Movip

Modeling van Initiele Product Kwaliteit

Modeling of System or Component Initial Quality (ZHDR)

When assembling systems its evident that the risk of a failure increases with increasing the number of parts and/of assembling activities. This number of failures is the Zero Hour Defect Rate (ZHDR) of a system. Zero Hour Defect Rate (ZHDR) reflects the number of systems or components which failed, with respect to the number of system or components delivered during an agreed period of time. A “failure” is the unplanned occurrence that prevents the system or component from meeting its functional requirements under the specified operating conditions. The time period in which a system or component fails can be as short as on receipt of the component or as long as the build and test time when this system or component is used in a larger system.
Failure mechanisms are system or component Assembly, Design, Handling and Transport.

After this agreed period the system or component to determine the Zero Hour Defect Rate (ZHDR), the failure mechanism is life time related.

Assembling Failure Contributions

The assembling quality of systems or components is described by the number of good systems or components at the end of the assembling process before the systems or components are tested divided by the number of delivered systems. This quality is a measure of the First Pass Yield (FPY) (1 of the system or component. The definition of First Pass Yield (FPY) is the number of systems or components out of the manufacturing process without any rework or corrections divided by the number of delivered systems or components.

First Pass Yield (FPY)

The yield has many definitions so the Zero Hour Defect Rate (ZHDR) is introduced to overcome the confusing definitions of the Yield (2. Yield is defined in a number of (un)ambiguous definitions such as First Pass Yield (FPY) (3, First Time Yield (FTY), Througput Yield (TPY) (4, Rolled Throughput Yield (RTY) (5, Yield after test, Test Pass Yield, Normalized Yield (6, Final process Yield, Overall Yield, Traditional Yield, Final Yield, Input Yield, Yield, .... . The Zero Hour Defect Rate (ZHDR) is a measure of the number of failed systems or components divided by the number of delivered systems or components.

Zero Hour Defect rate (ZHDR)

When a system or component is an assembly of parts (Bike, laptop, lamp, chair, photo camera, ...) the Zero Hour Defect Rate (ZHDR) is an add up of all individual part ZHDR's and the added workmanship ZHDR's. When the assemblies become larger, it will be more difficult to determine the Zero Hour Defect Rate (ZHDR) of the assembly. Due to the large number of parts used in the electronic industry of building Printed Circuit Board Assemblies (PCBA) a method to determine the Zero Hour Defect Rate (ZHDR) is available at the Institute for Interconnecting and Packaging Electronic Circuits(IPC) IPC-7912A (7. For other disciplines such as Electromechanical, Mechatronical, Optical, ... no standardized methodologies are available other than Process Failure Mode and Effects Analysis (PFMEA)(8, yield calculations, ..). All those methodologies didn't lead to a simple and unambiguous methodology to determine the Zero Hour Defect Rate (ZHDR) of an assembly.

Zero Hour Defect Rate (ZHDR) Model

To get more insight into the Zero Hour Defect Rate (ZHDR), a number of OEM's (ASML, Philips Healthcare, Assembleon and FEI) with a group of assembly builders (Neways, Techno TBP, Prodrive, KMWE, NTS, Sioux, Mi-Partners, CCM, Fibreworks, Bicore) and 2 knowledge institutes (IMEC and TNO) have come up with a methodology: Modeling of Prediction of initial Product Quality (MOVIP) to determine the Zero Hour Defect Rate (ZHDR) of all types of assemblies. The (Movip) methodology resulted in a model which categorizes all assembly risk in a limited number of unambiguous bucketed Defect Opportunities (DO) which, when complete, results in a Zero Hour Defect Rate (ZHDR) based on the assembly Risks.
The outcome is a quantized Zero Hour Defect Rate (ZHDR) and above all an insight in which category the assembly risks are the highest.
This Zero Hour Defect Rate (ZHDR) is no measure for the life time risk, which is dictated by other fail mechanisms during use and is only a measure for the acceptance of the system or component.
The MoVip methodology can be used starting in the design phase, based on the Bill Of Materials (BOM) and the schematics (electronics) or the 3D design in the mechanics/mechatronics. The outcome of the model is an insight in the risks of the assembly before the design is finished. With this insight the Zero Hour Defect Rate (ZHDR) can be influenced. Based on this, the consequences of the remaining risks are clear and the mitigation strategy can be started. This can result in a design change which influences the ZHDR. This is a continuous process so the Zero Hour Defect Rate (ZHDR) improves during the design of a system or component.

