Printed circuit board assemblies (PCBAs) form the backbone of modern electronics. From smartphones to satellites, PCB assemblies make our devices function. But how do all those tiny components end up on the circuit board? There are two primary technologies, SMT and through-hole, along with a mixed assembly approach that combines both on a single board. Each has real trade-offs, and knowing when to use each one can make a meaningful difference in cost, reliability, and manufacturability on your next project.
What Are SMT and THT in PCB Assembly?
SMT (Surface Mount Technology) and THT (Through-Hole Technology) are the two overarching manufacturing technologies used to attach components to a printed circuit board. Each encompasses its own specific mounting methods and soldering techniques: SMT relies on automated pick-and-place placement and reflow soldering to bond components to copper pads on the board’s surface, while THT relies on lead insertion and wave or manual soldering to anchor component leads through the board’s substrate. Mixed assembly combines both technologies on a single board where a design calls for it.
Surface Mount Technology (SMT)
Surface Mount Technology revolutionized PCB assembly in the 1980s. Its core method involves applying solder paste to the board, placing components with automated pick-and-place machinery, then heating the assembly in a reflow oven to melt the paste and bond components to the board’s surface copper pads; no drilled holes required.
SMT supports much smaller, denser components than through-hole, resistors and capacitors in 0402 or even smaller packages, fine-pitch ICs, and leadless packages like QFNs and BGAs, which is why it’s the dominant technology for compact, high-density devices like smartphones and tablets. It also allows faster, fully automated assembly at high volume. The tradeoffs: SMT components are harder to replace or hand-repair than through-hole parts, and small surface-mount packages generally aren’t suited to high-power or high-current applications, where a stronger physical connection matters more than density.
Through-Hole Technology (THT) or Insertion Mount Technology (IMT)
Through-hole technology predates SMT and remains essential for certain applications. Its core method involves inserting component leads through pre-drilled holes in the board, then anchoring them with wave soldering (for automated, high-volume runs) or manual soldering (for hand assembly or rework). This creates a mechanically stronger bond than a surface-mount connection.
That mechanical strength is the whole point: through-hole is the standard technology for components subject to physical stress, vibration, or repeated handling, connectors that get plugged and unplugged, transformers, large electrolytic capacitors, and other parts where a surface-mount joint alone wouldn’t hold up. Through-hole is also often preferred for large components, high-power or high-current parts, and prototyping, since it allows easier manual assembly, inspection, and component replacement than SMT does. The tradeoff is component density and speed: through-hole boards generally hold fewer components per square inch and take longer to assemble than an equivalent SMT board.
Mixed Assembly
Mixed assembly combines SMT and through-hole technologies on a single board, using each where it makes sense rather than forcing one to do a job it’s not suited for. In practice, SMT components are placed and reflowed first, then through-hole components are inserted and soldered afterward, typically with wave soldering or by hand, depending on volume.
This is common on boards that need both high-density digital circuitry and a handful of connectors or power components that require through-hole’s mechanical strength; a power supply board with dense SMT logic ICs alongside through-hole connectors and transformers is a typical example. Mixed assembly also lets designers pair legacy through-hole parts with modern SMT components, useful when backward compatibility with existing designs matters. The tradeoff is added manufacturing complexity and cost compared to a single-technology board, since it requires both an SMT reflow line and a separate through-hole soldering step.
Choosing the Right Technology for Your Project
Selecting between SMT, through-hole, and mixed assembly comes down to a few concrete factors:
- Component density and board size. Compact, high-density designs point toward SMT; larger, lower-density boards have more flexibility.
- Mechanical stress and vibration. Connectors, transformers, and components subject to repeated physical handling favor through-hole’s stronger bond.
- Power and current requirements. High-power components often require through-hole’s larger contact area and current-carrying capacity.
- Production volume. High-volume runs favor SMT’s automated speed; low-volume or prototype runs may lean on through-hole’s easier manual assembly and rework.
- A mix of the above usually points to mixed assembly rather than forcing a single technology across the whole board.
Frequently Asked Questions
Are SMT and THT technologies, methods, or techniques?
SMT and THT are best understood as overarching manufacturing technologies. Within each, more specific methods and techniques apply: SMT’s method centers on automated pick-and-place placement and reflow soldering, while THT’s method centers on lead insertion followed by wave or manual soldering. In everyday conversation, the terms are often used loosely and interchangeably, but the technology sits above the specific method or technique used to carry it out.
What's the difference between SMT and SMD?
SMT (Surface Mount Technology) refers to the manufacturing technology itself, mounting components onto the board’s surface rather than through drilled holes. SMD (Surface Mount Device) refers to the physical component designed for that technology. The two terms are closely related but describe different things: SMT is the technology, SMD is the part.
When does mixed assembly make more sense than pure SMT or pure through-hole?
Mixed assembly is worth the added complexity when a single board genuinely needs both high-density digital circuitry, which SMT handles well, and a handful of components that require through-hole’s mechanical strength, such as connectors or transformers. Forcing an entire board onto one technology when it doesn’t fit both needs typically means either sacrificing density or sacrificing mechanical reliability where it matters.
Can through-hole and SMT components be reworked or replaced after assembly?
Yes, both can be reworked, though they call for different equipment and expertise. Through-hole components are straightforward to hand-desolder given their accessible leads and larger solder joints. SMT components, especially fine-pitch or leadless packages like BGAs, require more specialized rework equipment and trained technicians. These are the exact capabilities BESTProto has extensive experience with, backed by advanced pick-and-place equipment and IPC-trained technicians. That means design iterations on SMT-heavy prototypes aren’t a barrier to rework at BESTProto the way they might be with a less-equipped shop.
Work with BESTProto for SMT, Through-Hole, and Mixed Assembly
BESTProto’s assembly capabilities span from through-hole to fine-pitch surface mount, so the right technology for your design, not the technology we happen to offer, drives the decision. Whether your project needs pure SMT, pure through-hole, or a mixed approach, see our Quick-Turn PCB Assembly services or contact us today to get started.