What is the difference between through-hole and SMT in mixed assembly

When a printed circuit board needs both compact, high-density circuitry and physically robust connectors or power components, manufacturers turn to mixed assembly — a combination of Surface Mount Technology (SMT) and Through-Hole Technology (THT) on the same board. Understanding the difference between these two methods is essential for making informed decisions about board design, production cost, and long-term reliability. This article breaks down how each technology works, how they differ, and why they are combined in mixed technology PCB assembly.

What Is Surface Mount Technology (SMT)?

Surface Mount Technology is an assembly method where components are placed directly onto solder pads on the surface of the PCB. No drilled holes are required for the component leads. The process begins with solder paste printing through a stainless steel stencil, followed by automated pick-and-place machines positioning components at high speed, and finally a reflow oven that melts the solder paste to form permanent joints.

SMT components — including ICs, resistors, capacitors, diodes, and small packages like 0402 and 0201 — are significantly smaller and lighter than their through-hole equivalents. This allows for much higher component density, double-sided placement, and designs that would be physically impossible with through-hole alone. For high-frequency circuits, the short lead lengths of SMT parts also reduce parasitic inductance, which is critical for signal integrity above 100 MHz.

What Is Through-Hole Technology (THT)?

Through-Hole Technology is an assembly method where component leads are inserted through drilled, plated holes in the PCB and soldered on the opposite side. The mechanical engagement of the lead passing completely through the board creates a joint that is anchored in three dimensions, providing substantially greater mechanical strength than a surface-only solder connection.

THT is the preferred choice for components that will experience physical stress, vibration, repeated plugging force, or high current. Connectors, terminal blocks, transformers, relays, large electrolytic capacitors, and switches are commonly assembled using through-hole methods. The soldering can be done via wave soldering for volume production, selective soldering for localized joints, or manual soldering for low-volume or specialized work.

Key Differences: SMT vs. Through-Hole in Mixed Assembly

The table below summarizes the most important differences between the two technologies when used together on a mixed assembly board:

Why Mixed Assembly Combines Both Technologies

Most real-world electronic products cannot be built with SMT alone. A consumer device might use SMT for all of its ICs, passives, and RF components while relying on through-hole for its USB connector, power terminals, and mechanical mounting points. Trying to force these components into SMT packages would compromise reliability — surface pads alone cannot reliably hold a connector that will be plugged and unplugged thousands of times.

Mixed assembly gives designers the freedom to use the right technology for each component. SMT handles the dense logic and signal circuitry, while through-hole handles the parts that need physical durability, high current capacity, or thermal mass. This approach is standard in industrial controls, automotive electronics, medical devices, communication equipment, and power supplies.

For projects that need both technologies on the same board, working with a provider that offers a reliable SMT assembly service alongside through-hole capability ensures that the two processes are coordinated under one production flow, reducing handoffs and quality risks.

The Mixed Assembly Process Sequence

Mixed technology assembly must follow a carefully ordered production sequence because SMT and through-hole components use different soldering methods. The typical workflow is:

1. Review production files — Check Gerber files, BOM, pick-and-place data, assembly drawings, and testing requirements. Verify hole sizes, pad designs, component spacing, and soldering access for both SMT and THT parts.
2. Confirm DFM and assembly sequence — Validate that the PCB layout supports both technologies. SMT pad size, through-hole diameter, annular rings, and solder mask openings must all meet manufacturing requirements.
3. Prepare PCBs and components — Inspect bare boards for finish, warpage, and hole quality. Verify SMT parts in reels or trays and check through-hole components for lead length, shape, and insertion direction.
4. Print solder paste — Apply paste to SMT pads through a stencil. Paste volume must match pad size and component type to prevent bridging, insufficient solder, or poor wetting.
5. Place SMT components — Pick-and-place machines position parts according to coordinate files. Accurate placement is critical for fine-pitch ICs, BGA, and QFN packages.
6. Reflow soldering — The board passes through a reflow oven where solder paste melts to form SMT joints. The temperature profile must match PCB thickness, copper weight, and component heat tolerance.
7. SMT inspection — AOI checks for missing parts, wrong polarity, skewed components, and solder defects. X-ray inspection may be used for BGA and hidden joints. SMT inspection must be completed before through-hole insertion, because large THT parts can block inspection access.
8. ｉｎｓｅｒｔ through-hole components — After SMT inspection, through-hole parts such as connectors, terminal blocks, relays, and transformers are inserted. Direction, seating height, and lead fit are verified.
9. Solder through-hole components — Depending on the board layout and component density, this step uses wave soldering, selective soldering, or manual soldering. Selective soldering is often chosen when SMT components on the bottom side need protection from the wave.
10. Clean and inspect — The completed board is checked for solder joint shape, barrel fill, lead trimming, component height, and flux residue.
11. Electrical and functional testing — Testing may include ICT, functional test, power-on test, programming, and custom fixture tests to verify that both SMT and THT circuits perform correctly.
12. Final inspection and packing — Boards are inspected for appearance, labels, connector direction, and packing requirements, then packed with ESD protection and moisture barriers as needed.

