Overview of highly accelerated life test (halt)

OVERVIEW OF HIGHLY OVERVIEW OF HIGHLY ACCELERATED LIFE ACCELERATED LIFE TEST Chet Haibel ©2011 ASQ & Presentation Chet Presented live on Jan 18th, 2012http://reliabilitycalendar.org/The_Reliability_Calendar/Webinars_liability Calendar/Webinars ‐_English/Webinars_‐_English.html

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What Is Reliability? CLASSICAL DEFINITION Reliability is the probability that a component, subassembly, instrument, or system will perform its specified function for a specified period of time under specified environmental and use conditions. 1Chet Haibel ©2012 Hobbs Engineering Corp.

What is a Product Failure?Failure is the inability of a device to perform its intended functionsunder stated environmental conditions for a specified time.Failures are classified into three types based on time: • Early-Life (Infant Mortality) • Useful-Life (Random-in-time) • Wear-Out (End of useful life)Each failure type has different kinds of causes and therefore differenttests to discover them and different methods of correction / prevention. 2Chet Haibel ©2012 Hobbs Engineering Corp.

What is a Product Failure? Failures are also classified into three types based on their persistence: • Hard Failure (Persistent) Typically a component must be replaced, but trouble-shooting may be done at room temperature with no vibration or other stimulus • Soft Failure (Temporary) Often merely removing the environmental stimulus clears the problem, but sometimes it is necessary to cycle power, clear fault logs, etc. Product must be stressed to duplicate and trouble-shoot soft failures Many very important reliability issues are SOFT FAILURES. • Intermittent Failure (Elusive) This is permanent but the failure mode must be put into a detectable state 3Chet Haibel ©2012 Hobbs Engineering Corp.

What Causes Product Failure? A component fails when applied load exceeds design strength. Applied Load Design Strength Failure Units of Applied Load, Strength 4Chet Haibel ©2012 Hobbs Engineering Corp.

Applied LoadsExamples of applied load might be:  Force  Voltage  Torque  Current  Tension  Wattage  Shear  Clock Speed  Pressure  Electrostatic Discharge  Electromagnetic Interference 5Chet Haibel ©2012 Hobbs Engineering Corp.

Design Strength Examples of design strength:  Torque rating of a bolt  Voltage rating of a capacitor  Current rating of a diode  Power rating of a resistor  Shear strength of solder  Tensile rating of plastic  Temperature rating of transformer insulation 6Chet Haibel ©2012 Hobbs Engineering Corp.

Bathtub Curve Early-Life Wear-Out Hazard Rate - h(t) Failures Failures Useful-Life Failures Random-in-Time Failures Life to the Beginning of Wear-Out Operating Time (t) 9Chet Haibel ©2012 Hobbs Engineering Corp.

Wear-Out Failures with time Load Strength Increasing Hazard Rate h(t) Failures due to cycle fatigue Corrosion Hazard Rate Frictional wear Shrinkage, cracking in plastic components Time Typical of mechanical systems 10Chet Haibel ©2012 Hobbs Engineering Corp.

Cycle Fatigue Stresses: Cycled by: • Pressure • Product Operation • Tension • Thermal Cycling • Torsion • Vibration • Shear • Shock • Etc. • Etc. Use up Fatigue Life 11Chet Haibel ©2012 Hobbs Engineering Corp.

Observed Failure Behavior For a given stress level, the number of cycles to failure in a sample will occur in a distribution due to specimen variation 16 14 12 10 8 6 4 2 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 Cycles to Failure 12Chet Haibel ©2012 Hobbs Engineering Corp.

Observed Failure Behavior Higher stress level requires fewer cycles to failure Higher Stress Lower Stress 16 14 12 10 8 6 4 2 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 Cycles to Failure 13Chet Haibel ©2012 Hobbs Engineering Corp.

Observed Failure Behavior For the same failure mode, stress level and the number of cycles to failure are related by a straight line on log scales S - N Diagram 1.6 1.5 Log S, Stress 1.4  S1 N1 1.3 1.2 S2 N2 1.1  1.0 0.9 0 1 2 3 4 5 Log N, Cycles to Failure 14Chet Haibel ©2012 Hobbs Engineering Corp.

One Failure Mode: Fatigue Damage Vibration Analysis of Electronic Equipment by Dave Steinberg, Wiley, 1973 D  n  b, where • D is the Miner’s Criterion fatigue damage accumulation, • n is the number of cycles of stress, •  is the stress in force per unit area, • b is the negative, inverse slope of the S-N diagram for the material. For wrought Aluminum, doubling the stress decreases the fatigue cycles by a factor of 1000 b is approximately 10 15Chet Haibel ©2012 Hobbs Engineering Corp.

