What Is Surface Mount Technology (SMT)?
Also known as: SMT, SMD assembly, surface mount assembly
Definition
Surface Mount Technology (SMT) is the method of placing and soldering electronic components directly onto the surface of a printed circuit board, instead of inserting leads through holes. It is the dominant way modern electronics are assembled.
Surface Mount Technology (SMT) Explained
In SMT, components called surface-mount devices (SMDs) have flat contacts or small terminals that sit on copper pads on the board. A typical SMT line runs in sequence: solder paste printing through a stencil, solder paste inspection (SPI), pick-and-place machines that position components, reflow soldering in a multi-zone oven, and automated optical inspection (AOI). Double-sided boards repeat the sequence for the second side.
SMT replaced most through-hole assembly because parts are smaller, boards are denser, and machines can place thousands of components per hour with consistent accuracy. Through-hole parts still appear for connectors, large capacitors and mechanical stress points, so many boards are mixed technology: SMT first, then wave or selective soldering for the through-hole parts.
From an ERP point of view, an SMT line is a routing of tightly coupled operations, usually one work center per line rather than per machine. The bill of materials can carry hundreds of reference designators per board, and the feeders loaded on the placement machine must match the BOM revision. Machine programs, stencil IDs and oven profiles are controlled alongside the item revision.
ERPs such as Infor SyteLine, Infor LN and NetSuite hold the work order, BOM and inventory transactions for the SMT build, while the line itself is normally driven by machine software and a manufacturing execution system (MES) that reports completions and scrap back through an integration.
Why It Matters
- SMT yield problems (tombstoning, bridging, insufficient solder) show up as scrap and rework cost that ERP standard costing must absorb.
- Feeder setup and changeover time on an SMT line often decides how small a lot size can be run economically, which feeds lot sizing and scheduling.
- Reel-level component tracking is needed to trace a failing board back to the exact parts and the line that built it.
- Moisture sensitive parts and limited reel shelf life mean SMT planning is tied to purchasing and inventory discipline, not just production.
In Practice
A 4-layer controller board has 180 SMT placements across 40 unique part numbers. The work order for 500 boards releases kitted reels to the line. The first-article board is inspected, then the lot runs. If AOI flags 6 percent of boards for bridging on one fine-pitch part, the cause is usually stencil aperture or paste volume, and the lot is held for rework before final test.
Frequently Asked Questions
What is the difference between SMT and through-hole assembly?
SMT places components on the board surface and solders them in a reflow oven. Through-hole assembly inserts leads through drilled holes and solders them, usually by wave or selective soldering. Many boards use both.
What are the main steps of an SMT line?
Solder paste printing, paste inspection, pick-and-place, reflow soldering and automated optical inspection, with X-ray or in-circuit test added for hidden joints.
How does an ERP connect to an SMT line?
The ERP issues the work order and BOM, while machine software or an MES reports quantities, scrap and component usage back, typically through an API or file-based integration.
Related Terms
PCBA (Printed Circuit Board Assembly)
PCBA is a printed circuit board assembly: the bare board plus all electronic components soldered onto it. A PCB is only the fabricated board with copper traces; it becomes a PCBA once it is populated, soldered and usually tested.
Reflow Soldering
Reflow soldering is the process that melts pre-applied solder paste in a controlled-temperature oven so surface-mount components bond to the board pads. The time and temperature curve is called the reflow profile.
BOM (Bill of Materials)
A BOM (bill of materials) is the structured, hierarchical list of components, sub-assemblies, and quantities required to produce one unit of a parent item, and it is the primary input MRP explodes to calculate material requirements.
Traceability
Traceability is the ability to follow the history, location, and application of a part or material through the supply chain and production process, both backward to its sources and forward to every product and customer it reached.
First Pass Yield
First pass yield is the percentage of units that complete a process correctly the first time, without rework, repair, or retest. Units that eventually pass after rework are excluded, so the metric captures hidden factory loss.
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SMT Line Balancing Worksheet
List each SMT line station with its cycle time and see the bottleneck, line UPH and balance efficiency.
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Free tools
- SMT Line Balancing WorksheetList each SMT line station with its cycle time and see the bottleneck, line UPH and balance efficiency.
- SMT Process Control Maturity AssessmentScore how well your SMT line controls printing, placement, reflow, traceability and feedback, and see where to improve first.
- PCB Assembly Cost EstimatorEstimate SMT and through-hole assembly cost per board and total order cost from placements, complexity, NRE, and test overhead.
Guides
- SMT Line Scheduling and Capacity Planning for EMS ShopsSchedule an SMT line in a high-mix EMS shop: changeover and feeder setup, clear to build, stencil and reflow bottlenecks, and honest capacity numbers.
- How to Quote PCB Assembly From a Customer BOMStep by step PCB assembly quoting from a customer BOM: part matching, price breaks, lead time risk, overage and attrition, and quote validity for EMS shops.
- Plating Pricing Methods: Surface Area, Rack, Barrel or Per Piece?How metal finishing shops price plating and anodizing by surface area, rack, barrel load or piece, with a worked example and the margin traps in each method.
Manufacturing hubs
- SMTA and the surface mount ecosystem: process knowledge, events and the data behind an SMT lineSMTA explained for SMT and electronics assembly teams: conferences, chapters, process topics, and the ERP, traceability and private AI issues on SMT lines.
- NASF and surface finishing shops: standards, specs and ERP realitiesNASF and the surface finishing ecosystem: plating, anodizing and coating job shops, Nadcap and customer specs, and the ERP, quoting and private AI problems they face.
Software
- Cogiscan TTCWhat Cogiscan Track, Trace and Control is: SMT material tracking, line control and traceability, how it links to ERP and MES, limits and alternatives.
- Aegis FactoryLogixAegis FactoryLogix is a manufacturing execution platform strong in electronics and high-mix assembly. See modules, integrations, limits and alternatives.