risk assessment using design review based on failure mode

8/10/2019 Risk Assessment Using Design Review Based on Failure Mode

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Risk Assessment Using Design Review Based On Failure Mode

Roland Schmidt, PhD, ABB Switzerland

Gernot J. Riedel, PhD, ABB Switzerland

Klaus Kangas, ABB Oy Drives

Key Words: DRBFM, FMEA, Risk Assessment

SUMMARY & CO N CLUSIO N S

In order to improve sustainability of industrialapplications, big efforts are underway to increase efficiency ofelectrical power transmission, power conversion, or powerusage. The availability of electrical converters used in theseapplications is crucial for high efficiency. Therefore, riskassessment has to be performed during product developmentto improve their reliability. The method typically used is

Failure Mode and Effects Analysis (FMEA). In this paperanother method called Design Review Based on Failure Mode(DRBFM) is applied to a typical power converter andevaluated. The DRBFM approach used was found to be muchmore efficient, if the focus lies on the identification of thecritical problems and on improvements. On the other hand,FMEA is the method of choice for focusing on a complete listof possible failures.

1 I N TRODUCTIO N

Improving sustainability is a key goal in the developmentand enhancement of electrical power generation, powertransmission, and electrical machines [1]. Big efforts areunder way to reduce losses, to increase efficiency, and toimprove the stability and availability of electrical powertransmission. The interconnection of power systems withhigh-voltage direct current (HVDC) technology and the use offlexible AC technologies (FACTS) reduce losses. Electricalconverters optimized for high efficiency increase thesustainability of photovoltaic power plants or wind parks.Variable speed drives for motors also reduce the waste ofelectrical energy [2] [3].

For the overall efficiency of sustainable energies ingeneral and wind power plants in particular high availability iscrucial. Therefore the reliability of the wind turbines and their

components is highly important [4]. This applies especiallyfor offshore wind park installations, since maintenance andrepair of equipment need high logistic effort, depend onweather conditions, and are consequently very expensive [5].Electrical equipment is the main contributor to system failuresof wind parks [6]. In order to increase the service andmaintenance intervals and to reduce instant failures, improvedreliability of the electrical equipment is necessary.

To reach this goal, quality and reliability methods have to be introduced into design and production of large electrical

equipment. Automotive industry faced a similar situation,when it started to ask for zero defect products and zero defectstrategies [7]. This goal cannot be achieved by quality controland reliability testing alone. Knowledge and understanding of

possible failures and their mechanisms are necessary. Thisresulted in physics-of-failure approaches [8] and design-inreliability [9]. Even more important became risk assessmentmethods which identify critical or key failures in a structured

way.Such risk evaluations should be included within the full production process of products. Already at the early designsteps reliability issues and possible failures should beaddressed. The later a failure is detected the higher are effortsand costs to eliminate it. A rule of thumb states that with eachstep during the product development process the costs offailures increase by a factor of 10 [10].

Several approaches for risk assessment duringdevelopment and production are available, including FailureMode and Effect Analysis (FMEA), Fault Tree Analysis(FTA), Ishikawa diagram, etc. They follow a strict procedureand are highly structured. Whereas FTA and Ishikawa

diagram start from an observed or expected failure, FMEA is amore general approach trying to identify all possible risks.

FMEA [11] is the most widely used tool to assess risks of products. It is a team-oriented and cross-functional methodused for failure prevention. The result of an FMEA is athoroughly filled-in form (Fig. 1).

However, there are some serious drawbacks in performingan FMEA. It is very time consuming and binds a lot ofengineering and management manpower. Participants areoften not really interested to gain the best benefit, but want toget over and done with it as soon as possible. Especially inthe early development stages, FMEA is often not performeddue to a lack of design information, which is needed to fill inthe FMEA form thoroughly. Additionally, if an FMEA is notreviewed and updated regularly, it will loose its maincapability, namely giving an actual overview of risks andtaken countermeasures for improvement. Instead ofidentifying possible failures and defining countermeasures,time and energy is wasted to discuss intensively the ratings forseverity, occurrency, and detection of failures. Due to thesefacts, some reluctance is often observed against theimplementation and performance of FMEA within a company.