S-N Diagram for 7075 Aluminum Vibration Analysis of Electronic Equipment by Dave Steinberg, Wiley, 1973 O O~ 2 thousand cycles at 80 KSI, but at 40 KSI it takes 2 million cycles 16Chet Haibel ©2012 Hobbs Engineering Corp.

Fatigue Damage from Vibration Assume a resonance at 1,000 Hz At 40,000 psi, failure would occur at 2 million cycles 2 million ÷ 1 kHz = 2000 seconds or 33 minutes At 80,000 psi, failure would occur in 2 seconds Doubling the G rms level would achieve a time compression factor of 1,000. This TIME COMPRESSION is normal for HALT 17Chet Haibel ©2012 Hobbs Engineering Corp.

Time Compression Reference: GE Lighting, private telecon with Jim Harsa in 2000 Dtvb Increased voltage stress shortens time to see the same dominant t is time Wear-Out failure mode v is the voltage b =13 for incandescent lights b = 8 for fluorescent lights 18Chet Haibel ©2012 Hobbs Engineering Corp.

Discovering Wear-Out Failures Without Using HALT If possible, set up a repetitive “cycle test” which removes the “dead time” between cycles. But brainstorm what test artifact may be added and / or what the test may be concealing Test until a minimum of five failures are produced [Haibel’s rule] Use Weibull Analysis to fit a distribution to the failure data If life is not sufficient, determine the reservoir of material and the process consuming the reservoir. Increase the reservoir of material and / or slow down the process consuming it If necessary, replace the reservoir of material periodically with a scheduled preventive maintenance program 19Chet Haibel ©2012 Hobbs Engineering Corp.

Discovering Wear-Out Failures Without Using HALT Electromigration (photo courtesy Alcatel-Lucent) Standard test for electromigration in MIL-STD-883 is Dynamic Burn-In: 125°C for 160 hours with all voltages, currents, and clock speed maximized 20Chet Haibel ©2012 Hobbs Engineering Corp.

Useful-Life Failures Load Strength Constant Hazard Rate h(t) Random-in-time failures Hazard Parts are new until they fail Rate Strength-Load interference Insufficient design margin Time Typical of electronic hardware 21Chet Haibel ©2012 Hobbs Engineering Corp.

Quantifying Strength / Load Interference Subtracting two Normal distributions produces another Normal distribution whose mean is the difference of the means, but whose standard deviation is the root-sum-square of the two standard deviations We define Safety Margin MS  ML SM  ( S   L )1/ 2 2 2 22Chet Haibel ©2012 Hobbs Engineering Corp.

Useful-Life Failures Load Strength For simple mechanical products with few parts, we can calculate reliability one part at a time using Safety Margin for Normal distributions, or using Monte Carlo simulations for non-Normal distributions. For electro-mechanical products with thousands of components (each of which may have several relevant strength characteristics), we need an efficient technique to catch the few component applications that have marginal strength / load relationships. So far, the most efficient technique is Highly Accelerated Life Test (HALT). 23Chet Haibel ©2012 Hobbs Engineering Corp.

HALT HALT is the method of seeing the existing failure modes with the minimum number of prototypes (4 or 8) in the minimum time (typically a week) By experience with early prototypes or with similar products, determine which environmental factors will “stimulate” the relevant failure modes Many failure modes in typical electromechanical products are well stimulated by temperature and rapid temperature cycling simultaneous with six degree-of-freedom random vibration 26Chet Haibel ©2012 Hobbs Engineering Corp.

HALT Every stimulus of potential value is used during New Product Development to find the weak links in the product design These stresses are not meant to simulate field environments but to find the weak links in the design using only a few units in a very short period of time Stress levels are taken well beyond the normal mission profile Sometimes one kind of stress will produce a failure mode in HALT, but a different kind of stress will produce that same failure mode in the hands of customers Crossover Effect Focus on fixing the failure mode, don’t focus on the stimulus 28Chet Haibel ©2012 Hobbs Engineering Corp.

Stimulus-Flaw Precipitation Relationships Reference: “Flaw-Stimulus Relationships”, G. K. Hobbs, Sound and Vibration, August 1986 All Combined Vibration High Temp Burn in Thermal Voltage Cycle Cycle Margining 30Chet Haibel ©2012 Hobbs Engineering Corp.

Perhaps a Different Order More than one failure mode may be affected by the same stress Failure modes will not necessarily be exposed according to the field Pareto chart, but maybe in some other order Field HALT Pareto Order The time compression factor for the failure modes will be different 31Chet Haibel ©2012 Hobbs Engineering Corp.

