Surface mount technology is the foundation of modern electronics manufacturing. Whether you are holding a smartphone, a medical monitor, or an industrial control unit, the circuit board inside almost certainly owes its existence to SMT. Yet for many people commissioning electronics for the first time, the term remains a little opaque.
This guide covers everything you need to know. What SMT is, how it works, why it has become the dominant assembly method, where its limits lie, and what to look for when choosing a manufacturing partner. By the end, you will have a clear picture of the process and how it applies to your project.
What is surface mount technology?
Surface mount technology is a method of assembling electronic components by placing them directly onto the surface of a printed circuit board. The components used are called surface-mount devices (SMDs). They are designed without wire leads, sitting flat against the board and held in place by solder connections on small metal pads.
Before SMT became widespread, through-hole assembly was the standard approach. Components had long wire leads that passed through drilled holes in the board and were soldered on the reverse side. Through-hole assembly is still used today for certain component types, but it has clear limitations: it is slower, takes up more space, and is far harder to automate.
SMT removes those constraints. Because no holes are needed, components can be placed on both sides of the board, packed much more closely together, and handled entirely by automated machinery. The boards that come out of an SMT line are smaller, more capable, and produced more consistently than anything through-hole assembly alone could achieve.
A brief history of SMT
SMT has its roots in the 1960s, when IBM developed early surface mount techniques for use in the guidance computers of the Saturn V programme.
The technology was refined through the 1970s and began to enter commercial electronics production in the early 1980s, driven largely by the demand for smaller consumer electronics.
By the 1990s, SMT had become the dominant assembly method across the industry. Component sizes shrank dramatically, pick-and-place machines became faster and more precise, and the infrastructure around SMT, including standardised components, stencil printing, and automated inspection, matured into the reliable industrial process it is today.
For a UK contract manufacturer like Intercole, that evolution has meant decades of investment in equipment, process knowledge, and engineering expertise. SMT is not a single technique but an ecosystem of interconnected processes, and doing it well requires experience at every stage.
How the SMT assembly process works
SMT assembly follows a defined sequence of stages. Each step is critical, and the quality of the finished board depends on everything being done correctly throughout.
Solder paste application
The process begins with solder paste printing. A thin metal stencil is aligned precisely over the bare PCB. The stencil has apertures that correspond exactly to the solder pads on the board. Solder paste, a mixture of fine solder particles suspended in flux, is pushed through the stencil using a squeegee blade, depositing a controlled amount of paste onto each pad.
The accuracy of this stage sets the tone for everything that follows. Too much paste or a misaligned stencil leads to bridging faults or poor joints. Many assembly lines include solder paste inspection at this point to catch any issues before components are placed.
Component placement
With paste applied, the board moves to the pick-and-place machine. This is where the speed and precision of SMT becomes apparent. Modern pick-and-place machines can place thousands of components per hour, drawing from reels, tubes, or trays, and positioning each one onto its corresponding pads to within fractions of a millimetre.
The machine uses a vision system to verify component orientation and position before placement. For complex boards with fine-pitch components or BGAs, this precision is what makes reliable assembly possible at all.
Reflow soldering
Once all components are placed, the board passes through a reflow oven. The oven runs a carefully controlled temperature profile, gradually raising the board temperature until the solder paste melts and flows around the component leads and pads. As the board cools, the solder solidifies to form reliable electrical and mechanical connections.
The temperature profile is tailored to the board and the components on it. Getting this right matters: too much heat can damage sensitive components, whilst too little leaves cold joints that fail prematurely.
Inspection
After reflow, automated optical inspection checks every board. AOI machines use cameras and image processing to verify that each component is present, correctly oriented, and properly soldered.
Any board that does not pass is flagged for manual review and rework before it progresses.
Depending on the application, additional testing follows inspection. Board and final product testing confirm that the assembled board performs as intended under real operating conditions, not just that it looks correct visually.
Types of components used in SMT
SMT components come in a wide range of package types, each suited to different applications and board densities. Understanding the main categories helps you appreciate what is involved in assembling a complex board.
Passive components
Resistors, capacitors, and inductors make up the majority of components on most SMT boards. They are available in standardised sizes, known as imperial codes such as 0402 or 0201, which refer to their dimensions in hundredths of an inch. The smallest modern SMT passives are barely visible to the naked eye.
Active components and ICs
Integrated circuits, transistors, and other active components come in a range of surface-mount packages. Flat packages, such as SOICs and QFPs, have leads around their perimeters. More advanced packages, such as BGAs, have an array of solder balls underneath the component, allowing a very high density of connections in a small footprint.
BGAs and fine-pitch components
Ball grid array components require particular care during assembly. The solder joints are hidden beneath the component, so they cannot be inspected visually after reflow in the conventional sense. X-ray inspection is used to verify BGA joints, and the reflow profile must be controlled carefully to ensure all balls reflow evenly. This is an area where experience and the right equipment make a significant difference.
