Surface Mount vs Through Hole: A Complete Technical Comparison for PCB Engineers

If you’ve ever opened a vintage electronic device, such as an old tube radio or classic amplifier, you’ll notice a forest of components with long leads plunging through the PCB, soldered securely on the opposite side. This is Through-Hole Technology (THT), the foundation of electronics for decades.

Now, look inside a modern smartphone or high-speed embedded system. You’ll find a dense array of Surface Mount Technology (SMT) components sitting flat on the PCB surface, forming a miniature city of circuits.

The evolution from THT to SMT illustrates the relentless drive toward miniaturization, automation, and higher-performance electronics. While SMT dominates today’s high-volume products, engineers still need to master both assembly methods.

The choice between surface mount vs through hole is not merely a matter of old versus new; it’s a critical design trade-off affecting electrical performance, mechanical reliability, thermal management, and overall cost. Understanding when to use a robust THT connector versus a high-density SMT processor is essential for designing products that are both functional and durable.

This guide provides a comprehensive technical comparison of SMT and THT technologies to help engineers make informed PCB design decisions.

Surface Mount Technology VS Through-Hole Technology: Technical Differences

While we've looked at each technology individually, the real engineering decisions happen when you compare them head-to-head. The choice between Surface Mount Technology (SMT) and Through-Hole Technology (THT) directly affects signal integrity, reliability, manufacturability, and cost.

Based on the provided charts, the analysis is clear:

Through-Hole Technology's high parasitic inductance (10 nH) creates a rising impedance that blocks high-frequency signals, while its high parasitic capacitance (2.0 pF) creates a falling impedance that allows noise to cause crosstalk.

Surface Mount Technology, with its order-of-magnitude lower parasitics (1.0 nH and 0.5 pF), avoids both of these signal integrity issues. This makes SMT the essential and technically superior choice for any high-speed digital or RF design.

Changes of Inductive Reactance with Frequency

Changes of Capacitive Reactance with Frequency

SMT vs THT: Key Performance Factors

SMT vs THT in the Manufacturing Process

Where to Use SMT, THT, or Hybrid Assembly in Modern Electronics Design

Typically, the decision to use SMT or THT should not be a question of preference, but rather the environmental and technical requirements of the application. Although SMT is undoubtedly the strongest presence in the majority of modern electronics, THT technology is still a necessity in some limited applications.

The decision on which technology to choose should be based on electrical performance requirements, mechanical constraints, power handling, and manufacturing volumes. In many instances, using a combination of both THT coatings and SMT packages will be most sensible.

Surface Mount Technology (SMT) Applications

SMT has been designed for situations where the objective is to reduce size and improve high-frequency performance.

• High-Density Electronics: The tiny footprints in SMT packages allow for miniaturized products such as smartphones, IoT modules, and wearable medical devices to be created. Space is often at a premium.
• High-Speed and RF Circuits: The low parasitic inductance of SMT packages is a physical requirement for keeping signal integrity in high-speed digital interfaces (Ex, DDR memory) and RF systems (Ex, Wi-Fi and 5G).

Through-Hole Technology (THT) Applications

THT is the solution for components that will experience mechanical loads or high power ratings.

• Power Electronics: THT is used for large capacitors, transformers, and power packages (such as TO-220) that can be bolted onto heatsinks to handle high currents and heat dissipation.
• Mechanical Interfaces & Harsh Environments: The mechanical anchoring of THT is a distinct advantage for connectors, large switches, and components designed for auto or other industrial-type applications to endure vibration and shock as well as physical tension.
• Prototyping and Repair: THT lends itself to hand-soldering, which is an appropriate method for lab or educational kits and circuits where operational serviceability in the field is an important aspect of the design.

