing and component attachment.³⁴

Surface-mount technology has its own long history. SMT concepts are commonly traced to IBM’s 1960s “planar mounting” and hybrid packaging work, including the broader technology shift represented by IBM’s System/360 era.⁵ By the mid-1980s, SMT had become important enough that industry groups such as EIA, IPC, and SMTA formed the Surface Mount Council to help standardize and mature the technology.⁶

The takeaway is simple: THT is older than SMT, but both are mature technologies. SMT became the dominant answer for dense, automated electronics. THT remained the right answer for certain mechanical, electrical, and lifecycle problems.

Why Connectors and Harness Interfaces Still Favor THT

The most important reason THT still matters is mechanical loading.

A small SMT resistor tucked inside a protected assembly is solving a very different problem than a board-edge connector that has to survive repeated mating, cable movement, shock, vibration, and operator handling. Harwin notes that applications exposed to vibration, shock, or tension may require the added mechanical stability of a terminal passing through the PCB.⁷ IPC mixed-technology guidance makes the same point from a manufacturing perspective: while SMT is usually preferred for cost and density, mechanical strength keeps through-hole relevant for many connector applications.²

This is especially important when the PCB is part of a larger electromechanical system. Many assemblies are not just “boards.” They are interfaces between electronics, cables, harnesses, sensors, actuators, power systems, enclosures, and operators. In those situations, the connector is often one of the most physically stressed parts of the assembly.

That is where THT remains valuable. A through-hole connector is not only soldered to a pad on the surface of the board. Its leads pass through the board, giving the solder joint and plated barrel a different mechanical structure than a surface-only attachment. In rugged applications, that difference can matter.

Reliability Depends on the Application, Not the Label

Reliability discussions around THT and SMT can get oversimplified. It is not accurate to say “THT is always reliable” or “SMT is always fragile.” The real question is whether the attachment method fits the part, the board, and the environment.

IPC research on SAC305 through-hole solder joints found that joint strength correlated strongly with pin wetted length, not just nominal barrel-fill percentage. In the same study, random vibration caused the largest observed reduction in residual strength for low-wetted-length joints, while the authors still noted that residual through-hole strengths were generally much higher than surface-mount joints in that specific test context.⁸

A related IPC/Flextronics finite element analysis study found that many plated-through-hole configurations modeled beyond 10,000 accelerated thermal cycles without solder-joint failure once wetted length moved above very low values.⁹

The lesson is not that every THT joint is automatically rugged. The lesson is that good THT process control can create large reliability margins where the application needs them most.

Power Components and Larger Parts Still Have a Place

Current and power are another major reason THT remains relevant. Many real-world power and wire-to-board solutions still favor through-hole geometry because it combines electrical capacity with stronger board attachment and better resistance to user-induced mechanical stress.

Phoenix Contact positions PCB connectors for signal, data, and power transmission with current ratings up to 125 A in certain product families.¹⁰ Harwin’s high-reliability connector families include options for rugged aerospace and defense applications with published current ratings such as 20 A, 40 A, and 60 A depending on the connector family.¹¹ TE Connectivity’s heavy-duty sealed connector headers use through-hole PCB mounting for harsh-duty vehicle electronics in applications such as trucks, buses, agriculture, and construction equipment.¹²

This does not mean SMT can never handle power. Specialized SMT power packages absolutely exist. But many connectorized, high-current, serviceable, or harness-integrated designs still benefit from through-hole attachment because the electrical and mechanical requirements are tied together.

Mixed-Technology Boards Are the Real World

The real manufacturing question is rarely “SMT or THT?” More often, the answer is both.

A mixed-technology assembly may use SMT for dense control circuitry, microcontrollers, passives, ICs, and high-speed signal sections, while using THT for connectors, relays, power components, terminal blocks, transformers, or components that require additional mechanical support.

That is not a compromise. It is good engineering.

Industrial, Automotive, and Defense Electronics Change the Priorities

If you work in industrial, automotive, or defense electronics, this is not theoretical.

Phoenix Contact highlights PCB connector use cases in automation, battery storage, renewable energy, and machine building.¹⁰ TE Connectivity’s sealed headers target harsh-duty vehicle environments, including trucks, buses, agriculture, and construction equipment.¹² Harwin’s aerospace and defense materials point to applications such as radar, avionics controls, UAVs, satellites, launch systems, and power control modules.¹¹

In these environments, the board is not just carrying signals. It is part of a larger mechanical system made up of connectors, harnesses, power paths, mounting loads, vibration exposure, thermal cycling, service expectations, and inspection requirements.

