What is PCB assembly?
PCB assembly is the placing and soldering of electronic components onto a bare printed circuit board. The result is called an assembly or PCBA (printed circuit board assembly), the bare board on its own is not one yet. There are two placement technologies: SMD, where components sit on pads on the surface, and THT, where component leads are pushed through drilled holes. Most assemblies are mixed.
SMD assembly
SMD stands for surface-mounted device. The component is not pushed through the board but set onto pads on its surface. The process runs as one continuous line: a stencil printer first prints solder paste through a metal stencil onto the pads; a pick-and-place machine then sets the components into the still-soft paste, at tens of thousands of placements per hour on a modern machine; the assembly passes through a reflow oven where the paste melts along a defined temperature profile and every joint forms at once; automated optical inspection (AOI) checks the result. SMD dominates today because components can be placed on both sides, densely, and without a drilled hole, package sizes down to 0201 or 01005 are beyond manual handling.
THT assembly
THT stands for through-hole technology. The component's leads go through drilled holes in the board and are soldered on the far side, in a wave solder bath, joint by joint in selective soldering, or by hand. THT costs more effort and is still needed wherever a joint carries mechanical load or high current: connectors, relays, power devices, larger electrolytic capacitors, terminal blocks. The difference from SMD is not only technical but commercial: one THT position costs a multiple of one SMD position, because it usually pulls in its own handling and its own process step.
What makes assembly expensive or cheap
The price does not come from a flat rate per component but from the work the layout forces. What matters: the number of placements and the package sizes (very small and very large parts run slower); whether both sides are populated, which means a second complete pass; the number of THT positions and whether they are selectively soldered or hand-fitted; the number of distinct part numbers, because each one occupies a feeder slot and a setup action; the panelization, meaning how many boards are combined into one production panel; and the lot size, across which the setup cost is spread. On small quantities the one-off costs therefore dominate regularly, see NRE costs, not the component prices.
Common misconceptions
A populated board is an assembly, not a "board": in everyday use, "board" usually means the bare one. Assembly is also not the same as board fabrication: the bare PCB is manufactured by a board house, and populated afterwards, often by a different company. And "SMD assembly from lot size 1" describes technical feasibility, not economics: it is feasible, but it only makes sense once setup effort and batch size fit each other.
Where this shows up in PCBoffer
PCBoffer does not sell assembly, it calculates it: placement time per component from the pick-and-place data, selective soldering time, manual work per THT position with the component's packaging taken into account, each of them mapped onto the machine park of one specific plant rather than onto industry averages. For a development team that means the cost difference between two layout variants is visible before an enquiry goes out.
PCBoffer for OEMsWhether the machine can still place a package, and how many feeder lanes one reel occupies, is decided before the first solder joint. The package table lists 50 classes with their feeder lanes, plus size, tape width and placement accuracy wherever the costing holds them; free and without a sign-up.
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