Hybrid Applications (SMT + THT on Same Board)

Modern PCBs may frequently use mixed technology to maximize density, reliability, and power requirements as follows:

• Consumer Devices: SMT for all ICs and passive components; THT for I/O connectors and charging ports.
• Industrial Controllers: SMT microcontrollers and sensors + THT relays and terminal blocks.
• Telecommunications Equipment: SMT for high-frequency circuits, THT for optical connectors, and high-current DC power inputs.

JLCPCB offers hybrid assembly capabilities, allowing designers to incorporate SMT ICs and THT power devices and connectors in a single workflow, without any manual rework.

What Is Surface Mount Technology?

Unlike through-hole technology (THT), which requires drilled holes, surface mount technology (SMT) directly solders surface mount devices (SMDs) on PCB copper pads. SMT does not use long leads, but rather short terminals, pads, or solder balls, thus limiting the parasitic effects and allowing for high-density and multilayered PCB layouts.

If THT is the foundation of electronics from a long time ago (and perhaps the more robust way to do PCB and electronics), then SMT is the high-tech engine of miniaturization and high performance. SMT is the dominant PCB design and assembly technology being used today. PCB designers or embedded systems engineers need to have a basic knowledge and experience of SMT design.

SMD Components Mounted on a PCB

The true value of Surface Mount Technology (SMT) in modern PCB assembly lies in scalability and accessibility. Instead of investing heavily in SMT lines and equipment, engineers can rely on professional services like JLCPCB SMT Assembly—which combines high-speed pick-and-place automation, reflow soldering, and AOI/X-ray inspection—to quickly turn designs into reliable prototypes and full-scale production boards. This approach minimizes cost, shortens lead time, and ensures consistent manufacturing quality.

The PCB SMT assembly process involves:

1. Stencil Printing: A stainless steel stencil with laser-cut holes is positioned above the PCB. An application machine is employed to deposit solder paste - a paste of tiny solder spheres and flux onto the stencil, onto a predetermined amount on each component pad.

2. Pick-and-Place: The board, now printed with solder paste, moves to a pick-and-place machine, a high-speed robot that uses vacuum nozzles to pick individual SMDs from reels and trays and places them, with very high precision, over the tacky solder paste.

3. Reflow Soldering: The assembly board, still with solder paste on it, undergoes a reflow process whereby the entire board assembly moves through a long, multi-zone reflow oven. The temperature is carefully controlled to first activate the flux and then melt the solder paste.

4. Inspection: Automated Optical Inspection (AOI) and X-ray (for BGAs, QFNs) assure the accuracy of placement and quality of solder joints.

Electrical Performance of Surface Mount Technology (SMT)

The short interconnect length of SMT components dramatically reduces parasitic effects:

• Lower Inductance: For a 0.2 mm solder joint, parasitic inductance is typically < 1 nH, compared to ~10 nH for a 5 mm THT lead.
• Reduced Capacitance: Smaller pad spacing minimizes parasitic capacitance, improving signal integrity in GHz-range circuits.
• Controlled Impedance: SMT packages (e.g., 50 Ω RF connectors, chip resistors) allow for precise impedance matching in high-speed digital and RF designs.

Example:
At 100 MHz, a 1 nH inductance results in:

This is manageable compared to 62.8 Ω reactance from a 10 nH THT lead at the same frequency.

Thermal and Mechanical Reliability of Surface Mount Technology

• Thermal Cycling: SMT's small solder joints are more sensitive to expansion mismatch between the component body (ceramic, plastic) and PCB substrate (FR-4, polyimide) than traditional THT components. Repeated thermal padding can induce solder fatigue cracks, particularly in BGA packages.

• Vibration and Shock: Compared to THT, SMT joints have less anchoring strength. Those components, like MLCCs (ceramic capacitors), are brittle and likely to crack if bent or dropped. Designers commonly use underfill materials (epoxy under BGAs) or stiffeners on boards to minimize vibration or drop shock.