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General details on this method, and especially on the rating problem, can be found in [12] [13]. Most FMEAs focus on acomplete filled-in failure list. This is meanwhile often donewith software tools. Improvement will not be achieved by

perfectly filled-in forms, but through actions derived from anFMEA.

To overcome some of these drawbacks, the FMEAmethod is changed or adapted to the internal needs andexperiences of a company or product, e.g. [14]. Othermethods derived from FMEA may come up and will beestablished within industrial sectors. For example, Design

Review Based on Failure Mode (DRBFM) is one of thesemethods, which is often used in the automotive industry.

2 DRBFM METHOD

Design Review Based on Failure Mode (DRBFM) wasdeveloped and established within Toyota Motor Corporation

by Tatsuhiko Yoshimura [15]. He realized that often theFMEA table is blocking a creative design review. As changesinvolve the highest failure potential, DRBFM was developedmainly to identify failures due to a design change.Background is the concept of Mizen Boushi (reliability

problem prevention) and the innovation process GD3 (G.D.Cube) [16]. GD3 stands for

good design good discussion good design reviewGood design means that parts should be kept, if they

already have a proven good design and if they do not have to be changed necessarily. Good discussion stands for creativeand open minded discussions during which risks and failures

can be revealed without any reservation. It is the difficult taskof the leader or moderator to establish such a good discussion.Good design review means, that all design modifications arecritical and are in depth analyzed and discussed.

'DRBFM is a method of discovering problems anddeveloping countermeasures by taking notice of anddiscussing intentional changes (design modifications) andincidental changes (changes in part environment). Themethod encourages the human ability to find problems, and isa practical tool based on FMEA and FTA' [16].

DRBFM consists of two parts. In the first part, theanalytical phase, the designers put together the changes andidentify possible failures for the function and implementation

related to them. They also define countermeasures. In thesecond part, the design review, an interdisciplinary teamreviews the design and the failures identified by the designers.Additional failures will be identified and actions defined [16][17]. To summarize and structure the results a form derivedfrom FMEA form is used (Fig. 2). The filled in form shouldgive a thorough overview of components (column 1), theaffected functions and requirements (column 2), possiblefailures or loss of function (column 3), the root causes of thefailure (column 4) and the impact or effect of a failure(column 5). A small column "Evaluation" belongs to column

Fig. 2 - Headline of a DRBFM form. The light fields should be prepared by the design team.The dark fields will be filled in during the design review meeting [17].

Fig. 1 - Example of an FMEA form.


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5. In this column it is possible to rate the effect (severity) orto identify it as hazardous or critical. Measures already takenin the design phase or production to eliminate the failure rootcauses or to detect them are given in column 6. Threecolumns are reserved for corrective actions. These actionswill be assigned to the design phase (column 7), the test orverification phase (column 8) and the production phase(column 9). Each action is assigned to a responsible persontogether with a deadline. Results and follow up results can

later on be put in column 10. Especially the question 'Anyother concern?' and 'Any other causes?' should lead to a deeperthinking on failures and root causes. For very importantfailures FTA, 5-Why method or Ishikawa diagram can be usedto identify the real root causes.

Fig. 3 shows a DRBFM from during the discussion process [17]. Removable sticky notes are used instead of thefixed fields of a table. This allows a more creative and livelydiscussion. The focus is not any more on the task of filling inthe blank fields of the table, but on the subject underdiscussion.

Fig. 3 - Typical picture of a DRBFM table

The sticky notes can easily be grouped and regroupedduring the team session supporting an open discussion.Failure (3), failure root cause (4), failure effect (5),countermeasures (6) and recommended actions (7-9) do not

have to stay necessarily within one line. If necessary orwished relations can be indicated alternatively with numbers.The most important result is the list of recommended actions,which gives the input to improve the quality and reliability ofthe product.

3 APPLICATIO N

The reliability of power electrical converters is veryimportant as discussed above. Therefore methods were looked

at to perform risk assessments with the focus on identifying possible failures and risks and to achieve productimprovement. As described, FMEA shows some drawbacks,if used as tool. Here DRBFM was chosen to be evaluatedduring risk appraisal. To perform a DRBFM of powerconverters, a team was assembled consisting of several veryexperienced product engineers. The aim was to identify risks,evaluate them, and define possible actions for improvement.The assessment was not performed as a functional or system

risk estimation. Instead, it was looked at components, groupsof components, design, and manufacturing. Therefore, theconverter was divided in several main parts (Fig. 4). Boardassembly was dealt with as a separate part, as it is related to all

printed circuit boards.

