1.2 The Bill of Materials (BOM) & Revision Control

The Bill of Materials (BOM) is not merely a purchasing list; it is the primary driver for SMT programming, supply chain logic, and cost control. In the Golden Data Pack (GDP), the BOM functions as Master Data. If the BOM is ambiguous, the production line stops.

Treat the BOM as a database export, not a casual spreadsheet. It must be machine-readable, deterministic, and synchronized with the physical board design.

The Component Identity: MPN is King

The Manufacturer Part Number (MPN) is the only valid identifier for a physical component. Internal part numbers (IPNs) or generic descriptions are insufficient for manufacturing because they require translation, which introduces human error.

The Data Hierarchy:

1. Primary Key: Manufacturer Part Number (MPN).
2. Validator: Manufacturer Name (must match the MPN).
3. Reference: Description (e.g., "RES 10k 1% 0402").
4. Locator: Reference Designators (RefDes) (e.g., R1, R2, R3).

Logic for Intake:

• If the BOM lists only "10k Resistor" without an MPN ⭢ Reject. The EMS cannot guess tolerance, voltage rating, or thermal coefficient.
• If the Description conflicts with the MPN (e.g., Desc says "10k" but MPN is for "100k") ⭢ Flag for EQ. The MPN generally takes precedence, but this is a critical data mismatch.

AVL Strategy: Approved Vendors & Alternates

Supply chain resilience relies on pre-validated alternates. However, "or equivalent" is not a specification—it is a liability.

Defining Alternates:

Do not leave alternate selection to the purchasing agent's discretion during a shortage. Define them explicitly in the GDP.

• Explicit Method: Add columns for Alt_1_Mfr, Alt_1_MPN, Alt_2_Mfr, etc.
• Generic Method: If the part is a commodity (e.g., standard pull-up resistor), reference a controlled "Internal Standard Spec" that the EMS can map to their house stock (e.g., "Must meet Spec 100-001: 0402 Resistor, 1%").

Pro-Tip: Never use the phrase "Or Equivalent" for capacitors or MOSFETs. DC bias characteristics and ESR vary wildly between manufacturers even if voltage and capacitance match.

DNP Management (Do Not Populate)

"Do Not Populate" (DNP) or "Do Not Install" (DNI) components must be explicitly managed to prevent placement errors and false-failure reports at AOI (Automated Optical Inspection).

The Rules of Omission:

1. Explicit Status: The BOM must have a column labeled FITTING_STATUS (or similar).
2. Values: Use FITTED for populated parts and DNP (or NO_FIT) for unpopulated parts.
3. No Deletions: Never simply delete DNP rows from the BOM. If R5 is in the schematic but missing from the BOM, the EMS assumes it is a data export error.
   • If RefDes exists in CAD ⭢ Then RefDes must exist in BOM (marked DNP).

The Triad of Revision Control

A valid manufacturing build requires the synchronization of three artifacts. If these revisions are not aligned, the data pack is invalid ("The Frankenstein Pack").

The Synchronization Rule:

The Project Revision (e.g., REV_05) must be stamped on:

1. The BOM Header: Indicates the list of parts.
2. The PCB Fabrication Data: Indicates the copper geometry.
3. The Assembly Drawing: Indicates the visual instruction.

Scenario ⭢ Action:

• If BOM is Rev B but Gerber data is Rev A ⭢ Stop Build. There is a high risk that footprint locations (X,Y) in the old Gerber do not match the new BOM component package (e.g., 0603 vs 0402).

ECO Cut-Ins & Running Changes

An Engineering Change Order (ECO) issued after the GDP release must follow a strict "Cut-In" logic to preserve traceability.

Cut-In Strategies:

• Clean Break (Preferred): The ECO triggers a new GDP Revision (Rev A ⭢ Rev B). Old stock is purged or quarantined.
• Running Change: The ECO is applied to the current build based on logic.
  • By Serial Number: "Apply ECO-101 starting at Unit SN #5000."
  • By Date Code: "Apply ECO-101 for all boards assembled after [Date]."

Pro-Tip: Avoid "Use Up Stock" dispositions for critical signal integrity components. It creates a fleet of mixed-performance devices that are impossible to debug later.

Final Checklist

Check	Requirement	Pass Metric
Header Info	Project Name, Revision, Date.	Matches GDP folder name.
MPN Integrity	Every line item has a Manufacturer + MPN.	No "Generic" or blank MPN fields.
RefDes Audit	Every RefDes is unique and matches CAD.	No duplicates; match count = CAD placement count.
DNP Clarity	Unpopulated parts are listed but marked DNP.	No missing RefDes sequences.
Alternates	Alternates are specific MPNs.	No "Or Equivalent" text fields.
Format	Machine-readable (CSV/XLSX).	One component per row (flat list preferred).