4.1 Metrics That Matter: Yield, FPY, Scrap, OTD

A factory generates two things: products and data. If you ignore the data, the product will eventually fail. However, not all data is useful. In EMS, "Vanity Metrics" make managers feel good but hide the bleeding wounds of the operation. You must learn to distinguish between metrics that measure activity and metrics that measure health. A healthy factory is not one that fixes defects quickly; it is one that does not create them in the first place.

First Pass Yield (FPY): The Only Truth

Definition: The percentage of units that pass the testing process on the very first attempt, without any rework or re-testing.

FPY = Units Passed First Time / Total Units Tested

The Engineering Reality

Most factories report "Final Yield" or "Ship Yield," which is often 99%+. This is a dangerous lie. It includes units that failed, were fixed (reworked), and then passed.

• If your FPY is 85% but your Final Yield is 100% → Then your factory is a repair shop, not a manufacturing line.
• If FPY drops below 95% → Then you have a systematic process problem (e.g., bad stencil, wrong profile) that requires immediate engineering intervention.
• Why it matters: Reworked units have lower reliability. A low FPY predicts high field failures, even if the units eventually "passed."

Scrap Rate: The Cost of Waste

Definition: The percentage of materials that are discarded because they cannot be repaired.

Scrap Rate = Value of Scrapped Material / Total Production Value

The Engineering Reality

Scrap is not just the cost of the lost component. It is the cost of the Component + Labor added so far + Disposal Fees + Opportunity Cost (you could have built a good unit in that time).

• If you scrap a populated PCB at the end of the line → Then you lose the most money.
• If you scrap a bare PCB at the start → Then the loss is minimal.
• Rule: Catch defects early. Scrapping a $50 chip is bad; scrapping a $5,000 finished system is a career-limiting event.

OTD (On-Time Delivery): The Supply Chain Promise

Definition: The percentage of shipments that leave the dock on or before the committed date.

The Engineering Reality

"On Time" is binary. You are either on time, or you are late. "Almost on time" is late.

• The Trap: Changing the date. If you promise Friday, but slip to Monday, and update the system date to Monday, the metric says "100% OTD." This is fraud against the customer relationship.
• If OTD is consistently < 90% → Then the factory has a planning or material shortage problem, not necessarily a production speed problem.

The "Bone Pile" and WIP (Work In Progress)

Definition: WIP is the inventory sitting on the floor, partially finished. The "Bone Pile" is the stack of failed boards waiting for debug.

The Engineering Reality

A large Bone Pile is a sign of a failing process. It represents trapped cash.

• If the Bone Pile grows faster than the repair tech can fix it → Then the production line must stop. You are producing garbage faster than you can clean it up.
• Rule: Measure "Days of WIP." If a board enters the line and typically leaves in 2 days, but currently takes 5 days, your flow is blocked.

The Vanity Metric Trap: "Testing into Compliance"

A common localized failure is the "Retest Loop."

• Scenario: A board fails the functional test. The operator hits "Retest." It fails again. They hit "Retest" again. It passes. They ship it.
• The Truth: This is a defective board with an intermittent fault. It will fail in the customer's hands.
• Critical Control: Configure test software to lock out a serial number after 2 consecutive failures. Force a technician to analyze it. DO NOT allow infinite retests.

Final Checklist