Soldering Methods in Mixed Assembly

Mixed assembly boards require multiple soldering methods because SMT and through-hole components cannot share a single process. The choice depends on component type, layout density, heat sensitivity, and production volume:

In most mixed assembly projects, SMT reflow is completed first, and through-hole soldering follows. This sequence prevents large THT components from blocking pick-and-place nozzles, AOI cameras, or rework tools during the SMT stages.

Quality Control and Inspection in Mixed Assembly

Quality control for mixed assembly must cover both technologies. SMT inspection focuses on solder paste volume (SPI), component placement accuracy, reflow joint quality (AOI), and hidden-joint verification (X-ray for BGA and QFN). Through-hole inspection focuses on barrel fill, lead protrusion, solder wetting, and joint shape.

Functional testing is particularly important for mixed boards because they often combine power input, signal connectors, control circuits, and mechanical interface components on the same board. ICT, FCT, thermal imaging, and high-low temperature reliability testing all play a role in verifying that the assembled board will perform under real operating conditions.

Standards such as IPC-A-610 for PCBA assembly quality and IPC-A-600 for PCB fabrication provide the framework for acceptable solder joints, component placement, and board condition. Working with a manufacturer that follows these standards — and holds certifications like ISO 9001, IATF 16949 for automotive, and ISO 13485 for medical devices — helps ensure consistent quality across mixed assembly production runs.

When to Choose Mixed Assembly

Mixed assembly is the right choice when a board contains components that cannot be reliably mounted with SMT alone. Common scenarios include:

• The product includes power connectors, terminal blocks, transformers, or relays that require strong mechanical anchoring
• The board will be exposed to vibration, repeated plugging, high current, or physical stress during operation
• The design calls for specific through-hole-packaged components that are not available in SMT equivalents
• The product needs both dense signal circuitry and robust power or interface connections on the same board

For these situations, a through-hole soldering service combined with SMT capability provides the manufacturing flexibility needed to handle both types of components in a single, controlled production flow.

How Farway Supports Mixed Assembly Projects

Farway Electronic, based in LongGang, Shenzhen, operates a 2,000-square-metre production facility equipped for both SMT and through-hole assembly. The company's manufacturing capabilities include two SMT production lines with Yamaha placement machines, two DIP plug-in lines with Nitto wave-soldering equipment, and a dedicated conformal coating line.

For mixed assembly boards, Farway follows the standard sequence — SMT first, then through-hole insertion and soldering — and backs it with a full inspection chain: SPI solder-paste inspection, AOI optical inspection, FAI first-article inspection, X-ray inspection, ICT, FCT functional testing, and thermal imaging. The company is certified to ISO 9001, ISO 13485, IATF 16949, and ISO 14001, and works to IPC-A-610 assembly standards.

Farway also offers conformal coating, low-pressure injection moulding, program burning, fixture production, and finished-product box-build assembly — giving customers a complete manufacturing path from bare board to packaged product. For customers who need a partner that can handle both SMT and DIP on the same board, Farway provides a one-stop smt + dip assembly service with component sourcing, testing, and traceability built into the process.