• Thermal Dissipation: Power handling on SMT packages is limited to thermal pads without through-hole leads that would aid heat conduction; that is, thermal pads have pin-to-pin thermal limits, whereas vias on pads conduct heat to internal copper planes. An example is a QFN package with an exposed pad; for the latter, a θJA (junction-to-ambient thermal resistance) reduction of 30–50% is realized for mounting over a thermal via array.

Common SMT Package Types

• Chip Resistors/Capacitors → 0201, 0402, 0603, 0805.
• IC Packages → SOIC, TQFP, QFN, BGA.
• Discrete Semiconductors → SOT-23, SOD-123.
• Specialized SMDs → RF filters, MEMS sensors, oscillators.

Common SMD Packages

Common Problems and Limitations in Surface Mount Technology (SMT) Assembly

• Manual Assembly Difficulty: Manually soldering or replacing small SMT components requires a steady hand and usually magnification, and of course, sometimes very specialized SMD soldering tools like hot-air rework stations to do the work safely without damaging the solder pads. This is particularly true of leadless packages like BGAs and QFNs.

• Heat Dissipation Limits: SMT power devices don’t have metal tabs to connect heatsinks, like TO-220 packages do. Instead, SMT devices use PCB copper thickness and vias to spread the heat. If the layout is poorly designed, thermal runaway can result.

• Inspection Complexity: The solder joints under a BGA (Ball Grid Array) or similar package are below the component body. Failed solder joint types, such as voids, head-in-pillow defects, and cold joints, cannot be inspected, so X-ray inspection systems are required to detect these defects.

• Rework Challenges: Hot-air, or infrared rework stations are required to remove/replace fine-pitch SMT ICs by first pre-heating the board, and any components are then removed by applying either hot-air or infrared heat. Too much heat can damage the pads and cause the circuit board to be scrapped.

• Moisture Sensitivity: Some SMT packages, specifically plastic BGAs and QFNs, can absorb moisture during storage. If these types of packages are reflowed without proper MSL (Moisture Sensitivity Level) baking specifications, these packages can experience "popcorning". Popcorning is caused by internal delamination due to the expansion of vapor inside the package.

What is Through-Hole Technology (THT)?

Through-hole technology (THT) is the initial standardized method for electronic component mounting. It involves drilling plated through-holes (PTHs) into the PCB, component lead placement, and soldering onto copper pads. THT remains the undisputed champion for applications involving mechanical strength and high power.

A Through-Hole Resistor mounted on a PCB

The THT assembly process is fundamentally different from SMT:

1. Drilling and Plating: The PCB is drilled through, and its inner surfaces are plated with copper to create the PTH. The remainder of the copper ring surrounding the hole on the top and bottom layers is known as the annular ring.

2. Component Lead Insertion: Component leads are inserted semi-automatically or manually through the PTHs. This is a fundamental aspect of its cost and scalability.

3. Soldering: The board is joined together to complete the final circuit. Mass production uses wave soldering, where the board bottom passes over a wave of molten solder. The disadvantage is that it exposes the whole board to large quantities of thermal stress. A more recent and more precise alternative is selective soldering, which uses a localized fountain of solder to connect only chosen leads, with reduced heat shock to fragile components elsewhere on the board.

The metallurgical bond created extends solder over the lead and barrel of the hole, creating a strong intermetallic compound (IMC) with the PCB pad, providing THT with improved joint reliability compared to SMT for certain stress states.

Example of Through-Hole soldering technique, showing solder joints on the bottom of the PCB.

Electrical Performance of Through-Hole Technology (THT)

Despite its mechanical strength, THT's long leads and hole geometries cause several parasitic effects:

• Parasitic Inductance(L): Longer leads raise loop inductance, which is troublesome in circuits operating at high speeds (>100 MHz), according to parasitic inductance (L).
• Parasitic Capacitance(C): Larger lead-to-lead and lead-to-ground spacing enhances capacitive coupling.
• Signal Reflection: In high-frequency PCBs, vias cause impedance mismatch by acting as transmission line stubs.

