Files Needed for PCB Assembly: A Checklist

Your PCB design may appear perfect in your EDA software. The traces are routed correctly, components are placed correctly, and simulations are passed. However, this finished design remains a digital concept. The most important step is to transform that digital file set into a physical, functional board through SMT Assembly (Surface Mount Technology Assembly).

A single error in your exported PCB files, whether it's a mismatched component footprint, an incorrect rotation, or a missing solder paste layer, can lead to significant problems. These issues can halt an automated SMT line, cause costly manual rework, or even a batch of non-functional boards.

This technical guide walks you step by step through each essential component, showing how to properly prepare an accurate PCB file package required for a fast, cost-efficient, and successful SMT assembly. The accuracy of your data input directly determines the quality, speed, and overall cost of your finished hardware.

The "Big Three" PCB Files Every PCB Assembly Order Requires

A bare board PCB fabrication order only needs Gerber files. However, a complete turnkey SMT Assembly order needs three separate data sets. Each file addresses a distinct, critical question relevant to automated SMT machines.

1. Gerber Files: Define the exact location of copper traces, pads, masks, and solder paste.

2. Bill of Materials (BOM) File: Lists the actual components required for placement.

3. Centroid File(Component Placement List): Lists the exact placement coordinates and rotation for each component.

Let's take a look at each of these files in the technical detail necessary.

#1 Generating Accurate Gerber Files for PCB Assembly

Gerber files are the universal blueprints for the PCB. They are a 2D vector format that describes each physical layer of the board. For SMT assembly, some of these layers are far more critical than others.

Also Read: How to generate Gerber files in different software?

Format and Naming: Always export your Gerbers in the RS-274X (Extended Gerber) format. This is the modern standard that includes aperture information (the shape and size of flashes and draws) within the file itself, eliminating the need for a separate aperture table.

While file names can vary, a clear convention is essential for maintaining consistency.  JLCPCB’s Gerber viewer system automatically detects standard file extensions. A typical (and ideal) set of files for a 2-layer board would be:

● Top Copper Layer: .GTL

● Bottom Copper Layer: .GBL

● Top Solder Mask: .GTS

● Bottom Solder Mask: .GBS

● Top Silkscreen: .GTO

● Bottom Silkscreen: .GBO

● Board Outline: .GKO or.GML

● Top Solder Paste: .GTP (Critically Important!)

● Bottom Solder Paste: .GBP (Critically Important!)

The Most Critical Layers for SMT Assembly: The solder paste layers (.GTP / .GBP) are essential for SMT assembly. These files are used to laser-cut the SMT Stencil. Solder paste is squeegeed through the apertures of the stencil onto the pads of the board during SMT PCB assembly. Without these files, a stencil cannot be made, and assembly cannot begin.

Common Gerber Files Mistakes and How to Fix Them

1. Missing Paste Layers: This is the most frequent issue that halts the SMT assembly process.

Solution: Within your EDA software (e.g., EasyEDA, KICAD, Altium, Eagle), verify that the "Paste" layers (F.Paste, B.Paste) are activated in your Gerber export configurations.

2. Silkscreen Over Pads: When silkscreen (legend) ink is printed on top of an SMT pad, it hinders proper solder wetting, which can lead to poor joints or "tombstoned" components.

Solution: To ensure silkscreen elements do not overlap pads, use your EDA tool's Design Rule Check (DRC) to locate and move them.

3. Confusion between Solder Mask and Solder Paste: A common point of confusion arises with the solder mask and solder paste layers. The solder mask layer is negative, meaning you're specifying where to leave or expose copper and have openings. Conversely, the solder paste layer is positive, requiring you to precisely draw where paste should be deposited, ideally in a 1:1 correspondence with the copper pad.

4. Incorrect Board Outline: The outline file must be a single, continuous, closed-loop path representing the board's exact routed shape. A simple box drawn on a mechanical layer is insufficient if your board has a complex shape.

Before you even submit your SMT assembly order, you can use JLCPCB’s free Gerber Viewer. Drag your zipped Gerber files into the viewer for an instant visual sanity check. This allows you to catch layer alignment issues, missing paste layers, or board outline problems.

JLCPCB Gerber Viewer, showing a rendered PCB with all layers (copper, mask, silk, paste) correctly aligned.

#2 Creating a Machine-Readable Bill of Materials (BOM) for PCB Assembly

The Bill of Materials (BOM) is your project's shopping list. For SMT assembly, a human-readable BOM (e.g., "10k 0402 resistor") is not sufficient. The sourcing machines need an exact, unambiguous machine-readable BOM.

