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

SMT programming is a precise coordinate translation task. If the engineering input data is relative, visually ambiguous, or mathematically incorrect, the pick-and-place machine will place components incorrectly.

The Assembly Output package must provide precise coordinates and a locked visual “truth source” to verify them against. The operational goal is to produce a machine program that runs perfectly on the first pass, eliminating unnecessary back-and-forth where the EMS must ask for rotation clarifications for every polarized IC.

The Controlled Placement Dataset

The Centroid, Pick and Place file (also called XY Data) is the primary numerical driver for assembly. It must be generated programmatically directly from the final CAD database revision. It should not be manually edited, sorted, or copy-pasted in Excel before sending.

Required Data Fields (CSV/ASCII):

RefDes: (e.g., U1, R1). The unique primary key that must perfectly match the BOM.
Layer: (Top/Bottom).
Location X: Center of the physical component body (Not Pin 1).
Location Y: Center of the physical component body.
Rotation: (0–360 degrees).
Package/Footprint: (e.g., SOIC-8). Highly useful for verifying machine nozzle selection.

Intake Review:

When the Centroid file separates the Top and Bottom sides into completely different files, they should be merged or explicitly labeled. Ambiguity here can lead to side-placement mismatches.
When coordinates are relative to an arbitrary local origin rather than the absolute board origin (0,0), the file will be rejected. Coordinates must refer to the global panel or board fiducials.

Rotation Architecture: The “Zero” State

Rotation data is the single most frequent data-driven cause of placement defects. Providing a value of “90 degrees” is incomplete unless the base “0 degree” state is explicitly defined.

The Root Problem:

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

Since these three “Zeros” frequently conflict, you are required to provide a Polarity Reference Map.

The Polarity Reference Map

Do not rely solely on the Silkscreen layer, as it may be clipped by pads or physically hidden under component bodies. You must include a dedicated .PDF Assembly Drawing that explicitly shows:

Pin 1 Dot: Clearly visible for all ICs.
Cathode Band: Clearly marked and unambiguous for Diodes and Tantalum capacitors.
Connector Keying: The notch or latch position heavily indicated.

Engineering Rule: For physically symmetric connectors (e.g., USB-C), use a non-symmetric mark on the assembly drawing layer to explicitly indicate the “Front” or “Pin 1” side to prevent a 180-degree reversal.

Stencil & Paste Inputs

The Solder Paste layer is not a simple automated copy of the copper pads. It is a critical fluid-dynamics process control layer.

Input Requirements:

1:1 Output: Export the paste layer perfectly 1:1 with the copper pads (unless your specific CAD library already includes advanced reduction). Let the EMS or the Stencil Vendor apply the global aperture reduction (typically 10% to 20%) based on their specific stainless steel foil thickness and blade chemistry.
Exclusions: You must ensure non-soldered features (e.g., test points, fiducials, gold edge fingers, thermal vias) are void of paste data.
Review: When optical fiducials have paste openings, the machine vision system will fail once shiny solder paste is printed over them, stopping the production line.

Data Validation Logic: The “Sanity Check”

Before you zip the GDP and send it to manufacturing, run these validation checks.

1. The BOM vs. Centroid Count

Process: Sum the row count of every RefDes in the BOM (excluding DNPs). Sum the count of coordinates in the Centroid file.
Logic:
- When the BOM count exceeds the Centroid count, verify your DNP list to ensure the missing parts are intentionally tagged as unpopulated.
- When the Centroid count exceeds the BOM count, this creates a critical error where the machine is instructed to place parts that do not exist in the purchasing list.

2. The Fiducial Audit

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

3. The Diode Direction Audit

Process: Randomly spot-check 3 diodes and 3 ICs in the output data.
Check: Compare the numerical Rotation angle in the file against the Assembly Drawing and the IPC footprint standard.
Action: If “0” degrees physically points West on the drawing but North in the standard, you must add a rotation correction note in the Fab/Assy Instructions.

Final Baseline Checklist

The Control Point	The Operational Requirement	The Go/No-Go Metric
File Format	CSV / ASCII Text.	Machine readable; perfectly clean headers.
Coordinates	Metric (mm) highly preferred.	Must be consistent with Gerber master units.
Origin Point	Board/Panel 0,0 location.	Matches the Gerber global origin exactly.
Fiducials	Included in dataset.	X, Y coordinates are physically present.
RefDes Match	100% Match to BOM (minus DNP parts).	Zero “orphan” coordinates allowed.
Polarity Map	Explicit PDF Assembly Drawing.	Pin 1/Cathode strictly and clearly visible.
Paste Layer	1:1 export (or explicitly specified otherwise).	Zero paste allowed on fiducials or test points.