Defect Opportunities (DO)

To determine the Zero Hour Defect rate (ZHDR) of a system or a component each defect opportunity has to be taken into account. The notation is done in Defects Per Million Opportunities (DPMO) Mcarthy, Thomas. The Six Sigma Black Belt Handbook, p. 383. (9

DPMO

To have unambiguous Defect Opportunities (DO) a bucketing of the Defect Opportunities (DO) has resulted in an unambigios set of Defect Opportunities (DO) with a corresponding risk description represented in a Defect Per Million Opportunities (DPMO).

bucketing

Buckets

Defect Opportunity (DO) bucketing is divided in Part Risks and Workmanship risks. Part risks are those risks related to the physical parts in the assembly without any assembly step done. the Workmanship risks are those risks that are occuring during the total assembly process.

Part Related Risks

Assembly added Risks

Part Risks

To determine the Zero Hour Defect Rate (ZHDR) of an assembly the risk analysis is divided between the parts and the activities. Each assembly is always a collection of parts reflected by the Bill Of Material (BOM). The Defect Opportunities (DO) are coupled to a part where each part has 3 Defect Opportunities (DO).

Part Related Risks

Part Related Risks

Part related risk is best described when the parts are ready for assembly. What's the risk on not working at all (defect) or not fitting in the assembly (physical out of specification) or not working correctly (functional out of specification).

Part Defect Risk

Part is Defect

A part is defect when the part is not conform specification. Two special cases are distinguished where the part is either physical or functional out of specification. These categories are used when more information is available over the defect type of the part.

Example: Part is damaged beyond salvation and will neither fit or function

Part is Defect

Part is Defect

Part is Physical out of Specification

Part is not conform the physical specifications. This can be a large number of physical defects.

  • Hole is missing
  • Hole is not threaded
  • Surface is scratched
  • Part is not clean
  • Part has bend legs (Integrated Circuit)
  • ...

Example: Part has missing thread for a screw mount

Part is Physical out of Spec

Part is Physical out of Spec

Part is Functional out of Specification

Part functions not conform specification but is still working.

  • Motor turns slower than specified
  • Transmission lens is lower than specified
  • Bandwidth/noise is not within specification
  • ...

Example: Motor should run at 3000rpm at 12V but rotates at 1000rpm.

Mounting Risks

Next step in the assembly process is the mounting of parts. Mounting has 4 defect Opportunities. The Mounting Defect Opportunities (DO) are activity based.

Mounting Related Risks

Mounting Related Risks

Risks of mounting the part in the assembly process on the wrong way.

Part Mounting Defect Risk

Part is Missing

Part is missing in the assembly. This can be either forgotten or due to the process steps in the assembly causing the missing part.

  • Ring is missing
  • Fastener is missing
  • Electronic parts missing on a Printed Circuit Board (PCB)
  • Cover missing
  • ...

Part is missing

Part is Missing

Part is Wrong

Part ready for assembly is not the part which is on the Bill Of Material (BOM). If it is a complete different part it is easily detected during assembly but if the shape is the same or slightly different the assembly can hold a wrong part.

  • Wrong electronic component (value) in a Printed Circuit Board Assembly (PCBA)
  • Magnetic fasteners where non magnetic fasteners are required
  • Bolt length
  • ...

Part is wrong

Part is Wrong

Part is Misoriented

Part is placed with the wrong orientation in the assembly.

  • Inside out
  • 90° rotated
  • ...

Part is misoriented

Part is Misoriented

Part is Misplaced

The part is not mounted on the right position.

Part is misplaced

Part is Misplaced

Mounting Reference Part

To avoid counting the risk more than once for the mounting a reference part is required. In an assembly the part where the assembly starts or the frame/mounting platform is the best candidate. All risks are referenced to that part. Mounting risks are 0 for this reference part. An example is a part misoriented to the reference part is only 1 misorientation risk.

Connection Risks

The next step in determining the assembly Zero Hour Defect Rate (ZHDR) is the connecting of the parts to form an assembly. For each connection (Solder, screw, glue, staple, rivet, bond, electrical connect, crimp, ......) there is a risk in not meeting the required connection specification. This could be a fastener (not tightened correctly), a solder joint (connection fault), a glue connection (not strong enough), a crimp (not pressed with the correct force), a ......
Each connection is a Defect Opportunity (DO) (fastener, solder joint, glue, bond, crimp, ...) with a corresponding risk and must be taken into account.

