Surface Mount Technology (SMT) assembly is the foundation of modern electronics manufacturing. SMT places components directly onto the PCB surface pads — no drilled holes required — enabling smaller components, higher packing density, faster automated placement and lower per-board assembly cost compared to through-hole technology. AstroPCB provides full SMT assembly services from prototype quantities through production volumes, handling components from 01005 passives (0.4 x 0.2 mm) to fine-pitch BGAs with 0.3 mm ball pitch.

SMT Assembly Process

The SMT process follows a defined sequence: solder paste printing, component placement, reflow soldering and inspection. Each step is controlled by process parameters that must be optimized for the specific board and component mix.

Solder paste printing: A laser-cut stainless steel stencil aligned over the board applies solder paste to every SMT pad in a single print stroke. Stencil thickness (typically 100-150 microns) and aperture design control the paste volume deposited on each pad. Solder paste inspection (SPI) measures the volume, height and alignment of paste on every pad before components are placed — catching printing defects at the earliest possible stage, where correction costs nothing.

Component placement: High-speed pick-and-place machines retrieve components from tape reels, trays and tube feeders and place them on the solder paste with positional accuracy of ±25-50 microns. Placement rates of 20,000-60,000 components per hour are typical. The machine vision system verifies component orientation and polarity during placement, rejecting misoriented or incorrect parts before they reach the board.

Reflow soldering: The populated board passes through a reflow oven with a controlled thermal profile: preheat (gradual temperature rise to activate flux), thermal soak (temperature equalization across the board), reflow (peak temperature melts solder paste and forms joints) and controlled cooling. The profile is optimized for the specific solder alloy, component thermal mass and board construction to produce reliable joints without thermal damage to components or the PCB.

Inspection and Quality

Automated optical inspection (AOI) after reflow examines every solder joint for presence, alignment, bridging, insufficient solder and tombstoning. X-ray inspection examines solder joints hidden beneath BGA, QFN, LGA and other bottom-terminated packages where optical inspection cannot see the connection. For production builds, first-article inspection validates the assembly setup before the full lot is processed.

Fine-Pitch and Advanced Package Assembly

BGAs with pitch down to 0.3 mm, QFNs with exposed thermal pads, chip-scale packages (CSP), Package-on-Package (PoP) and 01005 passives require tighter process control: thinner stencils with smaller apertures, higher placement accuracy, optimized reflow profiles and mandatory X-ray inspection. The engineering team reviews the BOM and board design before assembly to identify any components or pad geometries that require process adjustments.

Double-Sided SMT

Boards with SMT components on both sides are assembled in two passes. The first side is printed, placed and reflowed. The board is flipped and the second side undergoes the same sequence. The solder joints from the first pass must survive the second reflow cycle without defects — the reflow profile and component weight distribution are managed to prevent the first-side joints from reflowing and allowing components to shift or fall.

Request an SMT Assembly Quote

Upload Gerber files, BOM (with manufacturer part numbers) and centroid file. Specify quantity, turnkey/consigned preference and delivery target. The engineering team will review and return a quotation.

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