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SMT vs Through-Hole PCB Assembly (and Flexible PCBs)
SMT PCB assembly vs through-hole: per-joint cost, mechanical strength, when to keep THT, and what changes when the board is a flexible PCB.
by the HardwareMap editors4 min read
Contents
SMT PCB assembly mounts components on pads on the board surface and solders them all at once in a reflow oven; through-hole assembly feeds leads through plated holes and solders them separately. Most boards use both. This note compares cost, strength and design rules for each, covers flexible PCBs, and sits under the PCB assembly guide.
SMT PCB assembly vs through-hole, side by side
The cost row is the one founders feel. On JLCPCB's published price list, last updated 9 September 2026, a machine SMT joint is $0.0016est and a manual joint is $0.0164est, plus a $3.58est hand-soldering labor fee per order. Domestic and regional assemblers price differently, but the ratio holds: through-hole is the labor line.
A worked example: one connector vs twenty
Take a 100-board run of a sensor board with 180 SMT joints and either a 2-pin power jack or twenty through-hole parts (headers, a terminal block, two electrolytics) with 120 joints between them.
Arithmetic uses the JLCPCB rates above and its tiered manual rate ($0.015est per joint in the 10,001 to 30,000 band; the range covers whether the tier applies to the whole order or only the joints past 10,000). Setup, stencil, parts and the bare board are left out because they are the same in both rows. Twenty through-hole parts multiplied assembly labor about six times. On a PCB assembly cost quote, that shows up as the THT line, and it scales linearly with volume where SMT setup does not.
The fix is design, not negotiation. Swap headers for SMT equivalents, use SMT electrolytics or polymer capacitors, and keep only the parts that need through-hole strength. See design for manufacturing for the wider checklist.
When to keep through-hole
Keep a part through-hole when it does one of these jobs:
- Takes repeated insertion force. USB, barrel jacks, RJ45 and board-edge connectors that users plug in. Hybrid parts with SMT signal pins and through-hole hold-down pegs are a common middle ground.
- Carries high current or heat. Terminal blocks, power resistors, large inductors.
- Is heavy or tall. Transformers, large electrolytics, relays. Under vibration, mass on an SMT pad becomes a fatigue problem.
- Must be field-replaceable. Sockets and parts a technician swaps with an iron.
Mixed boards cost one extra process step. Assemblers run SMT first, then through-hole by selective solder, wave, or hand. Some use pin-in-paste (also called intrusive reflow): paste is printed into the plated hole and the through-hole part goes through the reflow oven with the SMT parts. It removes the second step, but the part body must survive reflow temperatures, so check the datasheet before asking for it.
Through-hole parts that stay need layout care too. Put them all on one side so the board sees one selective or wave pass, not two. Leave keep-out space around each pin for the selective solder nozzle; the assembler will give you its number. Size finished holes to the lead diameter plus a small clearance so solder can fill the barrel, and orient rows of pins parallel to each other if the board will go over a wave. Ask for the assembler's through-hole rules before layout freezes, not after the first quote.
Flexible PCB assembly
Flexible and rigid-flex boards add three constraints to SMT PCB assembly.
- Flatness. Polyimide film does not hold its shape. The assembler fixes the flex on a rigid carrier or a custom pallet for printing, placement and reflow, and that fixture may be a one-time charge on the quote.
- Moisture. Polyimide absorbs water. Assemblers commonly bake flex circuits before reflow so trapped moisture does not delaminate the stack. Ask whether the bake is in the quote.
- Stiffeners. Components need a rigid area under them. FR-4, polyimide or steel stiffeners are bonded under connector and component zones, and the bend areas are kept free of parts and vias.
Keep heavy and through-hole parts on the rigid sections of a rigid-flex design. Specify the bend radius and which areas flex dynamically on the fabrication drawing. Online services publish their flex options: PCBWay's flex and rigid-flex page is a fast sanity check on what a quick-turn shop will take before you ask for a quote.
Choosing for your board
The line itself is covered step by step in how a PCB assembly line works: stencil, pick-and-place, reflow, AOI and where through-hole slots in after them. For small first runs, see prototype PCB assembly.
Frequently asked questions
What is the difference between SMT and through-hole assembly?
Surface-mount technology (SMT) solders parts onto pads on the board surface, placed by machine and soldered in a reflow oven. Through-hole technology (THT) pushes component leads through drilled, plated holes and solders them on the far side, by wave, selective soldering or hand. SMT is smaller, faster and cheaper per joint. Through-hole is stronger against mechanical load and easier to rework by hand.
Is SMT PCB assembly cheaper than through-hole?
Per joint, usually by a wide margin. JLCPCB's published September 2026 rates put machine SMT joints at $0.0016 each and manual joints at $0.0164, about ten times more. The gap narrows at very low volume, where setup and stencil fees dominate, and on boards where one or two connectors are the only through-hole parts. Count joints before deciding.
Which parts should stay through-hole?
Parts that take mechanical force or carry high current: board-edge connectors that get plugged and unplugged, terminal blocks, large electrolytic capacitors, transformers, heavy inductors and some relays. A through-hole lead anchors the part in the laminate, while an SMT joint relies on pad adhesion. Many connectors now come in SMT bodies with through-hole hold-down pegs, which split the difference.
Can flexible PCBs be assembled with SMT?
Yes. Flexible and rigid-flex boards are routinely assembled on SMT lines, but they need extra handling. The polyimide film is held flat on a rigid carrier or pallet, the board is often baked to drive out moisture before reflow, and stiffeners are added under component areas so joints do not crack when the circuit bends. Expect higher setup cost and a longer quote cycle.
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