At its simplest, yield is an easy concept: your yield is P/(P+F), or put into plain English:
(total # of passing units) / (total # of units input)
This concept gets complicated much further to provide more insight into the health of our manufacturing line. We will review the following concepts in relation to our manufacturing yield:
- Retest
- Repair
- First Pass Yield = yield without including units that went to repair
- Overall Yield = yield including units that were successfully repaired
- Retest Rate = % of units that retested pass
Retest
When a unit fails a manufacturing test, it is not immediately removed from the line. CMs will retest the DUT multiple times before declaring it a failure and sending it to repair. CMs will usually retest/repair DUTS until they pass. Less upright CMs will find tricks or swap in other units in order to pass the failing DUT down the line.
This fact of retest means that units can be reclassified as a pass or fail over time. Your CM will usually tell you “we had 100% yield”, but this does not give you actual insight into how many units entered repair and how many units needed to retest pass.
Repair
When a unit retests fail enough times, it will enter into repair. This unit counts as a failure until it is made to pass again.
After repair, the unit will return to either the station or the start of the test line. This is determined by the repair policy you set with your CM.
First Pass Yield
In order to get a better picture of the yield/rework breakdown we can categorize our passing units into different groups.
“First pass yield” (FPY) is the measure of units that tested pass on the first test attempt (or retest-pass) before being checked into repair. Once a unit fails enough times to enter repair, it counts as a “fail” under FPY.
FPY is typically calculated for each process step.
FPY gives you a metric for your yield without repairs being included. Consider FPY as a time-zero metric for passing units – units may fail at a later time. Later failures do not remove it from FPY calculations.
Overall Yield
Overall yield is a measure of your actual output yield including repaired units. This number is the typical yield number we consider: P/(P+F).
Retest Rate
When we count passing units, we are actually counting “no retest pass” + “retested pass”.
Retest rate can have a large impact on your overall throughput, as DUTs must spend more time at a station before moving down the line. High retest rates can also be used to identify fixture, software, or SOP issues.
To calculate your retest rate, simply consider the number of units that retested pass.
Pass to Fail Transitions
Note well: it is possible for a unit to pass in your first pass yield but fail in your overall yield (or later a field failure).
Example Calculations
Consider the following example for calculating yield & retest rates:
- Input 30 boards
- 25 boards passed
- 4 boards failed once, but retested pass on the second try
- 5 boards went to repair
- 2 boards could not be repaired
- 3 boards were successfully repaired
- Two boards retested pass on the second try after repair
- First pass yield = 25/30 => 83%
- Overall yield = 28/30 => 93%
- Retest rate = 6/30 => 20% (This is high – look into it to improve UPH!)