This document discusses using physics of failure (PoF) methodology to develop optimized test plans that are tailored to a product's specific design, materials, use environment, and reliability needs. It provides an overview of key aspects of test plan development including defining reliability goals and the use environment, identifying failure inducing loads, developing a comprehensive test plan, and ensuring change control processes and ongoing reliability testing are in place. The document also presents a case study of how PoF modeling was used to develop a test plan for microinverters intended for a 25-year lifespan in harsh outdoor solar installations.

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Test Plan Development using Physics of Failure: The DfR Solutions Approach
Cheryl Tulkoff, ASQ CRE
ctulkoff@dfrsolutions.com

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Introduction
oAgenda
oIntroduction to Test Plan Development
oIntroduction to Physics of Failure Methodology for Test Plan Development
oVirtual Qualification Option
oAfter Release
oCase Study
Power Cycling
Vibration
Temperature/
Humidity
Mechanical Shock
Temperature Cycling

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oHow can you be sure that you have the best test plans for your specific need?
oDfR Solutions has worked closely with over 500 OEMs in multiple industries developing hundreds (thousands??) of test plans.
oDfR Solutions experts have written product specs for these organizations.
oThrough close collaboration with industry leading electronics manufacturers and deep industry knowledge, DfR Solutions delivers the right test for the right product every time!
Test Plan Development

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Test Plan Development
oProduct test plans are critical to the success of a new product or technology
oStressful enough to identify defects
oShow correlation to a realistic environment
oDfR Solutions approach
oIndustry Standards + Physics of Failure
oResults in an optimized test plan that is acceptable to management and customers
•MIL-STD-810,
•MIL-HDBK-310,
•SAE J1211,
•IPC-SM-785,
•Telcordia GR3108,
•IEC 60721-3, etc.
•PoF!

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oPoF Definition: The use of science (physics, chemistry, etc.) to capture an understanding of failure mechanisms and evaluate useful life under actual operating conditions
oUsing PoF, design, perform, and interpret the results of accelerated life tests
oStarting at design stage
oContinuing throughout the lifecycle of the product
oStart with standard industry specifications
oModify or exceed them
oTailor test strategies specifically for the individual product design and materials, the use environment, and reliability needs
Physics of Failure (PoF)

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Industry Testing Falls Short
oLimited degree of mechanism-appropriate testing
oOnly at transition to new technology nodes
oMechanism-specific coupons (not real devices)
oTest data is hidden from end-users
oQuestionable JEDEC tests are promoted to OEMs
oLimited duration (1000 hrs) hides wearout behavior
oUse of simple activation energy, with incorrect assumption that all mechanisms are thermally activated, can result in overestimation of FIT by 100X or more

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oFailure of a physical device or structure (i.e. hardware) can be attributed to the gradual or rapid degradation of the material(s) in the device in response to the stress or combination of stresses the device is exposed to, such as:
oThermal, Electrical, Chemical, Moisture, Vibration, Shock, Mechanical Loads . . .
oFailures May Occur:
oPrematurely
oGradually
oErratically
Physics of Failure Definitions

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Critical Elements for Developing Robust Test Plans
oTest Objectives!
oComparison
oQualification
oValidation
oResearch
oCompliance
oRegulatory
oFailure analysis
oElements
oReliability Goals
oDesign
oMaterials
oUse Environment
oBudget
oSchedule
oSample availability
oPracticality
oRisk

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Define Reliability Goals
oIdentify and document two key metrics
oDesired lifetime
oDefined as time the customer is satisfied with
oShould be actively used in development of part and product qualification
oProduct performance
oReturns during the warranty period
oSurvivability over lifetime at a set confidence level
oMTBF or MTTF (try to avoid unless required by customer)
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Test Plan Development – Define Use Environment
oThe critical first step is a good understanding of the shipping and use environment for the product.
oDo you really understand the customer and how they use your product (even the corner cases)?
oHow well is the product protected during shipping (truck, ship, plane, parachute, storage, etc.)?
oDo you have data or are you guessing?
oTemp/humidity, thermal cycling, ambient temp/operating temp.
oSalt, sulfur, dust, fluids, etc.
oMechanical cycles (lid cycling, connector cycling, torsion, etc.)
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Identify Use Environment
o Old School Approach: Use of industry/military specifications
oMilitary, IPC, Telcordia, ASTM…..
oAdvantages
oNo additional cost!
oSometimes very comprehensive
oAgreement throughout the industry
oMissing information? Consider standards from other industries
oDisadvantages
oMost more than 20 years old
oAlways less or greater than actual (by how much, unknown)
IPC SM785
MIL HDBK310

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Use Environment (cont.)
oBetter Approach: Based on actual measurements of similar products in similar environments
oDetermine average and realistic worst-case
oIdentify all failure-inducing loads
oInclude all environments
oManufacturing
oTransportation
oStorage
oField

