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 & Place machine will execute incorrect placements.

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. Manual editing, sorting, or copy-pasting in Excel before transmission is prohibited.

Required Data Fields (CSV/ASCII):

1. RefDes: (e.g., U1, R1). The unique primary key that must perfectly match the BOM.
2. Layer: (Top/Bottom).
3. Location X: Center of the physical component body (Not Pin 1).
4. Location Y: Center of the physical component body.
5. Rotation: (0–360 degrees).
6. 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 baseline “0 degree” state is explicitly defined.

The Root Problem:

• CAD Library Zero: The arbitrary orientation the component was drawn in the 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, a Polarity Reference Map is strictly required.

The Polarity Reference Map

Relying solely on the Silkscreen layer is insufficient, as it may be clipped by pads or physically hidden under component bodies. A dedicated ”.PDF” Assembly Drawing must be included 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.

Pro-Tip: For physically symmetric connectors (e.g., USB-C), a non-symmetric mark on the assembly drawing layer must be used 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: The paste layer must be exported perfectly 1:1 with the copper pads (unless the specific CAD library already incorporates advanced reduction). The EMS or Stencil Vendor will apply the global aperture reduction (typically 10% to 20%) based on their specific stainless steel foil thickness and blade chemistry.
• Exclusions: Non-soldered features (e.g., test points, fiducials, gold edge fingers, thermal vias) must be completely void of solder paste data.
• Review: When optical fiducials have solder 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 compiling the Gerber Data Package (GDP) for manufacturing, these specific validation checks must be executed.

1. The BOM vs. Centroid Count

• Process: The row count of every RefDes in the BOM (excluding DNPs) must be summed. The count of coordinates in the Centroid file must be summed.
• Logic:
  • When the BOM count exceeds the Centroid count, the DNP list must be verified 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 Pick & Place 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: Fiducials must be verified to 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: 3 diodes and 3 ICs in the output data must be randomly spot-checked.
• Check: The numerical Rotation angle in the file must be compared against the Assembly Drawing and the IPC footprint standard.
• Action: If “0” degrees physically points West on the drawing but North in the standard, a rotation correction note must be added to the Fabrication/Assembly Instructions.

Final Checkout: Assembly outputs: centroid, rotations, polarity and stencil inputs

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.