Fig. 4 - Principal parts of a converter.

The main focus of this risk assessment was not to puttogether all possible risks, but to identify main failure

possibilities, to investigate their root causes, and to define possible actions to eliminate these failures. This riskevaluation was not part of a product development with designreviews. Therefore, only the first part of the DRBFM wasused as a structured way to sum up and to order the upcomingideas. The form was modified, as it is shown in Fig. 5.

At the beginning of the assessment, a short introduction tothe method was given. For the DRBFM no software tool wasused, but boards with plain paper. The entries were written onsticky notes, pinned to the board, and freely placed during thediscussions. For each part of the converter an own DRBFMtable was used. These main parts were then divided in severalelectrical, mechanical, or optical components or componentgroups.

First, the main function of the group under investigationwas defined. Afterwards, possible failures were collected andthe root causes identified. It was not looked at the loss offunction in detail, Instead it was assumed, that the loss offunction is a failing converter no matter whether the converter

just stops or the power module explodes. The analyses of thedifferent main components were photographed fordocumentation.

Fig. 5 shows two examples of the numerous entriesachieved during the discussion. One example is the thermal

paste (1). It is applied between the power module and theheatsink. Its function (2) is to transfer the heat from the power


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module backside to the heatsink to cool the module. If theheat transfer is not good enough the power module will heatup during operation (3). This will lead to a power modulefailure (5). There are several root causes (4) given: too muchthermal grease, too little thermal grease, or bad material (notshowing a low thermal resistance). There are already sometools implemented (6) to guarantee the right amount andthickness of the thermal grease. As a possible action fordesign (7), it was defined to look into materials with improved

thermal conductivity and applicability. No entries were madeduring this DRBFM for evaluation (5). Instead importantentries were marked with an up-arrow and less importantentries with a down-arrow (see column 4). Also no entrieswere made in columns (8), (10), and (12). The column resultor follow-up (13) was kept empty as no decisions were madeduring the assessment.

The second example is taken from board assembly (1).The loss of function (3) looked at is simply that it is notworking. One root cause (4) out of many is that a wrongcomponent is used. A sequence of root causes was identified.Components were mixed during board assembly. This mayhappen, if the reels containing the devices are mixed or taken

falsely as they sometimes are not barcoded. Wrongcomponents may be detected during the performance ofelectrical tests (6), however not all wrong components might

be detected and there might be additional cost for rework.Therefore as a possible action concerning production (11) arequest to the board assemblers (often external suppliers) isattached to use barcoded reels for right identification. Thisexample shows that it is very easy to group and display asequence of root causes.

The results in the tables were evaluated and summarizedin a report for better understanding. They can be discussedwith other engineers and management defining next actions.

Such a discussion may take over the role of the design review.

4 EVALUATIO N OF THE APPROACH

After the risk assessment the method and the approachwere evaluated and compared with the standard FMEAmethod. The following advantages and disadvantages of theDRBFM approach were found.

The advantages are: Very time efficient.

Possible problems are identified and listed. It gives a good overall picture. It permits an overview of

possible failures and gives a list of possible actions: A clear overview of topics to be fixed is the result. The

result of the assessment is not a filled-in table, but possibilities for improvement.

All possible actions address realistic risks. The focus was on problems not on rating or calculating

the risk priority number (RPN). Therefore: No time was wasted for rating. The results and actions can directly be used for new

product development. It looks like brainstorming, but it is not. Brainstorming

has several drawbacks [14]. Therefore it was very positively registered that it is a different method. Theapproach is much more structured, makes it easier for themoderator to guide the discussion and leads to well-thought root causes and therefore possible actions.The disadvantages are:

The information of a risk priority number (RPN) islacking. A ranking of failures and actions is not so easily

possible. Failures and especially some root causes are not taken

into account, if they were assumed as not so important byteam members.

Fig. 5 - Adapted DRBFM form with some example entries of the risk assessment.


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The following ideas for future use were collected: The function of the component discussed should be more

in the center of the discussion. From there the failuresand the root causes can be derived easily.

