When we review a circuit board at XWONDER, one of the first questions is how its components should be mounted and soldered. A compact controller may use surface-mount parts across most of the board, while its connectors and larger components still use through-hole leads. Understanding why these technologies coexist helps buyers make better decisions about layout, assembly cost, and product reliability.
Through-hole assembly helped make PCB-based electronics repeatable and practical to manufacture. Surface mount technology then enabled smaller packages, higher component density, and more automated assembly. For a modern PCBA project, the right choice depends on the component, mechanical load, thermal needs, and production plan. Many boards benefit from a controlled mix of both technologies.
Here, we use the "two revolutions" as an engineering framework for those changes. They were gradual developments, with different adoption timelines across industries. The useful lesson is how each approach changed the way we design, build, and verify an electronic assembly.
How Did Through-Hole Assembly Change Electronics Manufacturing?
Before PCB-based assembly became widespread, many electronic products used point-to-point wiring. Component terminals were connected with individual wires, often around a chassis or terminal strips. The quality of the result depended heavily on assembly workmanship, and complex circuits could be difficult to reproduce consistently.
Printed circuit boards changed that foundation by defining connections as copper patterns. With through-hole technology, or THT, component leads pass through holes in the board and are soldered to the conductive features. Plated-through holes can also connect the leads to the required layers within a multilayer PCB. The combination of a defined circuit layout and repeatable component positions made assembly easier to organize and inspect.
Why the change mattered on the production floor
Through-hole parts can be inserted manually or with suitable insertion equipment. Wave soldering allows many joints to be formed in one controlled process, while selective or manual soldering can address specific parts and layouts. These options helped manufacturers move from individually wired circuits toward repeatable board-level production.
From an engineering perspective, through-hole construction still has useful strengths. A properly designed lead-and-hole connection can support components exposed to handling or connector mating forces. Many joints are also accessible for inspection and repair, although access becomes harder around dense layouts and large components.
Where through-hole assembly reaches its limits
The leads, holes, pads, and associated clearances take up space. On a multilayer board, a through-hole lead can also restrict routing across several layers. As the number of connections increases, these constraints make it harder to keep the board compact.
Through-hole technology has no single universal pitch or footprint. Package size varies by part family. When we compare options, we use the actual package drawing and recommended land pattern, including assembly and service clearances, rather than a fixed size-reduction percentage.
What Changed When Surface Mount Technology Became Widespread?
With surface mount technology, or SMT, component terminations are soldered to pads on the PCB surface. The components themselves are called surface-mount devices, or SMDs. SMT describes the assembly method; SMD describes the part.
Eliminating the need for a through-hole lead at each component termination makes much smaller packages possible. It also gives designers more freedom to place components on both sides of a board. SMT boards still use holes for vias, mounting, and any through-hole parts, so surface mounting does not eliminate drilling from PCB fabrication.
Smaller packages and more flexible layouts
Compact resistors, capacitors, and integrated circuits allow more functions to fit into a limited area. But the component body is only one part of the space requirement. Fanout routing, thermal copper, inspection access, and rework clearance still affect the usable layout.
Double-sided SMT can improve space use further, but it adds process considerations. The assembly sequence must account for component weight, solder behavior, and repeated heat exposure. A dense board is successful only when it can also be assembled, tested, and serviced as required.
Automation with different process controls
A common SMT route uses solder paste printing, component placement, and reflow soldering. Automated equipment supports repeatable placement of many small parts. However, actual output depends on the product mix, component count, feeder setup, inspection steps, and the slowest operation in the line.
Placement speed alone is not the same as finished-board throughput. We would evaluate a production plan against the complete assembly and test route. A high placement rate does not remove time spent on setup, through-hole operations, programming, or functional testing.
Shorter connections and signal performance
Shorter component connections can reduce unwanted inductance and help with high-speed or high-frequency design. That advantage still depends on the selected package and the complete signal path. PCB stackup, return paths, trace geometry, and connector design all remain important.
For this reason, there is no useful universal frequency limit separating SMT from through-hole construction. Electrical performance must be assessed for the actual circuit and layout. A mounting method alone does not establish the bandwidth or signal integrity of a finished product.
How Do Through-Hole and Surface-Mount Assembly Compare?
We treat THT and SMT as process choices with different strengths. Neither method guarantees lower cost or higher reliability for every board. The following comparison is a starting point for design review, not a substitute for part-specific requirements.
| Engineering factor | Through-hole assembly | Surface-mount assembly |
|---|---|---|
| Board space | Lead holes and pads can restrict placement and routing. | Compact packages support higher density; routing and rework space still matter. |
| Mechanical loading | Often useful for connectors and larger parts; support may still be needed. | Suitable parts and anchors can handle mechanical loads when properly designed. |
| Typical soldering route | Wave, selective, or manual soldering, depending on the design. | Usually solder paste printing followed by reflow. |
| Automation | Insertion can be manual or automated, depending on the parts and volume. | Well suited to automated placement of many small components. |
| Inspection access | Many joints are visible; hole fill and obstructed areas need attention. | Visible joints can be optically inspected; hidden joints need other methods. |
| Thermal design | Package, lead, copper, and heatsink design determine performance. | Exposed pads and PCB copper can support heat transfer when properly designed. |
| Cost drivers | Insertion labor, soldering route, tooling, and inspection. | Stencil, setup, placement, reflow, inspection, and package complexity. |
Our engineering takeaway: Specify the required outcome first. Mechanical strength, electrical performance, thermal behavior, and test coverage are product requirements. THT and SMT are tools for meeting them.
