Circuit Construction & Assembly Methods
From PCB to Finished Circuit
Producing the bare PCB is only the first half of circuit manufacture. The second half is circuit assembly — mounting components onto the board and creating reliable electrical connections between them. The OCR specification identifies three assembly methods: surface mount technology (SMT), through-hole construction, and manual soldering. Each has distinct characteristics, advantages, and disadvantages, and the right choice depends on the scale of production, the components used, and the application.
Surface Mount Technology (SMT)
Surface mount technology (SMT) is the dominant method in modern commercial electronics manufacture. SMT components have no wire leads — instead, they have small metal terminations on their bodies that sit directly on copper pads on the surface of the PCB. Two automated processes are used: first, solder paste (a mixture of tiny solder particles and flux) is printed onto the pads using a stencil. Then, a pick and place machine uses vacuum nozzles to pick individual components from reels or trays at high speed and place them precisely onto the solder paste. The board then passes through an oven that melts the solder paste and fixes the components; this is called reflow soldering (the specification calls it flow soldering). SMT allows very compact circuit layouts (components are much smaller than through-hole equivalents), very high assembly speeds, and both sides of a PCB to be populated. The disadvantages are that SMT components are extremely small and difficult to handle or solder by hand; specialised, expensive equipment is required; and reworking or replacing individual components is more challenging than with through-hole parts.
SMT assembly has two automated stages: (1) pick and place — a robotic machine places tiny components onto solder paste printed on the PCB pads; (2) reflow soldering (flow soldering in the specification) — the board passes through a reflow oven that melts the paste SMT advantage: very compact, fast, suited to mass production. SMT disadvantage: components are too small to solder by hand; expensive automated equipment is required.
Through-Hole Construction
Through-hole construction uses components with wire leads that are pushed through holes drilled in the PCB and soldered to copper pads on the underside. Through-hole components are physically larger than SMT equivalents and create mechanically very strong joints — the lead passing through the board provides a physical anchor. This makes through-hole construction suitable for components that experience mechanical stress, vibration, or repeated connection/disconnection — for example, connectors, power transistors, large capacitors, and switches. Through-hole assembly is also far easier to carry out by hand, making it the standard approach in schools, repair workshops, and for prototyping. The disadvantages are that the holes occupy board space on both layers, limiting routing density, and the components are bulkier, preventing the same miniaturisation achievable with SMT.
Modern commercial electronics predominantly use SMT because the small component size allows smartphones, laptops, and wearables to be as compact as they are. However, through-hole components are still found even on modern PCBs — particularly for connectors (USB ports, power sockets), high-power components (large electrolytic capacitors, power MOSFETs), and mechanical mounting points that benefit from a physically anchored joint. A PCB that uses both SMT and through-hole components is called a mixed-technology board. For the OCR exam, always compare the two based on size, mechanical strength, ease of manual assembly, and suitability for mass production.
Manual Soldering
Manual soldering involves a technician using a soldering iron (typically heated to between 320°C and 380°C) and solder wire to join component leads to PCB pads by hand. The iron tip heats both the component lead and the pad simultaneously; solder wire is then fed into the joint, melts, and flows around the heated surfaces. When the iron is removed, the solder cools and solidifies, creating both an electrical and a mechanical connection. A good solder joint is shiny, smooth, and volcano-shaped. A poor joint — called a cold (dry) joint — is dull, grainy, and has high resistance. Manual soldering is used for through-hole prototyping, repair work, and assembly of low-volume or bespoke circuits. It is unsuitable for SMT components at the normal production scale because the components are too small and the process is too slow for mass manufacture. Safety: the iron tip reaches temperatures that cause immediate burns — always use a stand, avoid touching the tip, and work in a ventilated area to avoid inhaling flux fumes.
Three assembly methods: SMT (tiny surface-mounted components, pick-and-place + reflow oven, mass production — but expensive equipment and difficult to hand-solder); through-hole (leads through drilled holes, strong mechanical joint, easy to hand-solder — but larger components, less dense layout); manual soldering (hand-applied with soldering iron, used for prototypes and repairs — but slow, requires skill, unsuitable for SMT at scale).
A good solder joint must be electrically conductive, mechanically secure, and resistant to fracture in use. In practice, a well-made through-hole joint should be shiny (indicating the solder flowed and bonded correctly), smooth, and volcano-shaped — broader at the pad and tapering up the component lead. A cold joint forms when the pad or lead is insufficiently heated before solder is applied; the solder solidifies before flowing properly, leaving a dull, crumbly surface and a high-resistance connection that may fail intermittently. Other defects include solder bridges (solder accidentally connecting two adjacent pads, causing a short circuit) and insufficient solder (leaving a weak joint). Visual inspection under magnification is the primary quality check for manual soldering in a workshop environment.