Knowing the First Pass Yield is key. Whether introducing a new process or making a change to a decades old process, no other metric is as useful. This article will provide an introduction to first pass yield and describe some ways you can use it to gain understanding in your tests.

What is first pass yield? The general definition of the first pass yield is the number of units coming out of a process divided by the number of units going into that process in a specified period of time. In other words, the percentage of units that successfully complete a process with no rework. Units that get reworked do not count on subsequent runs through the process. When talking about test, first pass yield is the percentage of units that pass on the first test.

First pass yield has a number of uses. Tracking the performance of a test system is important to a test engineer. Keeping an eye on the first pass yield can alert you to potential failures caused by equipment degradation. This gives you an opportunity to troubleshoot and fix the system before the operator or the line manager notices. Making changes to a test procedure is sometimes a hit and miss process. Often you make the change and wait to see if the complaints go away. Keeping track of the first pass yield will allow you to determine if the change is an improvement or a detriment. If the first pass yield goes up after the change is released, you can be sure the process has improved. Another valuable use of the first pass yield is to compare the performance of different test systems. When duplicating a test bench or test system, the first pass yield can alert you to problems caused by subtle differences in the equipment. Once you know they are different, you can dig further to try to determine the root cause of the difference. The first pass yield can also be used to compare the performance of different operators. You may then identify operators that have had inadequate training and are causing a loss of efficiency on the production line due to continual rework of units.

Unfortunately the first pass yield is not a magic bullet. It does not indicate true failure on it’s own. It can only be used as an indicator that a process has changed and may need further investigation. For those test engineers that need to support legacy products while keeping up with new product development, tracking first pass yield, especially with automated tools, is a good way to keep on top of the legacy products without sacrificing large amounts of time. First pass yield does not consider compound failure, when multiple items fail on a single unit, either. This makes it unusable for determining the defect rate in a process. And finally, while there are ways to calculate first pass yield on non-serialized data, in practice it is difficult to rely on the operator to tell the system when a unit is being re tested. First pass yield calculations are much easier on serialized data. Some people will use a flag in their test data to indicate if a test run is a retest of a unit. I’ve found it best to rely on the serial number as operators will tend to forget to mark a unit as a retest at times potentially corrupting the data.

Now that you’ve learned what first pass yield is, it’s time to put it to use. Find out how your test systems are performing. Start charting your first pass yield today on big visible charts. Get the insight you need to do your job more effectively.

Originally posted at CnC Software Solutions