SMT versus through-hole assembly
SMT and through-hole assembly are not competing methods so much as complementary ones. Most real-world PCBs require both.
Through-hole components remain the right choice for connectors, large power components, and parts that need to withstand mechanical stress. A USB socket or a power connector benefits from the physical strength that through-hole mounting provides. SMT components, by contrast, are not always well suited to high-stress mounting points.
Where boards combine both methods, conventional PCB assembly handles the through-hole stage, whether that is wave soldering or selective soldering, whilst SMT handles the surface mount population. Managing both processes within a single production flow is one of the things that distinguishes a capable contract manufacturer from a more limited one.
For a straightforward comparison:
- SMT: smaller components, higher density, faster automated assembly, lower cost at volume, suited to both sides of the board
- Through-hole: stronger mechanical joints, better for large or high-power components, slower and more labour-intensive, still essential for many designs
The benefits of SMT assembly for your product
If you are deciding how your product should be manufactured, here is a clearer look at what SMT delivers in practice.
Miniaturisation
SMT is what makes compact electronics possible. Without it, the devices we use every day would be many times larger and heavier. For product designers working in sectors such as medical devices, wearables, or aerospace, the ability to reduce board size without sacrificing functionality is often central to the whole design brief.
Production speed and scalability
Automated SMT lines can assemble boards at a rate that manual processes cannot approach. This matters both for time to market and for cost at volume. Once a programme is set up, it can be repeated consistently across thousands of boards with no meaningful variation in quality. Scaling production up or down is straightforward because the process is not dependent on individual operator skill.
Quality and consistency
Because every stage of SMT is controlled and verifiable, the quality of the output is more consistent than manual assembly. Automated inspection catches placement errors and soldering faults at the point they occur, not after the product has been built up into a finished assembly. The earlier a fault is caught, the cheaper it is to correct.
Cost efficiency
SMT components generally cost less than their through-hole equivalents. Assembly is faster. Inspection is automated. Rework rates are lower when processes are well controlled. The cumulative effect is that SMT is significantly more cost-efficient for most medium to high volume production runs, and increasingly competitive even at lower volumes as equipment and processes have matured.
Where SMT requires care
SMT is a mature and reliable process, but it is not without its demands. Understanding where care is required helps you ask the right questions of your manufacturing partner.
- Thermal management. The reflow profile must be controlled carefully. Mixed assemblies with heat-sensitive components alongside those that need full reflow require experience to get right.
- Fine-pitch and BGA assembly. High-density designs with very small or complex components need precise equipment and skilled process engineers to assemble reliably.
- Stencil design. The solder paste stencil must be designed correctly for the board. Poor aperture design leads to paste issues that no amount of downstream correction can fully recover.
- Component availability. SMT components are subject to supply chain pressures. A manufacturing partner with strong procurement capability can protect you from shortages that would otherwise halt production.
What to look for in an SMT assembly partner
Choosing the right contract manufacturer for your SMT assembly is one of the most important decisions in your production process. Here are the things worth looking at closely.
Equipment capability
Does the manufacturer have modern pick-and-place equipment capable of handling the component types in your design? Can they assemble BGAs, fine-pitch ICs, and very small passives? Equipment capability determines what is and is not possible.
Quality accreditation
ISO 9001 accreditation is a baseline indicator that a manufacturer has documented, audited quality processes. It does not guarantee good work, but it does mean the processes are structured and controlled. Ask about their inspection regime and what happens when a board fails.
Breadth of services
SMT assembly is rarely all that a project needs. PCB layout and design, component procurement, production engineering, and testing and validation are all part of the journey from design to finished product. A manufacturer that handles all of these in-house simplifies your supply chain and reduces the risk of things falling between the gaps.
Experience with your sector
Electronics manufacturing for medical devices, nuclear instrumentation, and industrial control systems carries different demands to consumer electronics. Make sure your manufacturer understands the standards and expectations of your sector, not just the assembly process itself.
SMT assembly at Intercole
Intercole has been manufacturing electronics from our facility in Chandlers Ford, Hampshire, for over 60 years. Our SMT assembly service covers everything from prototype builds through to volume production runs, with the full range of component types handled in-house.
We combine SMT with conventional through-hole assembly where designs require it, and our production engineering team works closely with customers to ensure designs are optimised for manufacture before production begins.
Our ISO 9001 accredited quality processes and automated optical inspection mean you get consistent, verified output at every stage. And because we offer procurement, assembly, testing, and box build all under one roof, you deal with one partner rather than managing a chain of suppliers.
We work with technology-driven businesses of all sizes, from entrepreneurial companies developing new products to large multinationals that need a reliable UK-based manufacturing partner they can depend on.
If you have a project that needs SMT assembly, we would be glad to help. Get in touch with the Intercole team to talk through your requirements.