File Format: We recommend exporting your BOM as a .CSV (Comma-Separated Values) or .XLSX (Excel) file, as these are easily parsed by our system.

Essential BOM Columns: Your BOM must be formatted with the following columns. While the order can vary, the data is essential:

1. Designator (e.g., C1, R1, U1): This marks the location of components on your board. This must exactly match the designators in your Centroid (CPL) file.

○ JLCPCB Guideline Note: Please do not use case distinctions (e.g., r1 vs R1) to differentiate your components. All part names will be converted to uppercase during production.

2. Comment (e.g., "100nF 80% -20% 50CV"): This field specifies the component's type, specification, and tolerance (value, voltage, etc.).

3. Footprint (e.g., 0402, 0805, SSOP-8): This is the part's package.

4. Manufacturer Part Number (MPN): This single piece of data (e.g., "RC0402FR-0710KL") tells our system the manufacturer's component to purchase.

An example of a perfect BOM list

Showing how JLCPCB handles the BOM file

Common BOM Issues and How to Avoid Them

Be careful with ambiguous "range" grouping (e.g., R1-R8). This can cause errors if some numbers in that range are not used on the board. The safest and clearest methods are to either:

1. List each part on its own line (e.g., one line for R1, one for R2, etc.), or

2. Use the comma-separated format for identical parts (e.g., R1, R2, R5).

Sourcing components from JLCPCB’s in-stock Parts Library is the best way to save time and money on your SMT Assembly order. With 440,000+ components in stock, we can also source on demand for your order.

Pro-Tip for SMT Cost Reduction: By designing your circuit from the start using components found in the JLCPCB Parts Library, you can verify stock, get exact pricing, and often reduce your total SMT Assembly cost by 30-60%. When you upload your BOM, our system automatically matches your MPNs against this library and instantly flags any parts that are out of stock, "Not for SMT," or have sourcing issues.

#3 The Crucial Centroid File (Component Placement List) for PCB Assembly

This document contains specific instructions for the Pick-and-Place (PnP) Machine. It instructs the robotic head on specific X & Y coordinates, layer, and rotation measure for each element.

JLCPCB refers to this as the Component Placement List (CPL). It is also commonly known as a Centroid File or Pick-and-Place (PnP) File.

File Format: Like the BOM, this should be a .CSV or .TXT file.

Essential CPL Columns: Your CPL must contain the following information:

1. Designator (e.g., U1): This must exactly match the reference designators in your BOM.

2. X-Coordinate: The X-axis position of the component's center.

3. Y-Coordinate: The Y-axis position of the component's center.

4. Layer (e.g., Top or Bottom): Which side of the board the component is on.

5. Rotation: The component's orientation, in degrees (0, 90, 180, 270).

Pick & Place File Sample

Common Centroid File Mistakes and How to Fix Them

● The Origin Point: The most common error is a mismatch between the (0,0) coordinate in your Component Placement List (CPL) and your Gerber files. For example, if your Gerbers use the bottom-left corner as the origin while your CPL uses the center of the board, every component will be incorrectly placed.

Solution: In your EDA tool's export settings, ensure you set a common "Origin Point" for both your Gerber and Centroid file exports. Using the absolute origin of your design space or the bottom-left corner of your board outline is recommended.

● Incorrect Rotation: This is the most visible failure. A diode, LED, or IC placed with a 90- or 180-degree error will cause total circuit failure. This often happens because the component's "0-degree" rotation in your EDA library doesn't match the PnP machine's expectation for that part's reel orientation.

Solution: There is no substitute for visual verification. This is where the JLCPCB advantage comes in.

● Wrong Units: Mixing millimeters (mm) and imperial units (mils/inches).

Solution: Standardize your units. Using millimeters (mm) for all your export files (Gerbers, CPL, and drill files) is recommended.

● Centroid vs. Pin 1: The coordinate must be for the geometric center of the component, not for Pin 1. Most modern EDA tools handle this correctly, but it's a common error with custom-made footprints.

When you upload your Gerbers, BOM, and CPL to the JLCPCB SMT Assembly quote page, our system provides you with more than just a price. It generates a high-resolution, interactive visual preview of your board.

The Component Placement Tool in Gerber Viewer is located on the top bar, where users can check, approve, or manually correct the rotation of a specific component.

