PCB Assembly Process: From BOM Review to Tested PCBA

Check that the BOM, centroid, fabrication files and assembly drawings share the same revision. Confirm manufacturer part numbers, package sizes, polarity, DNP positions and approved alternates. Resolve discrepancies before kitting; a correct part value in the wrong package cannot be fixed by placement programming.

Incoming material review should address quantity, packaging, traceability and moisture handling as applicable. Consigned and turnkey builds have different sourcing responsibilities, so document who supplies shortages, spares and replacements. See the turnkey RFQ checklist.

A stencil controls solder-paste deposition on SMT lands. Aperture size, stencil thickness, paste choice and board support must suit both large thermal pads and small components. A single global paste reduction is rarely a complete design rule for a mixed BOM.

Where specified, solder-paste inspection evaluates deposit location, area or volume before components hide the print. Printing defects can lead to bridges, opens or insufficient joints later. Confirm special aperture strategies for exposed pads with the assembler rather than changing the fabrication pad geometry to compensate informally.

Placement machines use package libraries, feeder assignments, centroid coordinates and fiducials. Rotation and origin conventions need verification, especially for polarized parts and bottom-side components. A first-article check should catch interpretation errors before the remaining lot is assembled.

Reflow heats the assembly through a controlled profile suitable for solder paste, components and the board. Use the material and component limits to develop the profile; one universal peak temperature is not appropriate for every BOM. Double-sided boards may require a second printing, placement and reflow sequence with a plan for already mounted parts.

Through-hole parts may be soldered by wave, selective or manual methods, depending on volume, access and thermal constraints. Fixtures, masking and process order should protect components already on the board. Press-fit connectors have their own tooling and acceptance needs and are not ordinary soldered joints.

Cleaning, conformal coating, staking, heatsinks and mechanical hardware belong in the process scope when required. “No-clean” flux does not automatically eliminate cleanliness review for every application, especially where coating adhesion or leakage is critical.

Agree inspection coverage and acceptance criteria before production. Optical inspection cannot see every hidden joint, and X-ray does not establish that firmware and circuit functions are correct. Functional test requires defined stimuli, equipment and limits.

Define what the delivery package must contain: assembly revision, quantity, inspection or test records, serial numbers where required, firmware version, deviations and packaging. First-article approval may be a release gate for the balance of the lot rather than a document produced after everything is finished.

For an RFQ, supply the BOM, centroid, assembly drawing, fabrication package, DNP list, sourcing model and requested test scope. Identify who owns fixtures and test software. The Gerber/BOM/placement checklist helps close file gaps before requesting an assembly quote.