SMT vs Through-Hole Assembly: Which Is Right for Your PCB?

When an engineer first starts laying out a PCB, one of the earliest structural decisions is how components will be mounted and soldered. Two dominant approaches have shaped the industry for decades: Surface Mount Technology (SMT) and Through-Hole Technology (THT). Each has a clearly defined set of strengths, limitations, and ideal use cases — and on many real-world boards, they coexist.

This guide works through the differences between SMT and through-hole assembly in practical terms: process mechanics, application suitability, cost structure, and how to make the call when your design requirements sit somewhere in the middle.

What Is SMT Assembly?

Surface Mount Technology places components directly onto the surface of a PCB. Components are soldered to copper pads without requiring holes through the board, which allows placement on both sides of the substrate.

The SMT assembly sequence follows a repeatable, highly automated flow:

1. Solder paste printing — a stencil deposits controlled volumes of solder paste onto each pad
2. Pick-and-place — automated machines position components onto the paste, typically at speeds of thousands of placements per hour
3. Reflow soldering — the board passes through a controlled-temperature oven; the paste melts and solidifies to form permanent joints
4. AOI inspection — automated optical inspection checks placement accuracy, solder coverage, and bridging

Reflow soldering profile is tightly controlled: for SAC305 (lead-free), preheat runs from 150–180°C, the soak zone extends this to activate flux, and the reflow peak reaches 235–260°C before a controlled cool-down. The entire thermal curve matters — both for joint quality and for component survivability.

SMT components use standardized package formats: 0402, 0603, 0805 passives; QFP, QFN, LGA for ICs; BGA for high-density packages. Miniaturized variants go down to 01005 (0.4 mm × 0.2 mm). For an in-depth breakdown of what SMT can achieve within a production environment, NextPCB's SMT assembly capabilities overview covers component pitch minimums, stencil requirements, and inspection standards.

What Is Through-Hole Assembly?

Through-Hole Technology inserts component leads through drilled holes in the PCB and solders them on the opposite side. The copper-plated through-holes create a mechanical and electrical bond that penetrates the full board thickness.

Three main soldering methods apply to through-hole work:

• Wave soldering — the PCB passes over a standing wave of molten solder; all exposed leads are soldered simultaneously. This is the high-throughput approach for production runs.
• Selective soldering — a controlled solder nozzle targets individual through-hole zones on boards that also carry SMT components, avoiding thermal damage to adjacent parts.
• Hand soldering — used for low-volume work, prototypes, or components with unusual geometries that automated processes cannot reliably reach.

Through-hole components occupy more board area per device than their SMT equivalents, and they require drilled, plated holes — which adds cost to the PCB fabrication stage. However, the through-board lead creates a joint with significantly higher pull-out strength and resistance to mechanical stress than a surface-pad connection.

SMT vs Through-Hole: Side-by-Side Comparison

When SMT Is the Appropriate Choice

SMT is the default for the majority of modern electronic product designs. Several specific conditions make it the technically and economically correct call:

High component density requirements. When a design must accommodate hundreds or thousands of passive components, ICs, and fine-pitch packages within a constrained PCB envelope, surface mounting is the only practical path. Placing the same bill of materials in through-hole format would require a board several times larger.

Volume production. SMT assembly is inherently suited to automation. A fully equipped SMT line handles solder paste printing, placement, and reflow with minimal manual intervention, which keeps per-unit labor costs low as quantities scale. This is a meaningful factor when comparing SMT assembly cost per unit at 1,000 pieces versus through-hole equivalents.

Boards requiring placement on both sides. Double-sided SMT assembly is standard — the bottom side is processed first with adhesive or light reflow to hold parts in place, and the top side follows. Through-hole components cannot occupy the second side without creating soldering conflicts with the first.

Signal integrity and high-frequency performance. SMT packages have shorter lead lengths and tighter parasitic inductance, which matters at frequencies above a few hundred MHz. Through-hole leads introduce inductance that degrades RF performance. PCB stackup considerations for SMT address how board construction affects signal behavior at the assembly level.

Low-profile or space-constrained enclosures. Surface-mounted devices sit flat against the board. Through-hole components project above and below the substrate, often significantly — a 1000 µF electrolytic capacitor standing 25 mm tall is incompatible with a 15 mm chassis height.

When Through-Hole Is the Right Technology

Through-hole assembly is not obsolete — it remains the correct choice in a well-defined set of situations where its structural properties outweigh the area and cost penalties.

Connectors subject to mechanical stress. Any connector that a user physically plugs into and unplugs repeatedly — USB, D-sub, barrel jacks, edge connectors — benefits from through-hole mounting. The solder joint in a surface-mounted connector carries the full insertion and extraction force. A through-hole connector distributes that load across the board thickness and surrounding barrel plating, delivering far better long-term reliability.

High-power components. Transformers, large filter inductors, high-wattage resistors, and power transistors often generate or dissipate significant heat. Through-hole mounting provides thermal mass and heat-spreading through the barrel, and many power components are only available in leaded packages.

Industrial and harsh-environment applications. Equipment that experiences mechanical shock, sustained vibration, wide temperature cycles, or high humidity — industrial motor drives, vehicle electronics, field instrumentation — has historically relied on through-hole construction for critical nodes. The lead compliance in a through-hole joint absorbs differential thermal expansion that would crack a surface pad.