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Examples of Failure Inducing Loads
•Temperature Cycling
–Tmax, Tmin, dwell, ramp times
•Sustained Temperature
–T and exposure time
•Humidity
–Controlled, condensation
•Corrosion
–Salt, corrosive gases (Cl2, etc.)
•Power cycling
–Duty cycles, power dissipation
•Electrical Loads
–Voltage, current, current density
–Static and transient
•Electrical Noise
•Mechanical Bending (Static and Cyclic)
–Board-level strain
•Random Vibration
–PSD, exposure time, kurtosis
•Harmonic Vibration
–G and frequency
•Mechanical shock
–G, wave form, # of events
Reliability Improvement with Design of Experiment, Second Edition, By Lloyd Condra

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Use Environment: Best Practice
oUse standards when…
oCertain aspects of your environment are common
oNo access to real use environment
oMeasure when…
oCertain aspects of your environment are unique
oStrong relationship with customer
oDo not mistake test specifications for the actual use environment
oCommon mistake

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Failure-Inducing Load Example: Electrical Environments
oOften very well-defined in developed countries
oIntroduction into developing countries can sometimes cause surprises
oRules of thumb
oChina: Can have issues with grounding (connected to rebar?)
oIndia: Numerous brownouts (several a day)
oMexico: Voltage surges

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Failure Inducing Temperature: Transport & Storage
Container and Ambient Temperature
15.0
25.0
35.0
45.0
55.0
65.0
75.0
0 50 100 150 200 250 300 350 400 450
Hours
Temperature (°C)
Container Temp (°C)
Outdoor Temp (°C) Temp.
Variation
In a
Trucking
Container

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Temperature: Long-Term Exposure
o For electronics used outside with minimal power dissipation, the diurnal
(daily) temperature cycle provides the primary degradation-inducing
load
Month Cycles/Year Ramp Dwell Max. Temp (oC) Min. Temp. (oC)
Jan.+Feb.+Dec. 90 6 hrs 6 hrs 20 5
March+November 60 6 hrs 6 hrs 25 10
April+October 60 6 hrs 6 hrs 30 15
May+September 60 6 hrs 6 hrs 35 20
June+July+August 90 6 hrs 6 hrs 40 25
Phoenix, AZ

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Humidity / Moisture (Rules of Thumb)
oNon-condensing
oStandard during operation, even in outdoor applications
oDue to power dissipation
oCondensing
oCan occur in sleep mode or non-powered
oDriven by mounting configuration (attached to something at lower temperature?)
oDriven by rapid change in environment
oCan lead to standing water if condensation on housing
oStanding water
oIndirect spray, dripping water, submersion, etc.
oOften driven by packaging

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General Test Plan Development Outline – PCBA Example
oComponent qualification (with end product in mind)
oThermal cycling, high temp, T&H, etc.
oPCBA qualification
oThermal cycling
oHALT/HAST
oDrop/shock
oHeat age
oSystem level qualification
oShock and Vibration
oDust testing
oTorsion
oEtc.
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Test Plan Development – for PCBAs continued…
oDevelop a comprehensive test plan
oAssemble boards at optimum conditions
oRework specified components on some boards
oVisually inspect and electrically test
oC-SAM & X-ray inspect critical components on 5 or more boards (+3 reworked for BGAs)
oUse these boards for further reliability testing (TC, HALT, S&V)
oPerform failure analysis
oCompile results and review

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Assemble at Optimum Conditions 66 pcs
Visual Inspection 66 pcs
Electrical Continuity Test (DC devices) 66 pcs
Send to Intel 10 pcs
X-Section Analysis (Devices ?) 2 pcs
Thermal Cycle Test (0/100C) 12 pcs
X-Section Analysis
(Devices ?)
1 pcs
Non-Op + Cont (S & V)
4 pcs
X-Section Analysis (Devices ?) 1 pcs
Square Wave (S & V) 4 pcs
X-Section Analysis (Devices ?) 1 pcs
2
4
4
1
1
4
HALT (Duration ?) 4 pcs
X-Section Analysis (Devices ?) 1 pcs
4
1
1
Dye / Pry Analysis
(Devices ?)
1 pcs
1
1
4
1
Dye / Pry Analysis (Devices ?) 1 pcs
Dye / Pry Analysis (Devices ?) 4 pcs
Dye / Pry Analysis (Devices ?) 1 pcs
Thermal + Non-Op +Cont (S & V)
4 pcs
X-Section Analysis
(Devices ?)
1 pcs
1
Dye / Pry Analysis (Devices ?) 1 pcs
1
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Example of a Test Plan (Notebooks)
Use for Rework Section*
25 pcs
25
1
2
2
7
2
2
NOTES:
* - Rework should follow the supplier’s standard
process for removing and replacing components
Number in lower right corner of box represents
Remaining quantity from that test element.

