PCBA vs bare PCB: what you are actually ordering from a fab

A PCB is the board; a PCBA is the populated assembly

A bare printed circuit board (PCB) is the laminated and patterned board before components are fitted. It contains dielectric material, copper layers, plated holes, solder mask, silkscreen, and the specified surface finish. Pads are empty.

A printed circuit board assembly (PCBA) is that board after the required components have been placed and soldered. Depending on the quote, assembly may also include component purchasing, stencil manufacture, inspection, programming, functional test, depanelization, cleaning, conformal coating, or packaging. None of those extras should be assumed from the letters PCBA alone.

When a low-cost PCB listing says “five boards,” it almost always means five bare boards. Components shown in a 3D preview are not included unless an assembly service is explicitly configured.

What a bare-board order consumes

The core bare-board data is:

• Gerber or another accepted fabrication-data format for every copper, solder-mask, silkscreen, and outline layer;
• Excellon or equivalent drill/rout data;
• order selections for material, layer count, stackup, thickness, copper weight, finish, mask, and special processes;
• a fabrication drawing or notes for tolerances and features the standard files do not communicate unambiguously.

The fab images and etches copper, laminates layers where applicable, drills and plates holes, applies finish, mask, and legend, routes or scores the outline, and electrically tests according to its service. It does not read your schematic to decide whether a missing trace should exist.

The two-layer stackup guide explains what is physically inside a typical simple bare board and which order choices alter it.

What assembly adds to the data package

An assembler needs to know what goes on every fitted location and where it goes. The usual additions are:

• BOM: references, quantities, exact manufacturer part numbers, and DNP state;
• pick-and-place or centroid file: reference, X/Y coordinate, board side, and rotation;
• paste layers or stencil data: apertures used to print solder paste for SMT;
• assembly drawings: polarity, pin one, DNP markings, special placement notes, and often board revision;
• programming and test instructions: only when those services are ordered.

Some assembly portals transform the BOM and placement CSV into their own templates. That formatting step must not change component identity or reference mapping. A KiCad BOM with real part numbers is the starting point; 100nF alone is not sufficient purchasing data.

Turnkey, consigned, and mixed sourcing

Assembly quotes differ in who buys parts:

• Turnkey: the assembler procures the components and boards.
• Consigned or kitted: you send some or all components to the assembler.
• Mixed: the assembler buys standard items while you supply constrained or preprogrammed parts.

Clarify acceptable distributors, alternates, date codes, moisture-sensitive handling, reel or cut-tape quantities, and ownership of unused parts. The price of components is separate from placement cost, and minimum purchase quantities or full-reel requirements can dominate a small run.

A turnkey quote does not transfer component-selection responsibility. You still approve exact MPNs and substitutions. A physically compatible “equivalent” can differ in pinout, voltage rating, oscillator tolerance, firmware, temperature grade, or regulatory status.

SMT, through-hole, and mixed assembly are different operations

For SMT, solder paste is printed through a stencil, components are placed, and the board is reflowed. Through-hole parts may be hand-soldered, selectively soldered, or wave-soldered. A mixed board can require multiple passes and manual steps.

Assembly pricing and feasibility depend on:

• unique line-item count and total placements;
• smallest package and fine-pitch requirements;
• single-sided versus double-sided placement;
• through-hole and hand operations;
• BGA, QFN, bottom-terminated, or unusual packages;
• panel format and fiducials;
• inspection and test requirements;
• cleaning or coating process.

Do not assume a fab’s bare-board trace capability implies its assembly line can place every package. Board fabrication and component assembly have different machines and design rules.

DNP and variants need explicit treatment

An optional resistor footprint may be present on the PCB but intentionally unpopulated. Mark it Do not populate in KiCad and carry that state consistently through BOM, placement, paste, and assembly drawing according to the assembler’s requirements.

For product variants, produce a controlled BOM and placement population for each variant. Avoid notes such as “fit R3 for version A unless U2 is type B.” A reference should have one clear state in a given build: fitted with an exact MPN, or DNP.

The bare board can often be shared among variants, which is useful, but the PCBA revision must identify both the board revision and the population variant.

Inspection is not functional test

Standard assembly may include automated optical inspection, X-ray for hidden joints, or a workmanship check. These processes can find placement, polarity, and solder defects. They do not prove the circuit boots, measures correctly, communicates, or survives load.

Functional test requires a specification, fixture or connection method, firmware, limits, expected results, and a policy for failures. Programming requires files, version identification, connector access, power conditions, and sometimes security keys. If these items are absent from the quote and work instructions, expect unprogrammed assemblies that have been visually inspected—not finished products.

Plan test points, programming access, fixture keepouts, and serial-number handling during design. Adding them after the PCBA arrives is expensive.

Quote the complete build you need

Before ordering, ask what the price includes:

• bare boards and any fabrication setup;
• stencil;
• component cost, attrition, and procurement fees;
• SMT and through-hole placement;
• special package handling;
• panelization and depanelization;
• inspection, X-ray, electrical test, programming, and functional test;
• cleaning, coating, labeling, and packaging;
• shipping of finished and unused materials.

Then run a DFM review for the exact order. A correct bare-board package is necessary for PCBA, but it is only the substrate. The assembly becomes orderable when part identity, placement, process, population, inspection, and test expectations are equally explicit.