Surface mount technology (SMT) and through-hole technology (THT) are the two primary methods used to assemble electronic components on PCBs.
Each method impacts assembly speed, board density, repairability, cost, and long-term reliability in different ways. Understanding their strengths and limitations helps you choose the right approach for your design.
Below is a simplified breakdown of how SMT and THT differ:
- Assembly speed
SMT offers the fastest assembly process thanks to its high level of automation. Pick-and-place machines can position tens of thousands of components per hour, and after placement, the entire board goes through a reflow oven where all solder joints form at once.
Since hundreds of parts are soldered simultaneously, SMT is ideal for high-volume production where speed, precision, and repeatability are essential. The process also minimizes manual handling, reducing errors and ensuring consistent quality across large batches.
THT takes longer because it requires drilling holes and inserting component leads through the board. Selective or wave soldering can automate a part of the process. However, the overall speed remains slower than SMT.
Through-hole technology is typically preferred for low-to-medium production volumes or applications where mechanical strength and durability are more important than assembly speed.
- Component density
SMT supports fine-pitch ICs, BGAs, CSPs, and tiny passives such as 01005 packages. This allows you to pack more functionality into smaller board areas. This technology also facilitates HDI layouts featuring microvias, via-in-pad structures, and dense inner-layer routing.
THT reduces routing space as each component lead requires a drilled, plated hole that spans multiple layers. These holes consume real estate and often increase board size. As a result, through-hole technology struggles to support miniaturized designs or products with aggressive space constraints.
- Repairability
SMT repairability is moderate. Rework stations and hot-air or IR tools enable the replacement of most components. However, very small packages, bottom-terminated parts (such as QFNs), and BGAs require specialized equipment and expertise. Dense layouts also make visual inspection and probing more difficult during troubleshooting.
THT offers excellent repairability. Components are easy to access, identify, probe, and replace using standard soldering tools. The large leads and visible solder joints simplify repairs and prototype debugging. This makes THT highly favorable for serviceable products and industrial hardware.
- Production suitability
SMT is ideal for medium-to-high-volume production runs. Its compatibility with HDI design, fine-pitch packages, and compact layouts makes it the standard choice for modern electronic products.
THT suits low-to-medium-volume production, prototyping, and designs where reliability or serviceability is prioritized over density or cost.
- Mechanical strength
Surface-mount solder joints mainly depend on pad adhesion, which makes them more vulnerable to mechanical stress compared to THT. However, modern techniques such as underfill and anchoring vias significantly improve joint strength and can make SMT assemblies highly reliable**.**
THT provides superior mechanical strength because component leads pass completely through the PCB and are soldered on the opposite side. This makes THT the preferred choice for connectors, relays, transformers, and any components exposed to frequent mechanical forces, cable pull, or high-vibration environments.
- Typical applications
SMT dominates modern electronics due to its high scalability, density, and automation. Typical applications include:
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Smartphones, laptops, tablets, IoTs, and wearables
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Computing hardware and networking equipment
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Automotive ECUs and industrial controllers
THT remains essential in applications with high current capacity. It is typically used for:
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High-power connectors and terminal blocks
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Electromechanical switches and relays
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Transformers, inductors, and large electrolytic capacitors
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High-reliability aerospace, defense, and avionics systems
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Equipment requiring field repair or extreme environmental reliability
THT components are larger and heavier, but their durability and electrical reliability make them indispensable for mission-critical equipment.
- Cost
SMT offers the lowest per-unit cost in medium-to-high-volume production due to minimal labor and reduced assembly steps. The absence of drilled holes also directly reduces the fabrication costs. Small, lightweight surface-mount devices enable compact layouts that reduce overall board size.
THT incurs higher costs as it involves manual insertion or selective soldering. Choose this technology when reliability or mechanical strength is a concern. Mixed assembly (SMT+THT) is a solution to strike a balance between cost and reliability.
Choosing between SMT and THT
The decision between SMT and THT depends on several factors, including component type, performance requirements, mechanical stress, and production scale. In many assemblies, a hybrid approach is used; SMT handles the majority of components, while THT supports connectors, power components, and mechanically stressed parts.
By understanding the strengths and limitations of both assembly methods, you can select the right combination to strike a balance between manufacturability and reliability.