Prototyping and development boards. Through-hole parts are easier to install, remove, and swap by hand. For breadboard-based development or first-article prototypes where individual component values need to change quickly, leaded components offer a practical advantage that automated SMT assembly does not.

Electromechanical components. Relays, switches, fuses, battery holders, and similar components that involve moving parts or require user access are almost always through-hole. The combination of mechanical retention and electrical connection in a single through-board joint is what these applications require.

Mixed Assembly: When Both Technologies Appear on the Same Board

Most industrial-grade and mid-complexity consumer boards use both. A mixed assembly PCB combines SMT components — typically the passive components, ICs, and low-profile parts — with through-hole components for connectors, power devices, and mechanically loaded parts.

The assembly sequence for a mixed board requires careful planning. The standard approach processes SMT components first (both sides as needed), then handles through-hole components via wave soldering, selective soldering, or hand assembly depending on density and thermal constraints. Introducing through-hole components before SMT reflow is generally avoided because the leaded parts block stencil printing and can interfere with pick-and-place.

Selective soldering is increasingly common in mixed assemblies. Rather than exposing the entire board to a wave solder bath — which risks thermally affecting nearby SMT components — a programmable nozzle targets only the through-hole zones. This adds process complexity and cost but gives precise control over which areas receive the solder. For more on how the full process sequence works, NextPCB's complete SMT assembly process guide covers the flow from paste printing through final inspection.

One DFM consideration specific to mixed boards: through-hole component placement should keep component bodies and leads away from SMT pads on the bottom side to avoid contaminating solder paste or creating shadowing effects during reflow.

Cost Implications

The cost difference between SMT and through-hole assembly is real and worth quantifying before committing to a design approach.

PCB fabrication cost. Through-hole components require drilled, copper-plated holes. Mechanical drilling at standard pitches adds cost relative to a board with fewer or no through-holes. For high-density through-hole designs, drill counts can be substantial.

Assembly labor. Automated SMT lines run at high throughput with low labor per unit. Through-hole insertion — particularly for non-standard or high-pin-count components — often requires manual placement, which increases labor time and cost per board. Wave soldering helps, but the insertion step itself is the bottleneck.

Stencil cost. SMT assembly requires a laser-cut stencil for solder paste printing. This is a one-time NRE cost per board revision, typically modest in absolute terms. Through-hole assembly does not require a stencil, but this rarely offsets the higher assembly labor on anything beyond very small quantities.

Per-unit cost at volume. At production quantities, SMT assembly typically yields lower per-unit cost because automation scales efficiently. The crossover point depends on board complexity and component mix, but as a general rule, SMT becomes more cost-effective above a few dozen units.

Component cost. SMT package variants of common components — resistors, capacitors, standard ICs — are often less expensive than their through-hole equivalents because SMT devices dominate current production volumes. For specialized power components, the comparison may reverse.

Frequently Asked Questions

Q: What are the main advantages of SMT over through-hole assembly?

SMT enables smaller component packages, higher placement density, placement on both board sides, and faster automated assembly. At volume, this translates to lower per-unit cost compared to through-hole equivalents.

Q: When is through-hole assembly preferred over SMT?

Through-hole is the correct choice for components subject to mechanical stress (connectors, switches), high-power devices, and applications where joint strength under vibration or thermal cycling is the primary reliability concern. Aerospace, defense, and industrial controls regularly specify through-hole for these reasons.

Q: Can SMT and through-hole components coexist on the same PCB?

Yes — this is called mixed assembly and is standard practice for industrial and mid-complexity boards. The assembly sequence processes SMT first, then handles through-hole components via wave, selective, or hand soldering.

Q: Is through-hole assembly more expensive than SMT?

Generally yes, primarily because manual component insertion costs more labor per unit than automated pick-and-place. This cost gap widens at higher volumes.

Q: Which component types always use through-hole?

Mechanical connectors, transformers, large filter capacitors, power transistors in TO-220 and similar packages, relays, and fused holders. These components require the structural retention that a through-board lead provides.

Choosing the Right Process for Your Design

The SMT vs. through-hole decision rarely comes down to a single factor. Most boards end up using SMT as the primary technology with through-hole used selectively — at connector locations, power stage components, and anywhere mechanical load is expected.

A few questions to work through when evaluating your design:

• Are any components subject to repeated mechanical insertion, vibration, or shock? → Through-hole for those positions
• Does the form factor constrain component height or double-sided placement? → SMT required
• Is volume production planned? → SMT assembly cost per unit favors automation
• Are any components only available in leaded/through-hole packages? → Mixed assembly likely

If you're uncertain about the right process split for your specific design, the SMT assembly FAQ covers common questions about process selection, minimum component sizes, and what to prepare for assembly. For specific design guidance on board dimensions that support efficient SMT processing, PCB size considerations for SMT assembly walks through the relevant constraints.

Have a design ready to evaluate? Talk to our engineering team — we review SMT, through-hole, and mixed assembly requirements and can flag potential process issues before your first production run.

Get a PCB Assembly Quote →