Example:
Consider a standard leaded resistor with a 5 mm lead length:

20 nHcm-1 Here is an empirical constant of typical straight-wire inductance in free space.

At high enough frequencies, this inductance converts to inductive reactance:

So, for 100 MHz, this inductance is ~6.28 Ω reactance, which is sufficient to distort RF circuit signals.

Mechanical and Thermal Reliability of Through-Hole Technology

• Vibration Resistance: Leads run through the board, gripping pieces more securely than surface solder pads. Aerospace and auto electronics still require THT for vibration modules.
• Thermal Cycling: The greater amount of solder has a greater thermal mass, making it less prone to solder fatigue.
• Heat Dissipation: TO-220-style packages rely on leads and heatsinks attached to them to dissipate power efficiently.

Comparative fatigue life of SMT vs THT solder joints under -40°C to +125°C thermal cycling

Common Through-Hole Package Types

• Axial (resistors, diodes): Suitable for automated lead insertion and forming equipment.
• Radial (capacitors, LEDs): Compact layout but still requires drilling.
• Dual In-line Package (DIP): Obsolete IC packaging, generally socketed.
• TO-220 / TO-247: Power semiconductors, widely used in SMPS and motor drives.
• High-current connectors & relays: Where mechanical strength outweighs density concerns.

Limitations of Through Hole Technology in Modern PCB Design

• Routing Blockage: Vias fill inner-layer routing channels, making dense multilayer PCB designs difficult.
• Assembly Cost: Hole drilling can be responsible for up to 30–40% of PCB manufacturing cost in high-volume production.
• Reduced Density: Component minimum pitch ~2.54 mm (0.1"), not suitable for today's compact design.
• Slower Automation: THT pick-and-place machines exist, but are much slower and more expensive than SMT counterparts.

Conclusion

The discussion of surface mount vs through hole is not a matter of picking a winner, but a matter of knowing what the right tool is for the job.

Surface Mount Technology (SMT) is the obvious champion for miniaturization and high-frequency performance and has become the default when designing the dense, high-speed core of modern electronics. While Through Hole Technology (THT), with its superior mechanical anchoring capability and higher power handling, will always be a critical technology when designing extremely rugged connectors and high-power electronics that will be the mechanical interface into the real world.

In the end, it won't be about either technology (Surface Mount or Through Hole) in any future work you do in electrical PCB design; it will be about the hybrid option, the fine art of utilizing Surface Mount and Through Hole together on a single PCB, to deliver end products that are still dense, high performing, and rugged.

FAQs

Q1: SMT or THT — Which technology provides better thermal management?

Neither technology is inherently superior; they simply handle heat differently. Through-Hole Technology (THT) components can be easily attached to external heatsinks or use their leads for better thermal conduction. Surface Mount Technology (SMT) components, on the other hand, rely on PCB-level thermal design—such as thermal vias, copper planes, and heat-spreading layers—to dissipate heat effectively.

Q2: Why are components sometimes placed on both sides of a PCB?

Double-sided component placement is a unique advantage of Surface Mount Technology (SMT) that allows designers to achieve maximum component density. This is particularly essential in miniaturized electronics such as smartphones, IoT devices, and compact medical instruments, where every square millimeter of board space matters.

Q3: What are the essential tools needed to start working with SMT?

At minimum, you’ll need a temperature-controlled soldering iron with a fine tip, fine-point tweezers, flux, and solder wick. For handling smaller packages, adding a magnifier or microscope, hot-air rework station, and solder paste stencil will greatly improve precision and safety.

Q4: Are SMT and THT versions of a component electrically identical?

For their primary electrical function (for example, a 1 kΩ resistor), yes—they behave the same. However, their parasitic characteristics differ significantly. THT resistors have longer leads, resulting in higher parasitic inductance, which can degrade performance in high-frequency or RF circuits. SMT resistors offer much lower parasitics, making them the preferred choice for high-speed designs.