Why Do Modern PCBAs Still Combine THT and SMT?
A mixed-technology board can use SMT for its control and signal circuitry while retaining through-hole connectors, relays, or larger components. This often reflects the available parts and physical demands of the product. It does not mean every connector or power component must be through-hole.
Consider an industrial controller with compact logic circuits and terminal blocks that receive repeated handling. SMT can help keep the logic section small. A through-hole terminal block may suit the mechanical design, but its solder joints should not automatically carry every external force; board support and enclosure design are part of the solution.
A typical route, with design-specific adjustments
- Review the board and BOM. Check package types, polarity, thermal limits, access, and any parts needing mechanical support.
- Assemble the SMT components. Print paste, inspect deposits as required, place parts, and run the validated reflow process. Complete a second-side process if the design requires it.
- Inspect the SMT assembly. Use appropriate optical checks and additional methods for joints that cannot be assessed visually.
- Insert and solder through-hole parts. Select the route according to access, thermal requirements, production volume, and protection of existing components.
- Complete verification and release. Inspect the finished assembly, perform the agreed electrical and functional tests, and retain the required production records.
The interaction between processes deserves as much attention as each individual process. A large connector can block access to nearby SMT parts. A soldering pallet needs clearance. A heat-sensitive part may have to be installed later. These details should be resolved before the layout and production route are frozen.
Control heat exposure across the whole route
For moisture-sensitive devices, handling time and storage conditions must be managed together with soldering exposure. The board's reflow profile must respect component ratings and solder paste requirements. Texas Instruments explains these relationships in its MSL Ratings and Reflow Profiles application report; the applicable documentation for each selected part should guide the final process.
Planning a Board with Both SMT and Through-Hole Parts?
Share your BOM, PCB files, and assembly requirements with XWONDER so we can discuss the process route and the details that need review.
What Should Engineers Check Before Choosing an Assembly Route?
We start with the operating environment and the real component choices. A small footprint may be attractive, but it should not remove needed test access or create an avoidable repair problem. The selection review should cover the complete product life cycle.
| Review area | Question to resolve | Useful input |
|---|---|---|
| Mechanical | Where will mating forces, vibration, or board flex occur? | Enclosure drawings, connector use, support locations, and test requirements. |
| Electrical | Which connections are sensitive to parasitics or layout changes? | Schematic, stackup, layout constraints, and performance targets. |
| Thermal | How will heat leave the components, and what soldering exposure is allowed? | Power dissipation, thermal design, and component specifications. |
| Manufacturing | Can the selected parts be placed, soldered, and inspected reliably? | BOM, placement data, fabrication data, and assembly drawings. |
| Test and service | How will faults be detected and repairs performed? | Test points, programming needs, functional criteria, and repair policy. |
| Commercial | What changes between prototypes and repeat production? | Build quantities, forecast, approved alternatives, and delivery targets. |
Compare total assembly cost
The cheapest component package does not always produce the lowest-cost assembly. An unusual part may need extra handling, a dedicated fixture, or a separate soldering step. A compact package can also increase inspection and rework difficulty.
Compare the complete route at the intended build quantity. Include setup, tooling, assembly, inspection, test, and likely rework. This gives purchasing and engineering teams a more useful basis for discussion than comparing component prices or machine speeds alone.
Define reliability through requirements and evidence
SMT does not have a universal defect rate, and through-hole construction does not automatically guarantee durability. Joint quality depends on design, materials, process control, and workmanship. Service reliability also depends on the stresses the assembly experiences after shipment.
Agree on acceptance criteria and the verification plan before production. Depending on the product, this may include optical inspection, hidden-joint inspection, electrical testing, functional testing, and environmental or mechanical validation. A traceability system supports investigation and control, but it does not replace these checks.
How Do We Approach THT and SMT Projects at XWONDER?
At XWONDER, we support surface-mount assembly, through-hole soldering, and final assembly. Our engineering approach is to connect those operations into a practical route for the actual board. Package selection, soldering access, inspection needs, and the final test plan should work together.
For an initial review, send the current BOM, PCB fabrication data, placement file where applicable, and assembly drawings. Include build quantities, operating conditions, programming needs, and functional acceptance criteria. Identify any special connector loads, thermal limits, or customer-controlled process requirements early.
The two revolutions of PCBA expanded what engineers could build. Through-hole assembly made PCB-based construction practical for a wide range of products, while SMT opened the way to much denser designs. Our goal is to choose and control the combination that supports your product's performance, manufacturability, and service needs.
Turn Your PCB Design into a Clear Assembly Plan
Talk with XWONDER about your SMT, through-hole, or mixed-technology PCBA requirements. Send your design files and project priorities to start the engineering discussion.
Technical reference: Texas Instruments: MSL Ratings and Reflow Profiles. Refer to the selected components' documentation and the agreed product requirements when defining soldering and handling conditions.
