Movip

Process Risks

Process

When manufacturing parts or systems each activity can be seen as a process. This means that the part or system being manufactured has undergone a number of process steps. Each process add a risk of not meeting the requirements to the final part. To see the consequences of these risks and how to mitigate those the following steps has to be taken.

Process Risk

A process step is meant to realize an activity which is required to manufacture or assemble a part or system. Each process step can add a risk in not meeting the required outcome of that process step. To determine the risk of an existing process measurements (SPC) can give insight in the process performance. When having done those measurements an insight in the process Quality can be obtained. When having a new product on an existing process a good starting point is to use the process quality of previous product. The process performance can be measurend to establish the real process performance. When having a new process there is no information on the process quality. To get insight a process FMEA can give insight on the expected process quality. When those identified risks are high a possibility to obtain information on those risks is to build a Functional model of the process. Based on the obtained results the process risks can be mitigated by a design change or process improvement.

Process

To calculate the Zero Hour Defect Rate (ZHDR) of a process the following formula can be used where the Defect Opportunities (DO) represent the process risks, which can be more than 1, and the Deffects Per Million Opportunities (DPMO) the risk in Parts Per Million (ppm) per Defect Opportunity (DO).

Zero Hour Defect Rate (ZHDR) Calculation

Example: An process has two identified risks. Risk 1: 200000 ppm and Risk 2: 400000 ppm. The end result of this process will be that the process Zero Hour Defecrt Rate (ZHDR) is 0.52. (this means that out of the 100 parts leaving the process 52 not meats the output requirement of this process.) Precondition is that the two risks are independent of each other. There are 3 options to Mitigate those risks entering the next process.

  • Change the Design
  • Improve the Process
  • Test
The design change and/or process improvement will result in new Risk figures and can be recalculated based on those new numbers. When the result is acceptable a test must be applied to ensure that those risks are not entering the new process step.

Test

To Mitigate the remaining risks after the design has changed and the process improved a test must be applied to mitigate the risks not entering the next process. Each test has 4 possible outcomes.

process test

  • OK
  • False OK
  • NOK
  • False NOK

Each applied test is not perfect in finding the process risks. This means that a risk is slipped through the test.

OK

OK means that the test has found no risks and the process output is meeting the requirements.

False OK

With a false OK the test is not detecting all risks and gives an OK. The test has not the 100% coverage needed to identify good systems. These false OK's can result in a defect later on in the following processes or a life time issue later on. The measure of false OK's is determined by the Slip of the test.

NOK

With a NOK the test found a Risks which resulted in a NOK.

For this the variable Slip is a measure of how much the risk is mitigated.

If the slip is 1 no mitigation takes place.
If the slip is 0 the risk is completely mitigated.

Slip

The not mitigated (detected) risk will result in a False OK.

False NOK

With a False NOK the test found a Risks which was not there. The process delivered an OK output but was given a NOK. A possibility is that the test has parameters which have a to tight window which results in a False NOK. This is unnecessary loss.

Zero Hour Defect Rate (ZHDR) calculation

To calculate the Zero Hour Defect rate (ZHDR) the Test slip has to taken into account where a Slip of 1 means no risk detected and a 0 all risk detected. An in between value means a incomplete risk detection.

Zero Hour Defect Rate (ZHDR) Calculation

Example: same 2 risks but with an applied test of Slip 1: 0.2 and Slip 2: 0.1. The Zero Hour Defect Rate (ZHDR) will be now dramatically reduced to an 0.0784. (This means that out of the 100 parts leaving the process only 7.84 parts not meets the requirements.)

The process output is:

  • OK = 50.4%
  • False Ok = 3.336%
  • NOK = 46.24% (16% test/slip 1 & 30.24% test/slip 2)
  • False NOK = 0%
Best way to calculate is making two tests where the test are placed in series and the test efficiency per test is taken.

Rework

The output of a NOK process step is not suitable to continue in the next process step. This output can be scrapped or reworked. Reworking is an activity which has as output a part where the risks are "repaired" and which can be tested again.

Rework

Total Process

Manufacturing comprises more than 1 process step. When more than 1 process step is required the following setup is true for sequential processes. Processes running parallel will be merged in 1 new process by treating them as an assembly process.

Total process

Test Efficiency

Test Efficiency is the capability to resolve the risks into a OK or NOK. The four outputs are influenced by the Slip which results in a Test efficiency. Based on the fact that the test detects only the risks the OK output equals the correct parts. Risk 1 is 200000ppm which will result in a OK output of 800000ppm or 0.8 of the batch size. The remaining 200000ppm is divided between NOK (detected risks) and False OK (not detected risks) and is based on the Slip.

Test

Example: 100 parts are entering the process of which 80 are OK and 20 * Slip (0.2) are false Ok and 20 * (1-Slip) are NOK. This results in:

  • OK = 80(%)
  • False OK = 4(%)
  • NOK = 16(%)
When also have insight in the False NOK this will influence the OK.

With more than 1 parameter to test the test will change in a OK and a NOK flow.

Test sequence