Movip

To be able to calculate the product Zero Hour Defect Rate (ZHDR) for each defect opportunity a risk on occuring must be known. For calculation purpose the Defect Parts per Milion Opportunities (DPMO) is used. DPMO's are widely used or could be calculed from the Cpk, Ppk or sigma's.
When all DPMO's which can occur are known a product risk calculation can be done.

ZHDR Calculations

This calculation result in a product ZHDR. Based on this also a list of Part Risk and process risks can be created.

When having calculated the Zero Hour Defect Rate (ZHDR) and this is larger than the specified product ZHDR there are three options to lower the ZHDR.

ZHDR Specified/Calculated

Design change is the first step in mitigation. Based on the list of part or process risks a design change must be made which avoids those risks. When not possible the second best option is to improve the processes which contribute most to the high ZHDr of the product. Last resort is to test the remaining risks which could not be solved with a design change or process improvement. Realize that a test when its detected a Not Ok lead to scrap and as such increasing the cost price.

• Design Change
• Process Improvement
• Test

Design Change

Select out of the list of risks those parts and/or processes which contribute to the not meeting the product ZHDR. For parts a replacement or avoidance is the only way to improve. For processes is take an other design which avoids those processes.

Process Improvement

When a design change is not sufficient to meet the product ZHDR process improvemnt is the next option.

ZHDR Specified/Calculated

In an assembly process, even with skilled/trained operators, perfect workinstructions, sign off lists, ideal environment or ... incidents happen. To avoid these process incidents the process must be controlled in itself. The most easy way is automating the process, or put in other words avoid manual labour. Worldwide is manual labour a risk. To get a feeling for the contribution of manual labour an average incident rate is 1000ppm (1 of the 1000).
When manufactured part processes contributes to not meeting the product ZHDR those identiefied process must be improved. When manual labour with manufactured part is the contributor the process must be improved. When there is no manual labour which can be improved an other strategy can be another machine or again avoid a process risk by relaxing the requirement which causes the risk.

Test

Even when the design changes and process improvements are not sufficient to meet the product ZHDR the last resort is testing.

ZHDR Specified/Calculated

Each part or process which is tested and is Not Ok results in scrapping the product. When testing not on product level but on part or (sub)assembly level the consequences are the same but the cost involved is lower. For design risks this will always lead to scrap.

Process & Test resulting in OK/FOK/NOK

Calculation of the Zero Hour Defect rate (ZHDR) includes now the test coupled to the defect opportunity.For the test the slip is defined as:

0 > No slip (no process failure slips through)
1 > No test (all process failures slips through to next stage)

ZHDR Calculations including test

When testing identifies a Not Ok this mabey can be reworked. When reworked this can solve the NoK but also can introduce new risks. Testing the reworked product is mostly done on another test fixture than the test used for production.

ZHDR Calculations including test & rework

If the calculated Zero Hour Defect Rate (ZHDR) is lower than the specified Zero Hour Defect Rate (ZHDR) then no mitigation of the risk is needed. If the calculated Zero Hour Defect Rate (ZHDR) is not within specification the risks must be mitigated.

If it is specified that a product requires a Zero Hour Defect Rate (ZHDR) of 1000ppm (1 of 1000 may fail) the risks must be mitigated to reach the specified Zero Hour Defect Rate. Starting point are the largest contributors of the Zero hour Defect Rate (ZHDR). Mitigation can be done in three ways.

• Design Change
• Manufacturing Process Change
• _{Testing (Last resort)}

Design Change

Avoiding the risks is the best way to mitigate. If the design can be changed in such way that the risks are avoided then this is to most preferable way of mitigation.

With the changed design, the calculation must be repeated with the new Bill Of Material (BOM) and/or the changed connections and/or the changed assembly process. The recalculated Zero Hour Defect Rate (ZHDR) must be checked if it is within specification.

Part Related Risk Mitigation

Part risks are design related due to the fact that the Bill Of Material (BOM) is dictated by design. Part related risks can only be mitigated by changing those parts which contributes to the Part related Zero Hour Defect Rate (ZHDR).

