What are No Fault Found (NFF) Events and How is NFF % Calculated?

A NFF event occurs when a board (identified by serial number) is tested repeatedly and its final result is a "pass."

Example: "fail then pass"

Here is what one kind of NFF event looks like in the data:

Time: 0 | SSN: 12345 | Result: Failed
Time: 1 | SSN: 12345 | Result: Failed
Time: 2 | SSN: 12345 | Result: Passed

In this example, the same serial number (board) failed the first two tests, but eventually passed on the third test. On an In-Circuit Test machine, this may happen when there is flux built up on the test pins (causing initial test failure) that is knocked off by the initial tests enabling a later "pass" result.

Example: "repeated pass"

Here is another kind of NFF event in the data:

Time: 0 | SSN: 12345 | Result: Passed
Time: 1 | SSN: 12345 | Result: Passed
Time: 2 | SSN: 12345 | Result: Passed

This commonly happens when multiple boards are in a panel, a different board experiences a failure, and then the entire panel of boards is retested. In this case, board 12345 never had a problem, but it was repeatedly tested as a different board failed the first two times.

NFF % formula

NFF % = (number of boards that had NFF events) * 100 / (total boards inspected)