1.4 Assembly Outputs: Centroid, Rotations, Polarity and Stencil Inputs

SMT programming is not an interpretative art; it is a coordinate translation task. If the input data is relative or ambiguous, the pick-and-place machine will place components sideways, reversed, or off-pad. The Assembly Output package must provide absolute coordinates and a visual "truth source" to verify them. The goal is to produce a machine program that runs correctly on the first pass, eliminating the "programming ping-pong" where the EMS asks for rotation clarifications for every IC.

The Controlled Placement Dataset

The Centroid file (also called Pick-and-Place or XY Data) is the numerical driver for assembly. It must be generated directly from the final CAD revision, never manually edited in Excel.

Required Data Fields (CSV/ASCII):

1. RefDes: (e.g., U1, R1). Unique key matching the BOM.
2. Layer: (Top/Bottom).
3. Location X: Center of component (Body center, not Pin 1).
4. Location Y: Center of component.
5. Rotation: (0–360 degrees).
6. Package/Footprint: (e.g., SOIC-8). Helpful for verifying nozzle selection.

Logic ⭢ Action:

• If the Centroid file separates Top and Bottom sides into different files ⭢ Merge or explicitly label them. Ambiguity here causes side-mismatches.
• If coordinates are relative to a local origin rather than the board origin (0,0) ⭢ Reject. Coordinates must refer to the panel or board fiducials.

Rotation Architecture: The "Zero" State

Rotation data is the most frequent cause of placement defects. A value of "90 degrees" is meaningless unless the "0 degree" state is defined.

The Challenge:

• CAD Library Zero: The orientation the component was drawn in the library.
• IPC-7351 Zero: The standard orientation (usually Pin 1 Upper Left or Top).
• Tape & Reel Zero: The orientation of the part in the pocket.

Since these often conflict, you must provide a Polarity Reference Map.

The Polarity Reference Map:

Do not rely on the Silkscreen alone, as it may be clipped or hidden under component bodies. Include a dedicated .PDF Assembly Drawing that explicitly shows:

• Pin 1 Dot: Clearly visible for all ICs.
• Cathode Band: Clearly marked for Diodes/Tantalums.
• Connector Keying: Notch position indicated.

Pro-Tip: For symmetric connectors (e.g., USB-C), add a non-symmetric mark on the assembly layer to indicate the "Front" or "Pin 1" side to prevent 180-degree reversal.

Stencil & Paste Inputs

The Solder Paste layer is not a simple copy of the copper pads. It is a process control layer.

Input Requirements:

• 1:1 Output: Export the paste layer 1:1 with the copper pad (unless the library already includes reduction). Let the EMS or Stencil Vendor apply the global aperture reduction (typically 10% to 20%) based on their specific foil thickness.
• Exclusions: Ensure non-soldered features (e.g., test points, fiducials, gold fingers) are void of paste data.
• Logic: If fiducials have paste openings ⭢ Then the vision system will fail once paste is printed, stopping the machine.

Data Validation Logic: The "Sanity Check"

Before zipping the GDP, run these three deterministic checks.

1. The BOM vs. Centroid Count

• Process: Sum the count of each RefDes in the BOM. Sum the count of coordinates in the Centroid file.
• Logic:
  • If Count(BOM) > Count(Centroid) ⭢ Check DNP list. Are the missing parts intentionally unpopulated?
  • If Count(Centroid) > Count(BOM) ⭢ Critical Error. You are placing parts that do not exist in the purchasing list.

2. The Fiducial Audit

• Requirement: Minimum 3 Global Fiducials per side (Top/Bottom) and per Panel rail.
• Check: Verify Fiducials exist in the Centroid file with coordinates, or distinct entry in the Drill/Gerber data.

3. The Diode Direction Audit

• Process: Randomly spot-check 3 diodes and 3 ICs.
• Check: Compare the Rotation angle in the file against the Assembly Drawing and the footprint standard.
• Action: If "0" degrees points West on the drawing but North in the standard ⭢ Add a rotation correction note in the Fab/Assy Instructions.

Final Checklist

Check	Requirement	Pass Metric
File Format	CSV / ASCII Text.	Machine readable; clean headers.
Coordinates	Metric (mm) preferred.	Consistent with Gerber units.
Origin	Board/Panel 0,0.	Matches Gerber origin.
Fiducials	Included in dataset.	X, Y coordinates present.
RefDes Match	100% Match to BOM (minus DNPs).	No "orphan" coordinates.
Polarity Map	PDF Assembly Drawing.	Pin 1/Cathode clearly visible.
Paste Layer	1:1 export (or specified).	No paste on fiducials/test points.