Mounting Related Risk Mitigation

Avoiding mounting risks is done by at least following the rules of Designing For Manufacturing (DFM), Designing For Assembly (DFA) and respecting the industrialization rules. Yang, Kai, Design for Six Sigma, Chapter 10 ⁽¹⁰ Tien-Chien Chang, Richard A. Wysk, and Hsu-Pin Wang. Computer-Aided Manufacturing, Second Edition, Pages 596 to 598 ⁽¹¹

Industrialization Rules:

• Develop a modular design
• Use of standard parts
• Minimize number of parts
• Minimize number of different parts
  • Use only one type of fastener, resistor, capacitor,.... (Wrong)
• Minimize adjustments
• Make adjustment accessible
• Design parts to be multi-functional
• Design for ease of manufacturing
• No symmetrical mounting (Misoriented)
• Minimize assembly directions
• Use gravity when mounting parts
• Use no hand tools
• Minimize handling

Connection Related Risk Mitigation

Mitigating connection risks is done by avoiding those connection technologies in the design which contributes most to the connection related Zero Hour Defect Rate (ZHDR).

Assembly Processing Related Risk Mitigation

Mitigating assembly processing risks is done by not using these assembly processing technologies in the design which contributes most to the assembly processing related Zero Hour Defect Rate (ZHDR).

Calculate New Zero Hour Defect Rate (ZHDR)

Based on the design changes a new Bill Of material (BOM) is created and the Zero Hour Defect Rate (ZHDR) must be (re)calculated.

This results in a new Zero Hour Defect Rate (ZHDR) which must be checked if the Zero Hour Defect Rate (ZHDR) is within specification.

Manufacturing Process Change

When the design changes alone are not sufficient to reach the specified Zero Hour Defect Rate (ZHDR), the risks in the manufacturers process can be mitigated.

Part Related Risk Mitigation

The only exception which can't be mitigated in the manufacturing process are the part related risks. These can only be mitigated in the design, so only the mounting, connection or assembly process risks can be mitigated by changing/improving the manufacturing process.

Mounting Related Risk Mitigation

Mitigation of only the contributing mounting risks in the manufacturing process are required. (good is good) The categories in the calculated Zero Hour Defect Rate (ZHDR) are missing, wrong, misoriented, misplaced and must lead to a change in the manufacturing process to minimize the risk.

Examples of mitigation strategies are :

• Missing (poka-yoke, lean process, ..)
• Wrong (logistics)
• Misoriented (poka-yoke, lean process, ..)
• Misplaced (poka-yoke, lean process, ..)

Connection Related Risk Mitigation

Improving of only the contributing connection risks in the manufacturing process are required. The category in the calculated Zero Hour Defect Rate (ZHDR) is connection and must lead to a change to minimize the risk.

• Screw (process improvements, Tools,...)
• Glue (process improvements, Tools,...)
• Solder (process improvements, Tools,...)
• Press (process improvements, Tools,...)
• Bond (process improvements, Tools,...)
• Rivet (process improvements, Tools,...)
• Staple (process improvements, Tools,...)
• ....

Assembly Processing Related Risk Mitigation

Improving of only the contributing assembly processing risks in the manufacturing process are required. (good is good) The categories in the calculated Zero Hour Defect Rate (ZHDR) are coating, cleaning, excess parts, adjusting, testing, greasing, ... and must lead to a change in the assembly process to minimize the risk.

• Coating (improve process,...)
• Cleaning (improve process,...)
• Excess parts
• Adjusting
• Greasing
• Testing
• .....

Calculate the New Zero Hour Defect Rate (ZHDR)

Based on the Manufacturing process change new processes are used and the Zero Hour Defect Rate (ZHDR) must be (re)calculated.

This results in a new Zero Hour Defect Rate (ZHDR) which must be checked if the Zero Hour Defect Rate (ZHDR) is within specification.

Testing

Testing is the last resort which must be used when the design or manufacturing process changes are not sufficient to reach the required Zero Hour Defect Rate (ZHDR). Testing will always brings added risk, scrap, rework, cost and time by introducing testers.