This preview tool is your single most powerful final check. You can zoom in and visually confirm that every component's rotation and position are correct before your order ever goes to a human for review. This feature alone saves thousands of hours of back-and-forth emails and prevents countless manufacturing errors.

JLCPCB Gerber Viewer Component Placement tool showing a board with components overlaid.

Final Checks: DFM Review & Packaging Your PCB Files for PCB Assembly

Once you have your "Big Three" files, run through this final Design for Manufacturability (DFM) checklist for SMT Assembly. JLCPCB offers a free DFM tool.

● Fiducials: Have you added fiducial markers? These are little exposed copper circles (often 1mm in size) located at the corners of your PCB. The camera for the PnP machine utilizes these as optical reference points to precisely align the board. Three fiducials on the top and bottom layers are recommended.

● Component Spacing: Ensure your components are not too dense. The PnP nozzle needs physical space to operate. Small components (SMD 0201) packed too tightly against a tall component (like a USB connector) may be impossible to place or rework.

● Panelization: If you are ordering many small boards, it's more efficient to assemble them as a "panel." You can either design this panel yourself (adding V-grooves or mouse-bites) or simply use the free JLCPCB panelization tool during the ordering process.

Panel Service by JLCPCB

A panelized PCB, clearly showing the “Mouse Bite” between boards and the four fiducial markers on the panel.

Packaging Your PCB Files for SMT Assembly: Your file set is finally ready.

1. Place all your Gerber files (GTL, GBL, GTP, etc.) into a single folder.

2. Zip this folder. Example: MyProject_Gerbers.zip.

3. Place your BOM (.csv or .xlsx) and CPL (.csv or .txt) files.

4. On the JLCPCB quote page, upload your Gerber zip file first. Then, upload your BOM and CPL files when prompted.

Conclusion

A successful PCB SMT assembly is not magic. It's a data-driven process that depends entirely on the quality of the files you provide. A meticulously prepared file set—with correct Gerbers, an unambiguous MPN-based BOM, and an accurate Component Placement List—is the foundation for a fast, low-cost, and high-yield manufacturing run.

By taking the time to verify your paste layers, cross-check your BOM against the JLCPCB Parts Library, and visually confirm your placements in our Component Placement tool of Gerber Viewer, you are eliminating guesswork and ensuring your PCB design is manufactured and assembled right, the first time.

Ready to see your PCB design come to life? Upload your complete file set to the JLCPCB SMT assembly instant quote page today. Our system will automatically parse your data, verify your parts, and give you a detailed price and lead time for your fully assembled boards - often in just seconds.

FAQs

Q1. What happens if a critical component in my BOM is not available in the JLCPCB Parts Library?

If JLCPCB’s system flags an MPN as “Not Found” during the quotation process, it means the component is not one that we currently stock or source directly. The fastest and most cost-effective solution is to find a suitable substitute that exists in our Parts Library.

Alternatively, you can contact our support team to arrange a consignment order, where you source and ship the specific components to us.

Also Read: How to consign parts to JLCPCB

Q2. My PCB design has SMT components on both sides. How does this work?

JLCPCB’s SMT assembly service fully supports double-sided SMT assembly. The board passes through the SMT production line twice:

● First pass: paste → pick-and-place → reflow on one side

● Second pass: the board is flipped → paste → pick-and-place → reflow on the other side

To ensure reliable assembly, the side with fewer components is always assembled first. This prevents larger or heavier components from dropping off during the second reflow.

This means the Top (T) side and Bottom (B) side do not have the same priority—the production team will arrange the order based on component distribution to ensure optimal yield.

Providing correct Top/Bottom paste files and centroid files is essential. JLCPCB’s quoting system automatically detects components on both layers and adjusts the assembly price accordingly.

Q3. Is it better to create the panel myself or use the JLCPCB panelization tool?

For most standard rectangular boards, using our panelization tool is the fastest and easiest option. Our tool automatically adds V-grooves and tooling strips with fiducials.

You should only create a panel yourself in your EDA software if your board has a complex, non-rectangular shape (e.g., round boards) that requires custom "mouse-bite" tabbing to fit together efficiently.

Q4. How should I format my PCB design files if I only want to order a stencil, not a full assembly?

If you are only ordering a stencil, the process is much simpler. We don't need your BOM or Centroid file. We only need your Gerber files. The most important files are the Solder Paste layers (GTP/GBP), as this is what we use to cut the stencil, and the Board Outline layer (GKO/GML), which we use to set the stencil's dimensions and alignment.