That is why THT remains important. Some electronics live in clean consumer environments. Others live in vehicles, industrial systems, control cabinets, power equipment, defense platforms, and fielded hardware. The assembly method should match the environment.

THT and Sustainment Support

There is also a lifecycle story.

Many long-service industrial and defense products outlive normal commercial component cycles by years or even decades. DLA DMSMS guidance highlights the mismatch between long-life military platforms and shorter commercial component lifecycles.¹³ MIL-STD-11991B also emphasizes parts, materials, and processes management across development, production, modification, and sustainment, including obsolescence management.¹⁴

That does not mean THT is automatically more reliable than SMT. But it does mean that when a proven fielded product already depends on through-hole connectors, terminal blocks, or leaded service parts, redesigning the assembly to eliminate THT may create new qualification, sourcing, and support risks.

In long-life programs, sustainment is not an afterthought. It is an engineering requirement.

What High-Reliability THT Manufacturing Requires

High-reliability THT is not simply “hand solder a connector and move on.” Depending on the product, it may involve wave soldering, selective soldering, hand soldering, solder preforms, or through-hole reflow/pin-in-paste processes.²

Each method has its own process controls. Selective soldering may be useful for mixed-technology boards where THT components need to be soldered without exposing the entire board to a wave process. Through-hole reflow can allow compatible leaded components to be processed during the SMT reflow cycle. Hand soldering may still be necessary for certain components, low-volume builds, rework, or specialized assemblies.

The quality system matters as much as the soldering method. J-STD-001 defines soldered electrical and electronic assembly process requirements. IPC-A-610 defines visual acceptability criteria. IPC-7711/7721 provides procedures for rework and repair. NASA’s soldered electrical connections standard emphasizes inspection access, trained personnel, solderability control, and stress-relief practices for leaded components.¹⁵¹⁶¹⁷

For high-reliability assemblies, THT quality depends on details such as:

• Proper component seating and orientation
• Appropriate lead forming and stress relief
• Controlled solder fill and wetting
• Thermal profiling for the board and component mass
• Cleanliness and flux control
• Inspection access and workmanship criteria
• Rework practices that protect plated-through holes and nearby components

The solder joint is also only part of the mechanical system. Rugged connector designs may use boardlocks, jackscrews, flanges, or other retention features to reduce stress on the solder connection. In demanding products, mechanical retention, PCB layout, connector selection, soldering process, and inspection all work together.

Choosing THT, SMT, or Both

The best assembly method depends on what each part needs to survive.

In a surface-mount world, THT still matters because some problems are still physical before they are electrical. If the assembly has to handle connector insertion, harness loading, high vibration, high current, legacy support, or field repair, through-hole remains a serious engineering tool — not a historical leftover.

And for manufacturers that live in connectors, harnesses, defense, and industrial builds, that distinction still matters every day.

Sources

1. IPC, IPC-AJ-820A Assembly and Joining Handbook.
2. IPC, Mixed Technology Assembly Guidance / Technical Resource.
3. Motorola, 1953 Motorola Annual Report.
4. Google Patents, US3099608A — Method of Making Printed Circuit Boards.
5. IBM, IBM System/360 History.
6. IPC, Surface Mount Council / SMT Status and Action Plan Technical Resource.
7. Harwin, 5 Considerations for PCB Layouts.
8. IPC / Electronics.org, Reliability of Partially Filled Through-Hole Solder Joints.
9. IPC / Electronics.org, FEA / Thermal Cycling Study of Through-Hole Solder Joints.
10. Phoenix Contact, PCB Connectors for Signal, Data, and Power Transmission.
11. Harwin / Avnet, Harwin Aerospace and Defence Solutions Brochure.
12. TE Connectivity, Heavy Duty Sealed Connector Headers.
13. Defense Standardization Program / DLA, DMSMS and Long-Life Military Platform Sustainment Guidance.
14. DLA, MIL-STD-11991B Parts, Materials, and Processes Management.
15. NASA, NASA-STD-8739.3 Soldered Electrical Connections.
16. IPC, IPC-A-610 Acceptability of Electronic Assemblies.
17. IPC, J-STD-001 Space and Military Applications Addendum.

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