Part Related Risk Mitigation

Part risks are design related due to the fact that the Bill Of Material (BOM) dictates these parts. Part related risks, which are not mitigated by a design change, needs to be tested to reduce those Part related Zero Hour Defect Rate (ZHDR).

• Part Defects (Incoming inspection, Test in assembly, ....)
• Part physical out of specification (Incoming inspection, Test in assembly, ....)
• Part functional out of specification (Incoming inspection, Test in assembly, ....)

Mounting Related Risk Mitigation

Mounting risks are manufacturing process related and can only be tested during or after assembly.

• Missing (Test in assembly, ....)
• Wrong (Test in assembly, ....)
• Misoriented (Test in assembly, ....)
• Misplaced (Test in assembly, ....)

Connection Related Risk Mitigation

Connection risks are assembly process related and therefore tested during or after assembly.

• Screw (test, ...)
• Glue (test, ...)
• Solder (test, ...)
• Press (test, ...)
• Bond (test, ...)
• Rivet (test, ...)
• Staple (test, ...)
• ....


 

Assembly Processing Related Risk Mitigation

Assembly process risks are manufacturing process related and therefore tested after assembly.

• Coating (test, ...)
• Cleaning (test, ...)
• Excess parts (test, ...)
• Adjusting (test, ...)
• Greasing (test, ...)
• .....

Test Mitigation

Each part, mounting, connection or assembly processing risk can be mitigated with a test. Each implemented test can influence one or more risks (defect opportunities) or one or more parts.

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.

Mitigated Zero Hour Defect Rate (ZHDR) Calculation

The part, mounting, connection and assembly process risks (DPMO's) are multiplied with the corresponding part, mounting, connection and assembly process slip.

This results in a new Zero Hour Defect Rate (ZHDR) which must be checked if the Zero Hour Defect Rate (ZHDR) is within specification.

Even when an assembly is manufactured perfectly, there is still a chance that the assembly will not meet its specified functionality and thus fails.

Methodologies to get insight in the design failures early in the design phase (architecture) is a Design Failure Mode Effect Analasys (DFMEA) where based on experiences of the past risks are identified. These risks can be quantized in a Zero Hour Defect Rate (ZHDR). New designs where there is no experiences have a high risk and those risks should be avoided or get insight in the risks by building a functional model. Furthermore each specification must be guaranteed by design or a test must be created to ensure this specification. When the design is finished a design verification must be done which must at least verify al the specifications. The same methodology also applies here that when a specification is not met a test must be applied to secure the specification. Underneath the DFMEA, Specification calculation and verification are the part and design related risks.

• Part related Risks
• Design related Risks

This could be caused by either a part or the design, even when the design is assembled correctly, resulting in not meeting the specifications.

Part Related Risks

A part could be used outside its specified area.
A special case is when the part is used outside its Safe Operating Area which causes wear and affects the reliability on long term. This will be explained in Life Time Risks.

• Part used outside specified area
• Usage of non specified part behaviour
• Part stress (internal, external mounting, thermal,..)

Part used Outside Specified Area

When a part is used outside its specified area the part is itself is correct but its usage not. Due to the not specified behaviour it is possible that the assembly will fail.

Examples:

• Sensor linearity
• Lens aberrations/transmission outside specified area

Usage of non specified Part Behaviour

All parts, even with the most extensive documented specifications, have hidden performances which are not guarantied.

Examples:

• OpAmp Noise behaviour is only specified for low frequencies
• Optical elements specifeid on optical axis
• ...

Design Related Risks

When assembled, the tolerances of all parts could lead to an assembly out of specification. (Worst case Risks)

• Timing violations (digital circuits)
• Noise behaviour (analog circuits)
• Offset (Sensor + acquisition system)
• Build dimensions (mechanic parts)

other Design risks are thermal stress induced in the design.

• Stress induced mounting
• Stress induced by thermal cycling during opteration. (Infant mortality)
• ....

Mitigate Part or Design related Risks

Mitigation of the part or design risk is possible by either changing the design or selecting other parts so the risks are avoided.

