SMT vs THT Assembly: Differences, Pros & Cons, and How to Choose

What Is SMT (Surface Mount Technology)?

Surface Mount Technology (SMT) places components directly onto the surface of a PCB. Component leads or terminations are soldered to pads on the board surface using a reflow oven. SMT is the dominant assembly method for modern electronics, accounting for over 90 % of all PCB component placements globally.

SMT components are available in standardised packages: resistors and capacitors in 0402, 0603, 0805 imperial sizes; ICs in QFP, QFN, BGA, SOP, and SOT packages. The smallest production-capable SMT package is 01005 (0.4 × 0.2 mm).

What Is THT (Through-Hole Technology)?

Through-Hole Technology (THT) inserts component leads through drilled holes in the PCB and solders them on the opposite side. THT predates SMT and was the dominant assembly method until the 1980s. Today it is used selectively for components where mechanical strength, high-voltage isolation, or large physical size makes SMT impractical.

Side-by-Side Comparison

When to Use SMT

SMT is the right choice for the vast majority of modern PCB designs. Use SMT when:

• Board space is limited and high component density is required
• You need the lowest possible assembly cost at volume
• Your design uses modern ICs (microcontrollers, FPGAs, RF chips) that are only available in SMT packages
• Double-sided assembly is needed to maximise component density
• The product will not be subject to extreme vibration or shock

When to Use THT

THT remains the best choice in specific scenarios:

• High-current connectors — board-to-wire connectors carrying more than 5 A benefit from the mechanical strength of through-hole mounting
• High-voltage isolation — mains-voltage transformers, optocouplers, and relay coils require the creepage distance that THT provides
• Harsh environments — automotive, industrial, and aerospace applications with high vibration or thermal cycling stress benefit from THT's stronger mechanical bond
• Large passive components — large electrolytic capacitors (>1000 µF), large inductors, and power resistors are typically only available in THT

Mixed SMT + THT Assembly

Most real-world PCBs use a mix of SMT and THT. The standard process is:

1. SMT paste printing and component placement (top side)
2. Reflow soldering (top side)
3. SMT paste printing and placement (bottom side, if applicable)
4. Reflow soldering (bottom side)
5. THT component insertion
6. Wave soldering or selective soldering for THT joints
7. Hand soldering for any components incompatible with wave solder

Mixed assembly is more expensive than pure SMT because it requires multiple process steps. If your design has only a few THT components, consider whether SMT equivalents exist — this can eliminate the THT process steps entirely.

Cost Comparison

SMT placements at Kingdta are priced at USD 0.01–0.03 per placement for standard packages. THT hand-soldering is priced at USD 0.05–0.20 per joint. For a board with 200 SMT placements and 10 THT joints, the SMT cost is USD 2–6 and the THT cost is USD 0.50–2.00. The THT joints represent a disproportionate share of labour cost relative to their count.