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oEffective failure analysis is critical to reliability!
oWithout identifying the root causes of failure, true corrective action cannot be implemented
oRisk of repeat occurrence increases
oUse a systematic approach to failure analysis
oProceed from non-destructive to destructive methods until all root causes are identified.
oTechniques based upon the failure information specific to the problem.
oFailure history, failure mode, failure site, failure mechanism
Don’t Overlook Failure Analysis!

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Virtual Qualification (VQ, Modeling)
oThis assessment uses physics-of-failure-based degradation models to predict time-to-failure
oModels include
oInterconnect fatigue (solder joint and plated-through hole)
oCapacitor failure (electrolytic and ceramic)
oIntegrated circuit wearout
oCustomers develop a degree of assurance that their product will survive for the desired lifetime in the expected use environment

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Sherlock ADA – A Reliability Assurance CAE Tool Suite - the Physics of Failure App.
It is not at the Iphone or Droid App store.
But yes there is now a Physics of Failure Durability Simulation App
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The 4 Parts of a Sherlock PoF Analysis
1)Design Capture - provide industry standard inputs to the modeling software and calculation tools
2)Life-Cycle Characterization - define the reliability/durability objectives and expected environmental & usage conditions (Field or Test) under which the device is required to operate
3)Load Transformation – automated calculations that translates and distributes the environmental and operational loads across a circuit board to the individual parts
4)PoF Durability Simulation/Reliability Analysis & Risk Assessment – Performs a design and application specific durability simulation to calculates life expectations, reliability distributions & prioritizes risks by applying PoF algorithms to the PCBA model
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oLighting products customer was attempting to develop a product qualification plan
oSherlock identified appropriate test time and test condition based on field environment and likely failure mechanism
PoF Modeling & Test Plan Development

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When to Repeat Testing: Change Control
oInadequate change control is responsible for many (some would say most) field failures.
oExamples would include
oBurning Li notebook batteries
oElectrolytic capacitor leakage
oRecent flip chip underfill problems
oCoin cell battery contact failure
oHeat sink clogging failure
oDDR2 Memory modules
oImAg corrosion
oAll changes need to be evaluated carefully (testing to failure recommended).
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On-Going Reliability Testing
oQualification shows that a limited number of early manufactured products (maybe even from a pilot line) are reliable.
oIt’s often not possible to create every permutation of component suppliers in the qual builds.
oHow do you know the product will remain reliable as you go to high volume and new component suppliers are introduced?
oThere is no perfect answer but an ORT program can help.
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Case Study: Solar Micro-Inverter Reliability
oThe electronic components used in a micro-inverter are commercial off-the-shelf (COTS)
oParts designed for consumer electronics but need to survive 25 years in solar installations
oOutdoor/Partially Protected & Temp Not Controlled
oLack of industry standards for testing

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What can be done?
oHow can a micro-inverter supplier design the product to meet the requirements AND convince the customer of this?
oOne new method: model the reliability of an electronic assembly in a variety of conditions based on the design (before building anything).
oDesign for Reliability (DfR) concepts and Physics of Failure (PoF) are used.

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Are there Methods to Model these Failure Mechanisms?
oYes!
oAlgorithms exist to estimate the failure rate from solder joint fatigue for different types of components.
oIPC TR-579 models PTH reliability
oRisk for CAF can be determined
oFinite Element Analysis can be used for Shock & Vibration risk.
oMTBF calculations can be performed to estimate component failure rates.

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Micro-Inverter Environment
oExtreme hot and cold locations (AZ to AK)
oPossible exposure to moisture/humidity
oLarge diurnal thermal cycle events (daily)
oLargest temp swings occur in desert locations where it can reach 64C in the direct sun down to 23C at night (Δ41C)

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Potential Inverter Failure Mechanisms
oSolder joint fatigue failure
oPlated through hole fatigue failure
oConductive anodic filament formation (CAF)
oShock or Vibration (during shipping)
oComponent wear out

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PTH Fatigue Results - Example

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Combined (SJ & PTH) Lifetime Prediction
oCombines analysis results into overall failure prediction curve
PTH
Solder Joint
Combined

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Combined Failure Rate is Provided

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Additional Uses for Modeling
oUse Sherlock to determine thermal cycle test requirements.
oUse to modify mount point locations
oUse to determine ESS conditions
oComponent Replacement
oDetermine impact of changing to Pb-free solder
oDetermine expected warranty costs

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Summary
oProduct test plans are critical to the success of a new product or technology
oStressful enough to identify defects
oShow correlation to a realistic environment
oPoF Knowledge can be used to develop test plans and profiles that can be correlated to the field.
oChange control processes and testing should not be overlooked (reliability engineer needs to stay involved in sustaining).
oOn-going reliability testing can be a useful (but admittedly imperfect) tool.
oPoF Modeling is an excellent tool to help tailor & optimize physical testing plans