Do You Know Your True First Pass Yield?
From Output Yield to First Pass Yield Explained.

True First Pass Yield

First Pass Yield (FPY) is one of the most important metrics in manufacturing, but the definitions vary significantly from company to company, and that variation has consequences.

A standard definition that is gaining traction is what some refer to as True First Pass Yield. In this article we look closely at what that means, why it matters, and what it takes to use FPY as something more than a reporting number.

In general, True First Pass Yield is based on all test results and includes even trivial test failures. It is unique to each test operation your product goes through. And it is not something you can manage through Microsoft Excel.

We'll discuss the benefits for companies to consistently track and optimize True FPY, and look at the premises needed to ensure it functions as a metric that guides informed continuous improvement rather than a selective vanity number.

What Are the Different Types of Yield?

Broadly speaking, there are four types of yield used in manufacturing. The difference between these lies in when a failure is first recognized.

The four types of yield:

• Output Yield
• Rework Yield
• First Pass Yield
• Batch Yield

Not all yield metrics are the same, and understanding where each one sits in relation to your test data is the starting point for using any of them meaningfully.

Output Yield

With Output Yield you only consider the failures that cause you to scrap a unit. This type of yield can easily run close to 100%.

Rework Yield

The Rework Yield metric is traditionally more common. Here you recognize a failure when the unit goes to rework or repair. Part of the reason why this is a common approach is that most EMS companies adopted MES software technology early on, and repair documentation is a core feature of these systems. With repair information and production quantities already in the MES system, this metric becomes very simple to calculate. Rework Yield can also easily trend high, often above 90%.

Rework Yield and Output Yield

Both Rework Yield and Output Yield are often referred to as First Pass Yield. That is incorrect. True First Pass Yield in the context of manufacturing is a test-centric metric.

Yield recognition should come from the results of the test stations and include any and all scenarios where the test sequencer flags the test as failing.

Without this view, your results when compared to a company actively managing True First Pass Yield will fall short when it comes to:

• Transparency
• Accountability for different stakeholders that contribute to building products
• Productivity
• Product quality
• Profitability

In any and all scenarios we are also including trivial test failures, such as the operator forgetting to power on an instrument. If these things happen often enough to have statistical significance they should definitely be on your radar.

Batch Yield

Batch Yield is a variant of First Pass Yield based on batch ID rather than serial numbers. This is most common very early in the build process when a unit has still not been assigned a unique serial number.

Because the serial number needed for a unit-centric yield calculation is either static (NOVALUE) or random, a random number for each test run including retests, your ability to extract actionable information from the First Pass Yield metric is significantly limited.

Either all failures except one will belong to test runs later than the first attempt, or all will be counted as first-attempt failures.

For a detailed breakdown of how yield is calculated at the unit level, including the distinction between test report yield and unit level yield and why the way a time filter is applied can significantly affect the accuracy of your numbers, read our article Unit Yield vs Test Report Yield.