Connection related Risk

  • Solder
  • Fasterner
  • Glue
  • Connect electrical
  • Connect Fluids (hoses, pipes, ..)
  • Connect gasses (hoses, pipes, ..)
  • Bond
  • Press
  • Crimp
  • Click
  • ....

Some connections contains a number of parts (fastener, rings, nut) or contain only one part (nut) or no parts at all (pressfit, glue, click).

Connection

Part is Misconnected

When a connection contains more than one part this connection is in itself a hierarchy and the result of this hierarchy is a new part which has not only part and mounting risks but also a connection risk. Connections containing only one part have the part and mounting related risks and a connection risk. Connections containing no parts (Glue, Crimp, press, ..) have only a connection risk.

Connection Risk

Assembly Processing Risks

The last phase on the assembly are processing steps on the complete assembly. Each step poses again a risk on the total assembly.

Assembly Processing Risks

  • Coating
  • Potting
  • Cleaning
  • Excess parts
  • Adjusting
  • Greasing
  • Testing
  • ....

Part is misplaced

Excess part

Each Process Step adds a risk to the final Zero Hour Defet Rate (ZHDR).

Assembly Risk

Zero Hour Defect Rate (ZHDR) calculation

The assembly Zero Hour Defect Rate (ZHDR) can be calculated by taking for each Bill Of Material (BOM) item the part and mounting Defect Opportunity (DO) corresponding risks (7 per BOM item), from the schematic (Printed Circuit Board Assembly (PCBA)) or 3D drawings (mechanics, mechatronics, optics) the connection risks followed by the assembly processing risks.
  • Part Risks
  • Mounting Risks
  • Connection Risks
  • Assembly Process Risks
When the DPMO's of the risks are within a small band (5-10ppm or 20-30ppm) use the graph to determine the Zero Hour Defect Rate (ZHDR)

Zero Hour Defect Rate (ZHDR) Graph

When the DPMO's vary outside a limited band use the formula below

Zero Hour Defect Rate (ZHDR) Calculation

All these Defect Opportunity (DO) corresponding risks together, give an insight in which risk contributes most to the Zero Hour Defect Rate (ZHDR) of the assembly.

Contributors:

  • Part Risks
  • Mounting Risks
  • Connection Risks
  • Assembly Process Risks

Hierarchy

With complex assemblies the build sequence is mostly done in a hierarchical way. When encountering such a hierarchy each hierarchical level results in itself in a part with all the part, mounting, connection and assembly related risks. This part is treated as a part one hierarchical level up. When out of the design phase the build sequence is done in takts and can be treated the same way as a hierarchy.

Hierarchical connections

When connecting cables, hoses (fluids, gasses, vacuum, ..), this could be treated as a separate hierarchical level. The cable, hose in itself is treated as a part with a risk and as a connection with its connection risks. This methodology is needed to avoid counting risks more than once.

Printed Cicuit Board Assemblies (PCBA) connections

When connecting Printed Circuit Board Assemblies (PCBA) to each other, counting the risk more than once is not allowed. An approach could be to select the male or female connectors for determining the connection risk.

Risk Mitigation

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.

Zero Hour Defect Rate (ZHDR) realized

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

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.

Zero Hour Defect Rate (ZHDR) Calculation

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

Zero Hour Defect Rate (ZHDR) realized

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.

Zero Hour Defect Rate (ZHDR) Calculation

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

Zero Hour Defect Rate (ZHDR) realized

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.

Slip

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.

Zero Hour Defect Rate (ZHDR) Calculation

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

Zero Hour Defect Rate (ZHDR) realized

Design Failure Contributions

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.

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.

Zero Hour Defect Rate (ZHDR) Calculation

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.

Zero Hour Defect Rate (ZHDR) Calculation

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

Handling and Transport Failure Contributions

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.

Zero Hour Defect Rate (ZHDR) Calculation

Mitigated Zero Hour Defect Rate (ZHDR) Calculation

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

Zero Hour Defect Rate (ZHDR) Calculation

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

Life Time Failure Contributions

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

Relia bathtub Curve

Infant Mortality Related Risks

Random Life Time related risks

Structural Life Time related risks

Wear Life Time related risks

  • Heat
  • References

    (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