How Does SMT PCB Assembly Work?

Detailed Explanation

Surface mount assembly (SMT assembly or PCBA — Printed Circuit Board Assembly) is the factory process that turns a bare, unpopulated PCB and a bag of components into a working circuit board. Understanding what happens at each stage helps designers make better decisions about component selection, footprint design, panelisation, and the fabrication output package they submit.

Stage 1 — Solder Paste Printing

A metal stencil (typically 0.1–0.15 mm thick laser-cut stainless steel) is aligned over the bare PCB. A squeegee pushes solder paste through the stencil apertures onto the exposed copper pads. Each aperture position and size matches a pad on the PCB layout; the paste volume deposited at each pad is controlled by the aperture area and stencil thickness.

Solder paste is a mixture of tiny solder alloy spheres (typically SAC305: 96.5% tin, 3% silver, 0.5% copper for lead-free) suspended in a flux carrier. The paste serves two functions: it positions the components before reflow (the surface tension of the wet paste holds small parts in place), and it provides the solder that forms the joint.

What can go wrong here: Insufficient paste (too small an aperture, paste bridging on fine-pitch pads), or paste deposited on the wrong pads. The stencil design page covers aperture ratio rules and how stencil thickness affects paste volume.

Stage 2 — Automated Component Placement (Pick-and-Place)

Pick-and-place machines use vacuum nozzles to pick components from tape-and-reel feeders (or trays for larger parts) and place them at precise positions on the paste-covered pads. Placement accuracy for modern machines is typically ±25–50 µm — accurate enough for 0201 passives and fine-pitch ICs.

The machine reads the centroid (pick-and-place) file to know each component's XY position, rotation, and layer (top or bottom). This is why the centroid file is mandatory for automated assembly — without it, the operator must locate every component manually.

Component placement rates range from a few thousand to over 100,000 components per hour for high-volume machines, which is why SMT is far more cost-effective at volume than hand assembly.

Stage 3 — Reflow Soldering

The populated board enters a reflow oven — a long tunnel with heating zones set to a controlled temperature profile. A typical lead-free profile has four zones:

The paste transitions from a grey slurry to shiny metallic solder joints. Flux residues (from the paste) remain on the board after reflow; some paste types produce "no-clean" residue that can remain without electrical effect; others require washing.

What can go wrong here: Tombstoning (small components standing on one end because one pad reaches liquidus before the other), solder bridging (paste from adjacent pads merging), cold joints (insufficient heat or bad wetting). These defects are closely related to footprint design, component placement, and stencil aperture design.

For how to tune the four zones to your specific board, paste alloy, and component mix, see How to Design a Reflow Profile for PCB Assembly.

Stage 4 — Automated Optical Inspection (AOI)

After reflow, the board passes through an AOI machine — a camera-based inspection system that photographs every component placement from multiple angles under controlled lighting. AOI can detect:

• Missing components
• Incorrect component polarity (for polarised parts — electrolytic capacitors, diodes, LEDs)
• Lifted leads on ICs
• Solder bridges between adjacent pads
• Insufficient solder (visually obvious cases)

AOI cannot detect: opens under BGAs or other components where the joint is hidden, functional failures, or marginal joints that look correct optically. The PCB assembly testing page covers the full inspection and testing sequence.

Bottom-Side and Mixed-Technology Assembly

If components are placed on both sides of the board, the assembly line runs twice — top side first (paste print, place, reflow), then flip the board, paste print the bottom side, place, and reflow again. The bottom-side reflow must be hot enough to solder the new components without dropping components already attached on the top side — which is why bottom-side reflow component weight and pad geometry matter.

For through-hole components mixed with SMT, the through-hole parts are typically hand-inserted or machine-inserted after SMT reflow and soldered by wave soldering or selective soldering.

What the Designer Must Provide

For a complete assembly job:

• Gerber files — bare board geometry (see PCB fabrication files)
• Pick-and-place (centroid) file — one row per component: reference designator, X centre, Y centre, rotation in degrees, side (top/bottom)
• Bill of materials (BOM) — reference designator, manufacturer part number, value, footprint, and quantity
• Stencil data — the paste layer (usually F.Paste / B.Paste in KiCad) is included in the Gerber package; the assembly house uses it to fabricate the stencil
• Assembly drawing — for any component with polarity, orientation, or insertion direction not obvious from the centroid data

Design Decisions That Affect Assembly

• Component orientation consistency — aligning all passives in the same direction reduces pick-and-place head rotation time and makes hand-inspection easier
• Minimum spacing between components — smaller spacing increases tombstoning risk and makes rework harder; PCB component placement best practices covers recommended clearances
• Panel vs individual boards — boards under ~50 mm in either dimension usually need to be panelised; see PCB panelisation
• Surface finish — the finish on the copper pads directly affects solderability; see PCB surface finishes

For complex or first-time assemblies, Zeus Design's rapid prototyping service manages the output package preparation, assembly house liaison, and first-article inspection.

Design Considerations

• Match footprints to IPC standards: IPC-7351 defines land pattern dimensions optimised for SMT assembly yields. Footprints copied from manufacturer drawings may be accurate for the component but not optimised for paste volume and wetting — EDA library-standard footprints are usually a safer starting point.
• Keep large thermal-mass components away from fine-pitch ICs: Heavy components near fine-pitch ICs can cause localised temperature lag in the reflow zone, producing cold joints on the IC pads. Review the component placement with thermal mass in mind, not just routing convenience.
• Design for rework access: Leave clearance around every IC and connector for a hot-air rework nozzle. A component placed against a board edge or tightly surrounded by other components may be effectively irreparable if it fails.

Common Mistakes

• Submitting the Gerber package without a centroid file, then expecting the assembly house to manually locate all components — this adds significant labour cost and is not possible for high-volume automated assembly.
• Using non-IPC footprints from component manufacturer drawings that maximise component position accuracy but under-size the pads for good solder joint formation.
• Mixing metric and imperial units in the centroid file — many pick-and-place machines require a consistent unit; confirm with the assembly house before submitting.
• Placing components too close to the board edge, preventing the PCB from riding the conveyor rails through the reflow oven without support fixtures.