The 'ideal team' will consist of:2-3 product (development) engineers,2-3 predevelopment engineers, anda moderator

The following skills and information were used or

needed: product example on displayschematicsdesign know-howknowledge of failing partsdeep understanding of converter production and

testing / testing performed externallyinformation and background of involved suppliers

For this risk assessment the consumed time was about 10hwork in three sessions within 1 week.

The comparison between this DRBFM approach and atypical FMEA was discussed and rated within the team. Theresult is shown in table 1.

DRBFM FMEAOverall Efficiency 4 2Time Efficiency 5 2Benefit (new ideas) 3-4 2-3Feeling 4-5 2Coverage 3 4Standard /Customer Request

2 5

Table 1 - Comparison between DRBFM and FMEA(from 1 = bad to 5 = very good).

The results in the table show clearly the benefits of theDRBFM approach. Especially the time efficiency was ratedvery high compared to a classical FMEA. This is mainly

because of the creative approach, the concentration on problems and the omittance of detailed rating. Also the benefit of new ideas was rated much higher. Regarding thecoverage of the possible failures the FMEA was rated better.This is due to the fact that all failures are listed in the FMEAtable, whereas in this approach less important or veryimprobable failures were skipped immediately and not put inthe table. Customers easily accept FMEA as a method for riskassessment. The DRBFM approach has to be explained tothem thoroughly.

Very interesting is the emotional rating (how did youfeel?), which is strongly in favor of the DRBFM. Therefore, itis a possibility to enhance the attention of the engineers. Thisleads to better discussion and better results. Finally the overallefficiency was also rated much more positive for the DRBFMapproach.

Summarizing the evaluation shows that the DRBFMapproach has advantages due to a faster possibility to reach

product improvements. This is for engineers more interesting

and worth spending time than to fill in an FMEA table. Thismethod is addressing more the creativity leading to more newideas. A disadvantage is the fact that possible failures are notcollected and summed up so completely as with an FMEA,which therefore shows a lot of redundancy not necessary withthe DRBFM method. The FMEA is a widely known andapplied method showing high acceptance. If a quick but stillthorough risk assessment should be performed or a teamshould improve a product or its reliability the DRBFM

approach appears to be the better choice.This DRBFM approach can be summarized as follows:1. Collect and prepare information on the prototype device

or change. This might include drawings, former productsor product information, concept images, material

properties, test results etc.2. Put together the team. Next to product development

engineers, engineers from predevelopment or otherexperts should be involved. Additionally to themoderator, it should consist of two or more productengineers and two or more predevelopment engineers.Their experience should cover the areas electrical,thermal, mechanical, material issues or others.

3. Perform the DRBFM. Several shorter sessions are moreconvenient than one long one. Establish a niceenvironment with coffee and snacks.

4. Summarize the results in a report and identify therecommended actions. Present the results of the DRBFM,discuss and define corrective actions. For future use thediscussion and results can also be documented in aspreadsheet or database.

5 CO N CLUSIO N

A DRBFM approach was used for risk assessment of a power converter. It was evaluated and compared withstandard FMEA method. The risk evaluation using theDRBFM method seems to be much more efficient, if the focuslies on the identification of the most critical problems and on

product improvement. In contrast, FMEA is the method ofchoice for assessments focusing at a complete list of possiblefailures.

REFERE N CES

1. P. K. Steimer, "Enabled by High Power Electronics -Energy Efficiency, Renewables and Smart Grids",

International Power Electronics Conference , (Jun.) 2010, pp. 11-15.

2. P. Barbosa, C. Haederli, P. Wilkstroem, M. Kauhanen, J.

Tolvanen, A. Savolainen, "Impact of Motor Drives onEnergy Efficiency", International Exhibition andConference for Power Electronics / Intelligent Motion /

Power Quality (PCIM), (May) 2007.3. G. D. Demetriades, H. Z. de la Parra, "Trends in Power

Electronics and Variable Speed Drives", IEEE Power Electronics Specialists Conference , (Jul.) 2007.

4. E. Echavarria, B. Hahn, G. J. W. van Bussel, T.Tomiyama, "Reliability of Wind Turbine TechnologyThrough Time", Journal of Solar Energy Engineering ,


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vol. 130, (Aug.) 2008.5. J. Twidell, G. Gaudiosi (Eds.), Offshore Wind Power ,

Multi-Science Publishing, 2009.6. G. J. W. van Bussel, Offshore Wind Energy, the

Reliability Dilemma", Proceedings of the 1st World Wind Energy Conference and Exhibition , Berlin, Germany,(Jul.) 2002, pp 1-4.

