Through-hole technology (THT) and surface-mount technology (SMT) are the two fundamental methods for attaching electronic components to a printed circuit board. THT inserts leads through drilled holes and solders them on the opposite side; SMT solders components directly to pads on the board surface. The choice between them affects board size, assembly cost, mechanical strength, thermal performance, and repairability. This guide compares both technologies across every parameter that matters to a PCB designer, with a decision framework to help you choose.
Key Takeaways
SMT dominates modern electronics. Over 90% of new PCB designs use predominantly SMT assembly. Surface-mount components are smaller, lighter, and enable automated placement at rates of thousands to tens of thousands per hour—versus under 1,000/hour for through-hole.
Through-hole provides 3–10× higher pull strength per solder joint because leads pass through the full board thickness. THT is the correct choice for connectors, power devices, transformers, and any component subject to mechanical stress.
Mixed-technology boards are the norm. Most real-world designs combine SMT for logic, passives, and signal ICs with through-hole for connectors, power semiconductors, and electromechanical components. Selective soldering handles the THT joints on an otherwise SMT-only board.
Cost depends on volume. At low volumes (<100 boards), through-hole hand assembly may be cheaper. At scale (>1,000 boards), SMT’s automated pick-and-place and reflow soldering reduce per-unit cost dramatically.
Through-Hole vs SMD: Head-to-Head Comparison
| Parameter | Through-Hole (THT) | Surface Mount (SMT) |
| Mounting method | Leads inserted through drilled PCB holes, soldered on opposite side | Components soldered directly to surface pads; no drilled holes needed |
| Component size (typical) | Large (DIP-14: ~19×8 mm) | Small (SOIC-14: ~9×6 mm; 0402 resistor: 1.0×0.5 mm) |
| Board density | Lower; drilled holes consume routing space on all layers | Higher; components on both sides; finer pitch enables more I/O |
| Mechanical strength | High; leads bonded through full board thickness (3–10× SMT pull force) | Lower; joints at surface only; adequate with proper pad design |
| Placement rate | <1,000 components/hour (per DigiSource) | Thousands to tens of thousands/hour |
| Soldering process | Wave solder, selective solder, or hand solder | Reflow oven (primary); wave solder for bottom-side SMD |
| Solder joint reliability | Very high under vibration and thermal cycling | Good; proper pad design and solder volume are critical |
| PCB fabrication cost | Higher (drilling adds time and cost) | Lower (no drills for SMD-only boards) |
| Assembly cost (low vol.) | Lower (hand assembly feasible) | Higher (stencil and reflow setup cost) |
| Assembly cost (high vol.) | Higher (slow placement, wave solder) | Lower (automated, fast, repeatable) |
| Repairability | Easy; components desolder and replace with basic tools | Harder; requires hot-air rework station or reflow tools |
| Power handling | Excellent; TO-220, TO-247 bolt to heatsinks | Good; DPAK, D2PAK use PCB copper for thermal path |
| High-frequency performance | Lower; longer leads add parasitic inductance and capacitance | Better; shorter leads, lower parasitics |
| Breadboard prototyping | Yes (DIP, TO-92 fit standard breadboards) | No (requires adapter boards or dedicated SMD prototyping tools) |
| Component availability | Declining; many new ICs are SMD-only | Dominant; widest selection for new parts |
| Typical components | DIP ICs, connectors, TO-220/TO-247, relays, transformers, electrolytics | SOIC, QFP, QFN, BGA, 0402/0603/0805 passives, SOT-23 |
Sources: DigiSource THT vs SMT blog (placement rate comparison), Components101 (mechanical strength ratio), Sierra Circuits (SMT advantages), Epectec (design differences). All values are representative ranges; exact figures depend on specific components and equipment.
[IMAGE 1: Split photo showing a through-hole PCB section (DIP ICs, TO-220 regulators, axial resistors) next to an SMT PCB section (SOIC ICs, 0402 passives, QFN) at the same scale | alt: “Through-hole vs SMD PCB comparison showing component size and density differences”]
When to Use Through-Hole
Mechanical stress: Connectors (USB, Ethernet, power jacks), switches, and relays experience insertion/removal forces that shear SMT joints. Through-hole leads bonded through the full PCB thickness survive these forces reliably.
High power: TO-220 and TO-247 packages bolt to external heatsinks, dissipating 50–250+ W. No SMT package of equivalent cost achieves this without custom thermal solutions.
Prototyping and education: DIP ICs and axial/radial passives plug into breadboards and solder with a basic iron. SMT prototyping requires solder paste, stencils, and reflow (or a hot-air station).
Field serviceability: Socketed through-hole ICs (DIP in a machined-pin socket) can be replaced by a field technician with no specialized equipment. SMT replacement requires a rework station.
