4.2 Change Control and Revisions

In manufacturing, the only thing more dangerous than a bad design is an ambiguous one. "Continuous Improvement" is a business goal, but uncontrolled change is a factory killer. Once the "Data Freeze" (established in the Virtual Build) is locked, you cannot simply email the factory to say, "Swap R5 for a 20k resistor." This creates a synchronization gap between the documentation, the procurement team, and the machine programmers. Engineering Change Orders (ECOs) are the formal legal mechanism to break the freeze safely. Without them, you do not have a product; you have a collection of random experiments.

The Vocabulary of Change: ECN vs. ECO

Precision is required. Do not confuse a request with an order.

1. ECR / ECN (Engineering Change Request / Notice)

• Definition: The Proposal. "We found a bug. We should change C4 to 10uF."
• Status: Pending. No action is taken on the line. It is a signal to pause and evaluate impact (cost, scrap, schedule).

2. ECO (Engineering Change Order)

• Definition: The Mandate. "Proceed with the change. Update BOM to Rev B. Scrap old material."
• Status: Active. The factory is authorized to spend money and alter the process.
• Rule: An ECO must be signed by Engineering (Technical Approval) and Operations (Cost/Schedule Approval).

The Revision Label: The DNA Marker

Every change must trigger a revision advancement. A board built today must be distinguishable from a board built yesterday if the silicon or copper changed.

The Revision Rule

• If form, fit, or function changes → Then the Revision letter must increment (e.g., Rev A → Rev B).
• If you change the PCB copper → Then the bare board part number and the assembly part number must update.
• If you only change a firmware load → Then the assembly part number updates, but the bare board remains the same.

The Implementation Strategy: "Cut-In" Timing

An ECO is not just what to change, but when to change it. Timing dictates cost.

1. Running Change (Use Up Stock)

• Logic: The old part is fine, the new one is just cheaper or better.
• Action: "Use up the remaining 5,000 units of the old resistor, then switch to the new one."
• Risk: Low. Minimizes scrap.

2. Hard Cut / Mandatory (Purge)

• Logic: The old part causes a failure. It is toxic.
• Action: "Stop the line. Remove all old parts from the feeder. Throw them away. Install new parts immediately."
• Risk: High Cost (Scrap) + Line Downtime.

3. Next Build

• Logic: The change is an improvement for the next batch.
• Action: Update the BOM for the next Purchase Order. Do not touch the current work in progress (WIP).

The "Mixed Build" Nightmare

The ultimate failure of change control is the Mixed Build: shipping units that are half-Rev A and half-Rev B.

The Scenario

You issue an ECO to change a microcontroller. The factory updates the Pick & Place file but forgets to throw away the old reels in the warehouse.

• Result: The machine loads the old reel. The new firmware is flashed. The device bricks immediately.
• If you do not physically quarantine and scrap old material → Then it will find its way back onto the line.

Pro-Tip: Never rely on "Visual" ECOs. If an ECO requires a technician to manually solder a jumper wire on 1,000 boards, you have created 1,000 opportunities for failure. Respin the board instead.

Final Checklist