The short answer

Surface mount technology (SMT) solders components directly onto pads on the surface of the board. Through-hole technology (THT) runs component leads through plated holes drilled in the board and solders them on the opposite side. Neither one is better across the board. They solve different problems.

Choose SMT when:

1. Board space is tight or you need components on both sides
2. You’re running production quantities and want the lowest cost per placement
3. Your design uses fine-pitch ICs, BGAs, or high-speed signals

Choose through-hole when:

1. A part takes mechanical stress: connectors, switches, anything that gets plugged, pressed, or pulled
2. A component is heavy or tall, like transformers, large electrolytic capacitors, or relays
3. The part carries high current and needs a heatsink or the extra copper a through-hole mount gives it

Most real boards use both. A typical control board has a few hundred SMT parts and a handful of through-hole connectors. That’s called mixed-technology assembly, and it’s common on the aerospace, defense, and industrial boards built here in Yaphank.

What’s the actual difference between SMT and through-hole?

The difference starts at the bare board and carries all the way through inspection.

A through-hole part has wire leads. Those leads go through holes drilled and plated in the board, then get soldered on the far side, usually by a wave soldering machine, a selective soldering system, or a technician with an iron. The joint wraps around the lead and fills the hole, so the part is physically anchored through the entire thickness of the board.

An SMT part has no leads to push through anything. It sits on copper pads printed with solder paste, gets placed by a pick-and-place machine, then goes through a reflow oven where the paste melts and forms the joint. On our lines that means Yamaha YSM20R placement machines handling parts down to 01005 size, with solder paste inspection before placement and automated optical inspection (AOI) after.

One thing the table doesn’t show is routing. Every through-hole part needs a hole through every layer of the board. On a 10-layer or 12-layer design, that eats routing space on layers the part never touches. It’s one of the biggest reasons designers moved to SMT for anything that doesn’t need to be through-hole.

Which is more reliable for high-stress applications?

It depends on the kind of stress, and that distinction matters when your board is going into an aircraft, a vehicle, or a piece of field equipment.

For mechanical stress (vibration, shock, repeated connector mating) through-hole wins. A lead soldered through a plated hole has far more holding strength than a surface joint. That’s why even the most modern, SMT-heavy boards still use through-hole connectors and mounting hardware. A USB connector that gets plugged in a thousand times can eventually tear a surface-mount joint off its pad if the design doesn’t account for it.

For electrical performance at high speeds, SMT is usually the better choice. Shorter leads mean less parasitic inductance, which matters on RF and high-speed digital designs.

For thermal cycling, it’s closer than people expect. Well-made SMT joints hold up to thermal cycling in demanding environments all the time. When SMT joints do fail, the cause is usually a process or design problem rather than the choice of SMT itself: an off reflow profile, voids under a BGA, or a heavy part on a small pad with nothing else holding it in place. That’s why high-reliability SMT work leans so hard on inspection. You can’t see the solder joints under a BGA with your eyes or with AOI. Our 3D X-ray system exists for exactly that.

If your product is built to Class 3, the IPC class for high-reliability electronics where failure in the field isn’t acceptable, both SMT and through-hole joints have to meet stricter requirements. IPC J-STD-001 sets how the joints are built, and IPC-A-610 sets how finished joints are inspected and accepted. Class 3 doesn’t tell you which method to pick. It tells you how good the joints have to be once you’ve picked. IPC’s IPC-A-610 program page covers the standard and its certification tracks.

A practical middle ground for heavy SMT parts in high-vibration builds is to add adhesive staking or corner bonding, with conformal coating on top for environmental protection. That’s worth raising at the design review stage, not after the first failure report.

Can one board use both SMT and through-hole?

Yes, and most production boards do. Mixed-technology assembly just means the build has two soldering stages instead of one.

The usual order goes like this:

1. Solder paste is printed on the SMT pads and inspected
2. SMT components are placed and the board runs through reflow
3. If the board has SMT parts on both sides, the second side is printed, placed, and reflowed
4. Through-hole parts are inserted
5. Through-hole joints are soldered by wave, selective, or hand soldering
6. The board is inspected and tested

Step 5 is where the decisions get interesting. Wave soldering is fast, but it sends the entire bottom of the board across a wave of molten solder. If you have SMT parts on that bottom side, they either need to be glued and rated to survive the wave, or masked off with a pallet. Selective soldering solves that by using a small programmable nozzle to solder only the through-hole joints, leaving the SMT parts around them alone. We run both, plus hand soldering for the parts that don’t suit either machine.