7. W. Kanert, "Qualification Strategies in the Age of ZeroDefect". AEC Reliability Workshop 2006.

8. M. G. Pecht and A. Dasgupta, "Physics-of-Failure: AnApproach to Reliable Product Development", Journal ofthe Institute of Environmental Sciences , vol. 38, 1995, pp.30-34.

9. J. Snook, J. M. Marshall, R. M. Newman. "Physics ofFailure - As an Integrated Part of Design for Reliability".

Annual Reliability and Maintainability Symposium , (Jan.)2003, pp. 46-54.

10. Tilo Pfeiffer, Quality Management: Strategies, Methods,Techniques , Mnchen, Hanser Fachbuchverlag, 3rdedition 2002.

11. D. H. Stamatis, Failure mode and effect analysis: FMEA from theory to execution , Milwaukee, American Society

for Quality, 2nd edition, 2003.12. N. Bidokhti, "FMEA is Not Enough", Proc. Ann.

Reliability & Maintainability Symp. , (Jan.) 2009.13. Z. Bluvband, P. Grabov, "Failure Analysis of FMEA",

Proc. Ann. Reliability & Maintainability Symp. , (Jan.)2009.

14. R. Schmidt, "Risk Assessment in Research Projects", Proc. Ann. Reliability & Maintainability Symp. , (Jan.)2010.

15. B. Haughey, T. Yoshimura, "Design Review Based onFailure Modes - DRBFM", International Applied

Reliability Symposium , San Diego, (Jun.) 2007.16. H. Shimizu, T. Imagawa, H. Noguchi, "Reliability

Problem Prevention Method for Automotive Components- Development of GD3 Activity and DRBFM (DesignReview Based on Failure Mode)", International Body

Engineering Conference , (Oct.) 2003, pp 371-376.17. A. Kapust, Design Review Based on Failure Mode

(DRBFM) , Training course of IMQ Consulting GmbH,Koepffstrae 17, D-74076 Heilbronn, Germany,www.tqm.com.

BIOGRAPHIES

Roland SchmidtABB Switzerland Ltd.

Corporate ResearchSegelhofstrasse 1 K

Baden 5 Dttwil, 5405, Switzerland

e-mail: [email protected]

Roland Schmidt has more than 15 years experience withreliability and quality. He works as principal scientist atCorporate Research of ABB Switzerland on reliability of

power electronics. He is dealing with the reliability ofcomponents, interconnection technologies and electricalsystems as well as quality and reliability methodologies. Prior

to joining ABB he worked on reliability and failure analyticsof semiconductor packages. He was also responsible forsupplier quality assurance in the semiconductors industry. Hereceived his diploma in physics from the University ofErlangen and his PhD in physical chemistry for his work at theMax-Planck-Institute for Polymer Research in Mainz.

Gernot J. RiedelABB Switzerland Ltd.Corporate ResearchSegelhofstrasse 1 KBaden 5 Dttwil, 5405, Switzerland


Gernot J. Riedel works as scientist at Corporate Research ofABB Switzerland on topics regarding reliability in powerelectronics since 2009. He mainly investigates the physics offailures on component level and the reliability of systems. Heis also interested in system diagnostics and prognostics. Hereceived his PhD in physics for his work at the University ofBristol, where he studied the thermal and electrical dynamicsof GaN based high frequency and high power electronicdevices. He received his diploma in physics from theUniversity of Karlsruhe (TH).

Klaus KangasABB Oy DrivesHiomotie 13Helsinki, 00380, Finland


Klaus Kangas has 8 years experience with reliabilityengineering. He works as a reliability specialist at ABB OyDrives on topics regarding reliability engineering andaccelerated testing. Improving reliability through global ABBreliability co-operation and projects is one part of his work.Besides reliability topics in ABB, he has been for many yearsa chairman of the reliability workgroup in Finnish nationalKOTEL association. He received his diploma in electrical

engineering from the Helsinki University of Technology.

risk assessment using design review based on failure mode

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