Two Ways to Attach a Component, Two Different Factories

Through-hole technology puts component leads through drilled holes and solders them on the opposite side. Surface mount technology places components onto pads on the board surface and reflows the whole assembly in an oven. Almost every product uses both, and the mix you end up with drives your board size, your assembly quote, and how quickly a factory can turn units.

Founders rarely need to choose a technology outright. What they do need to understand is that each additional process pass has a setup cost and a per-board cost, and that decisions made in schematic capture determine how many passes the factory runs. That is where the money is.

Through-Hole: Strong, Slow, and Still Necessary

THT parts are mechanically anchored through the board, so the joint carries load. That is why it survives in specific places: power connectors that get yanked, screw terminals, large electrolytic capacitors and transformers, relays, and edge connectors that see insertion cycles. A surface-mount USB connector on a product people plug in daily will eventually tear its pads off unless it has through-hole retention tabs, which is exactly why nearly every real USB connector has them.

The cost is process. THT components are inserted, often by hand, then soldered by wave or selective soldering, and boards with parts on both sides need masking or hand work. Expect roughly 0.05 to 0.30 USD per joint in labor at low to moderate volume, versus a small fraction of a cent per SMT joint. THT also consumes board area on both sides, because a hole blocks routing on every layer it passes through.

Surface Mount: Small, Cheap at Volume, Unforgiving

SMT is the default for a reason. A pick-and-place machine sets 20,000 to 100,000 components per hour, parts are far smaller, and both sides of the board are usable. Component packages have shrunk to 0201 and 01005 passives, and virtually all modern ICs, connectors, and modules are available only in surface-mount packages. If a design is entirely SMT, the factory runs paste, place, reflow, and inspection, and that is it.

The tradeoffs are real:

  • You need a stencil. A stainless steel solder paste stencil costs 100 to 300 USD and is design-specific, so it is re-cut for every board revision.
  • Setup dominates at low volume. Feeder setup and machine programming run 200 to 800 USD per build regardless of quantity, which is why 10 boards and 100 boards can cost nearly the same to assemble.
  • Inspection gets harder. BGA and QFN joints are hidden under the package, so verification requires X-ray rather than a microscope. Budget for it if you use them.
  • Rework is specialized. Replacing a fine-pitch part needs a hot air or IR rework station and a steady hand, not a soldering iron.
  • Thermal profiles matter. Mixed component thermal mass on a dense board can leave large parts cold while small parts overheat, and the fix is layout, not oven settings.

What the Mix Costs You in Process Passes

Think in passes, because that is how the assembler prices you:

  • Single-sided SMT: one paste, place, and reflow cycle. Cheapest possible board.
  • Double-sided SMT: two reflow cycles, with the second usually reserved for lighter parts so the first side's components do not fall off. Adds meaningfully to cost.
  • SMT plus THT: the SMT passes plus insertion and wave or selective soldering. Selective soldering is the modern answer and is cheaper than masking a whole board for wave.
  • Any hand soldering: the most expensive per joint and the least repeatable. Keep it to zero if you can.

Two design rules follow directly. First, put all through-hole parts on one side of the board. Second, choose SMT alternatives for anything that does not need mechanical strength, since most connectors, switches, and even many power components now come in both. Cutting a board from three passes to two is often a bigger unit-cost win than renegotiating component prices, which is the kind of tradeoff design for manufacturing is meant to surface early.

Prototypes Behave Differently From Production

At the breadboard and first-article stage, through-hole is friendlier: you can hand-solder it, probe it, and swap parts. That is fine for proving a concept, but a hand-built through-hole prototype tells you almost nothing about production cost, size, or reliability, and every part you validate that way may need a surface-mount equivalent later.

The better path for anything heading to market is to build prototypes with the production package set from the start, using a quick-turn assembler. At quantities of 5 to 20 boards the setup charge hurts, but you are buying information about the real design, not a proxy for it. The economics are covered in PCB prototype cost, and how those builds map onto formal hardware phases is described in EVT, DVT, and PVT.

What This Means for Your BOM and Your Factory Choice

Package selection is a sourcing decision as much as an electrical one. A part in an unusual package may have one supplier and a long lead time, while the same function in an 0603 or SOIC package has five sources. Record the package for every line item, because it drives both the assembly quote and the risk of a part going unavailable. Getting that document right from the start is described in how to build a bill of materials.

Assemblers also differ. A shop set up for high-mix low-volume SMT with selective soldering will quote a mixed-technology board reasonably. A high-volume line will charge heavily for the THT pass or decline the job. Ask specifically: do you have selective soldering, what is your minimum passive size, do you do X-ray inspection in house, and what is your setup charge per unique part number. If you are sourcing in Asia, the density of options is one of the reasons the region dominates, as described in Shenzhen electronics manufacturing.

Two more levers worth knowing. Reducing unique part numbers cuts feeder setup charges directly, so standardizing on a small set of resistor and capacitor values across the design saves real money. And every part you can eliminate saves both component and placement cost, the same logic applied to mechanical assemblies in design for assembly.

Design It for the Line It Will Be Built On

Projects House designs boards with the assembly process in mind, choosing packages, minimizing process passes, and matching the design to the right assembler for your volume. Send your schematic or BOM and target quantities through our contact form and we will review the assembly implications.