A design tip that saves real money: keep through-hole parts on one side of the board when you can. Through-hole parts scattered across both sides can mean extra hand work on every single board, and that labor shows up on the quote.

How does the choice affect cost and lead time?

SMT is almost always cheaper per component once a build is set up. A pick-and-place machine places tens of thousands of parts an hour with no one touching them. Through-hole insertion, even when partly automated, is slower and involves more hands-on labor.

But the per-part number isn’t the whole picture:

• Setup cost. SMT requires a stencil and machine programming. On a very small prototype run, that setup can outweigh the per-part savings.
• Board cost. Through-hole needs more drilled holes. SMT designs often allow a smaller board, which cuts the bare board cost.
• Secondary operations. A mostly SMT board with six through-hole connectors still needs a second soldering step. That step is often where the lead time and labor go, not the SMT side.
• Part availability. Some components are only made in one package style. If a through-hole part goes end-of-life and the replacement only exists in SMT, you’re looking at a board revision. This is worth checking at the bill of materials stage, not after parts are on order.

For small quantities, the math shifts again. We cover that in more detail in our guide to low-volume PCB assembly.

What to settle before you send the design out

Most assembly problems with SMT and through-hole aren’t really assembly problems. They’re design choices that nobody questioned before the files went out. Before you request a quote, it helps to have answers to these:

• Which through-hole parts are truly necessary, and which have a qualified SMT equivalent?
• Are all through-hole parts on the same side of the board?
• Do heavy SMT parts need staking, coating, or mechanical support for the environment they’ll see?
• Is the product built to IPC Class 2 or Class 3? This changes the acceptance criteria for every joint on the board.
• Are there BGAs or bottom-terminated parts that will need X-ray inspection?

A good contract manufacturer should raise these during a design for manufacturability (DFM) review rather than quietly building whatever shows up in the Gerbers. If you’re comparing suppliers, it’s also worth asking whether they handle parts procurement or expect you to supply the components. The turnkey vs. consigned choice affects how part substitutions like SMT-for-through-hole get handled.

Frequently Asked Questions About SMT and Through-Hole Assembly

Are through-hole components obsolete?

No. Through-hole parts are still standard for connectors, switches, transformers, large capacitors, and high-current components, because they handle mechanical stress better than surface joints. What has changed is that through-hole is now used where it’s needed rather than by default. Most modern boards are largely SMT, with through-hole reserved for a few parts.

What is a through-hole in a PCB?

A through-hole is a hole drilled completely through a circuit board and plated with copper on the inside wall. Component leads are inserted into it and soldered, which connects the part electrically to the traces on each layer and anchors it mechanically. Plated holes that only connect layers, carrying signals, power, or heat with no component lead in them, are called vias.

What is the difference between SMD and SMT?

SMD stands for surface mount device, and it’s the component itself: a resistor, capacitor, or IC made to sit on the board surface. SMT stands for surface mount technology, which is the assembly process used to place and solder those devices. In practice people often use the terms interchangeably, but SMD is the part and SMT is the method.

What are the disadvantages of SMD components?

SMD parts are harder to rework by hand, especially fine-pitch ICs and BGAs whose joints are hidden underneath the package. They need specialized equipment to place and solder, and some are sensitive to moisture and heat during reflow. Surface joints are also weaker under repeated mechanical force, which is why connectors are often still through-hole.

Is through-hole stronger than SMT?

Mechanically, yes. A through-hole lead is anchored through the full thickness of the board, so it resists pulling, bending, and vibration better than a surface-mount joint. That doesn’t make through-hole more reliable overall. Properly designed and inspected SMT assemblies hold up in aerospace and defense applications, where the strength advantage matters mainly for connectors and heavy parts.

Is SMT assembly cheaper than through-hole?

Usually, on a per-component basis, because SMT placement is highly automated. The total cost depends on the whole board: stencil and programming setup, how many through-hole parts need a second soldering step, and the order quantity. A mostly SMT board with a few through-hole connectors is a very common and cost-effective design.

East End Assemblies builds SMT, through-hole, and mixed-technology assemblies at our AS9100-certified, ITAR-registered facility in Yaphank, NY, with three SMT lines plus wave, selective, and hand soldering under one roof. If you’re weighing the tradeoffs on a specific design, see what goes into our PCB assembly work or send us your files for a DFM review and quote.