Calculate the New Zero Hour Defect Rate (ZHDR)

Based on the design changes a new Bill Of material (BOM) is created and the Zero Hour Defect Rate (ZHDR) must be (re)calculated.

When a design change is not possible, the design must be tested on the risks to make sure that no part which could fail reaches the customer.
The same methodology also applies for the design related risks as for the assembly related risks. A test must be applied on the risks and the test slip multiplied with the risk DPMO.

Mitigated Zero Hour Defect Rate (ZHDR) Calculation

The Zero Hour Defect Rate (ZHDR) design risks are multiplied with the corresponding slip of the applied test.

This results in a new Zero Hour Defect Rate (ZHDR) which must be checked if the Zero Hour Defect Rate (ZHDR) is within specification.

When an assembly is handled and transported, before its reaches the customer, the handling and transport risks must be taken into account.

Handling Related Risks

Assemblies are handled to go to a storage area or packaging area.

• Electro Static Discharge (ESD)
• Packaging Risks
• Storage risks

Transport Related Risks

• Transport damage risks

Calculate the New Zero Hour Defect Rate (ZHDR)

Based on the changes in the handling and transport process the Zero Hour Defect Rate (ZHDR) must be calculated.

Mitigated Zero Hour Defect Rate (ZHDR) Calculation

The Zero Hour Defect Rate (ZHDR) handling and transport risks are multiplied with the corresponding slip.

This results in a new Zero Hour Defect Rate (ZHDR) which must be checked if the Zero Hour Defect Rate (ZHDR) is within specification.

Reliabilty performance is mostly depicted by the bathtub curve which describes the three phases of the product life time (Initial, useful life, wear).

Infant Mortality Related Risks

Random Life Time related risks

Structural Life Time related risks

Wear Life Time related risks

• Heat

(1 First pass Yield (FPY)
(2 Yield
(3 Mcarthy, Thomas. The Six Sigma Black Belt Handbook, p. 307. 2004 McGraw Hill Education. ISBN10: 0071443290
(4 Througput Yield (TPY)
(5 Rolled Throughput Yield (RTY)
(6 Lunau, Stephan. Design for Six Sigma + Lean Toolset, p. 133. 2009 Springer-Verlag Berlin Heidelberg. ISBN 978-3-540-89513-8.
(7 Institute for Interconnecting and Packaging Electronic Circuits(IPC) IPC-7912A
(8 Yang, Kai, Design for Six Sigma, Chapter 11 2003 McGraw Hill, ISBN: 0-07-141208-5
(9 Mcarthy, Thomas. The Six Sigma Black Belt Handbook, p. 383. 2004 McGraw Hill Education. ISBN10: 0071443290.
(10 Yang, Kai, ''Design for Six Sigma'', Chapter 10 2003 McGraw Hill, ISBN: 0-07-141208-5
(11 Tien-Chien chang, Richard A. Wysk, and Hsu-Pin Wang. Computer-Aided Manufacturing, Second Edition, Pages 596 to 598. Prentice Hall 1998
(12 The bathtub Curve and Product Failure Behaviour (Part1)
(13 The bathtub Curve and Product Failure Behaviour (Part2)

Litirature on Quality

George, Michael l., ''What is Lean Six Sigma'', 2003, McGraw Hill, ISBN-10: 007142668X
George, Michael l., ''The Lean Six Sigma Pocket Toolbook'', 2004, McGraw Hill, ISBN-10: 0071441190
Morgan, John, ''Lean Six Sigma For Dummies'', 2012, Wiley Publishing, Inc., ISBN-10: 1119953707
Gygi, Craig, ''Six Sigma for Dummies'', 2005, Wiley Publishing, Inc., ISBN: 0-7645-6798-5
Webber, Larry, ''Quality Control for Dummies'', 2012, Wiley Publishing, Inc., ISBN-10: 0470069090
Kemp, Sid, ''Quality Management Dymistifeid'', 2006, McGraw Hill Education, ISBN: 0-07-144908-6