Large passives with no SMT equivalent: High-value electrolytic capacitors, large power inductors, board-mount transformers, and fuses are still predominantly through-hole.
When to Use Surface Mount
Board density and miniaturization: SMT components use a fraction of the board area. A 0402 resistor (1.0×0.5 mm) replaces an axial 1/4 W resistor (~6×2.5 mm + leads), freeing space for denser routing and additional functionality.
Automated high-volume production: Pick-and-place machines place SMD components at 10,000–50,000+ per hour. Reflow soldering is highly repeatable and produces consistent joint quality. At volumes above ~1,000 boards, SMT assembly cost per unit is significantly lower than THT.
High-frequency and high-speed design: SMT components have shorter leads and lower parasitic inductance/capacitance, improving signal integrity at frequencies above 100 MHz. Modern RF, networking, and high-speed digital designs are not feasible with through-hole.
Double-sided population: SMT allows components on both sides of the PCB, doubling available board area. Through-hole components can only be on one side (the insertion side).
Component availability: The majority of new IC designs ship only in SMD packages (SOIC, QFP, QFN, BGA). Choosing SMT ensures access to the widest selection of current parts.
Mixed-Technology Boards: The Real-World Default
Most production boards are mixed-technology: SMT for the majority of components, through-hole for connectors, power devices, and electromechanical parts. The assembly sequence is typically:
1. Print solder paste (stencil) on the top side.
2. Place SMT components (pick-and-place machine).
3. Reflow solder the top side.
4. Flip board; repeat for bottom-side SMT if needed.
5. Insert through-hole components (manual or auto-insertion).
6. Selective-solder or wave-solder the through-hole joints.
Selective soldering applies molten solder via a programmable nozzle to individual through-hole joints, avoiding reflow damage to adjacent SMT components. This is the standard approach for mixed boards in modern contract manufacturing.
[IMAGE 2: Flowchart showing mixed-technology assembly sequence: paste print → SMT place → reflow → THT insert → selective solder → inspect | alt: “Mixed-technology PCB assembly sequence from SMT reflow to through-hole selective solder”]
Cost Comparison: THT vs SMT by Volume
| Volume Range | THT Assembly | SMT Assembly | Recommendation |
| 1–10 boards | Cheapest (hand solder, no setup cost) | Expensive (stencil + reflow setup amortized over few units) | THT or hand-solder SMT |
| 10–100 boards | Moderate (still hand-feasible) | Moderate (setup amortized; placement fast) | SMT with selective solder for THT |
| 100–1,000 boards | Expensive (slow placement) | Cheap (fully automated) | SMT dominant; THT only where required |
| >1,000 boards | Very expensive (labor-limited) | Very cheap (per-unit assembly cost minimal) | SMT dominant; THT connectors only |
Cost estimates are directional. Exact figures depend on component count, board complexity, and contract manufacturer pricing. The key inflection point is typically around 100–500 boards, above which SMT becomes clearly cheaper per unit.
Frequently Asked Questions
What is the main difference between through-hole and SMD?
Through-hole (THT) inserts component leads through drilled holes in the PCB and solders them on the opposite side. Surface-mount (SMD/SMT) solders components directly to pads on the board surface without drilled holes. SMT enables smaller components, higher density, and faster automated assembly. THT provides stronger mechanical bonds.
Is through-hole stronger than SMT?
Yes, in terms of mechanical pull strength. Through-hole solder joints bond leads through the full PCB thickness, providing 3–10× higher pull force than equivalent SMT joints. This makes THT the correct choice for connectors, transformers, and any component subject to insertion forces or sustained vibration.
Why is SMT cheaper at high volume?
SMT uses automated pick-and-place machines that place thousands of components per hour, versus under 1,000/hour for through-hole. Reflow soldering is faster and more repeatable than wave or hand soldering. SMT boards require no drilled holes, reducing PCB fabrication cost. These advantages compound at scale.
Can you mix through-hole and SMT on the same board?
Yes, and most production boards do. The standard approach: reflow-solder SMT components first, then insert through-hole parts and selective-solder them. This is called mixed-technology assembly and is the real-world default for boards with connectors, power devices, or electromechanical components alongside SMD logic and passives.
Is through-hole obsolete?
No. Through-hole is declining in prevalence but remains essential for connectors, power semiconductors (TO-220, TO-247), large passives, electromechanical devices (relays, transformers), and any application requiring field-replaceable socketed ICs. THT is also unmatched for breadboard prototyping and education.
Making the Decision
Default to SMT for all components where an SMD package exists and the application does not specifically require through-hole mechanical strength, heatsink mounting, or breadboard compatibility. Use through-hole selectively for connectors, power devices, and field-replaceable parts. Budget for selective soldering if mixing technologies on the same board. For volumes above a few hundred units, the per-unit cost advantage of SMT assembly is decisive.