Failure % by Stress Type “Summary of HALT and HASS Results at an Accelerated Reliability Test Center” by Mike Silverman Based on 49 products from 19 different industries Order of application and discovery: Cold Step Stress 14% Hot Step Stress 17% Temperature Transition 4% 6-Axis Vibration 45% Combined Temp and Vibe 20% Without simultaneous, all axis vibration, 65% would have been missed! 32Chet Haibel ©2012 Hobbs Engineering Corp.

“Our Path to Reliability Using HALT” Chuck Laurenson, Parker Hannifin 1999 Hobbs Engineering ARTS USA Award Winning Paper Where Design Flaws Were Discovered Cold Step Stress 10% Hot Step Stress 12% Rapid Thermal Cycling 4% Vibration Step Stress 43% Combined Temp and Vibe 31% 74% of the flaws would have been missed without simultaneous, all axis vibration! 33Chet Haibel ©2012 Hobbs Engineering Corp.

Driven Harmonic Motion d 2z dz M 2  D  Kz  A cos 2ft dt dt Transfer Function 10 1 0.1 Z-axis 0.01 excitation A cos 2πft 0.001 1 10 100 1000 Shaker frequency in Hz 37Chet Haibel ©2012 Hobbs Engineering Corp.

Swept Sine Vibration Essentially one frequency at a time, sweeping at one octave per minute Typically uses a Hydraulic shaker (limited upper frequency) or an ElectroDynamic shaker (high powered voice coil) Using a Stroboscope, one can observe behavior at resonance But can only see one resonance at a time, in one translation axis at a time; must mount the product for X, Y, & Z Miss interactions between resonances at different frequencies or in different directions No guarantee of stimulating rotational resonances at all ! 38Chet Haibel ©2012 Hobbs Engineering Corp.

Random Vibration Broadband, Pseudo Random (noise-like) vibration generated by a computer Typically uses an ElectroDynamic shaker, therefore one translation axis at a time; still have to mount the product three times for X, Y, & Z and that doesn’t stimulate rotational resonances very well But this is a major improvement to see all frequencies at once, therefore see the interaction of resonances in one direction Crest factor (ratio of peak to average acceleration) is around 3 Major advantage is to shape the spectrum for qualifying to some external standard (e.g., RCTA/DO-160D Category U Helicopter) 41Chet Haibel ©2012 Hobbs Engineering Corp.

Random Vibration Shaped Spectrum 1.000 Vertical axis is Power Spectral Power Spectral Density 0.100 Density in units of g2/Hz g2/Hz To convert to G 0.010 rms, integrate the power (g2) over frequency 0.001 and take the 10 100 1000 Freqency (Hz) square root Shown is approximately 5G rms 42Chet Haibel ©2012 Hobbs Engineering Corp.

Features of the TC-1 Ocelot Temperature change rates of plus or minus 120 Celsius degrees per minute, the highest in the industry, from -100°C to +200°C Vibration will start and run anywhere from 0.1 to 150 G rms Low G levels are important for executing Modulated Excitation™ which is a breakthrough for detecting intermittent failures X, Y, and Z acceleration balance is near 1:1:1 Sound level is only 50 dBA at 30 G rms, the lowest in the industry, no ear protection is necessary, can be used on production lines Will operate on 110 volts, 50-60 Hz with reduced heating for trouble shooting – this is important for duplicating soft failures 44Chet Haibel ©2012 Hobbs Engineering Corp.

TC-1 Ocelot Vibration System These are pneumatically- driven pistons which generate six-axis (6 DoF) vibration from approximately 20Hz to 10kHz (one spring is removed to show the table Bottom View construction detail) 45Chet Haibel ©2012 Hobbs Engineering Corp.

Repetitive Shock Spectrum T d Mathematically, a string of rectangular pulses of period T and duration d in the Time Domain Time in seconds 1 Transforms into a “comb” of 0.1 frequencies whose fundamental 0.01 frequency is 1/T with harmonics 0.001 Sin df weighted by in the 0.0001 πdf Frequency Domain 0.00001 Frequency in Hz 46Chet Haibel ©2012 Hobbs Engineering Corp.

Six-Axis Random Vibration Using several pneumatic pistons, with air flow modulated in a proprietary fashion, produces overlapping smeared spectrums The different angles of the pneumatic pistons generate a feedback controlled, broadband level of random vibration in X, Y, and Z translational directions and yaw, pitch, and roll angular directions Feedback for the control system is provided from one z-direction accelerometer on the bottom (piston side) of the table This results in all frequencies in all directions, simultaneously exciting all resonances for complete failure mode stimulus The Crest Factor, the ratio of peak to average acceleration is ~10, which rapidly precipitates design and manufacturing flaws 47Chet Haibel ©2012 Hobbs Engineering Corp.

Some Defects Precipitated by Vibration Poorly mounted components Poorly formed leads Poor solder joints Fretting Corrosion Loose hardware Loose wires Adjacent parts contacting Wires over sharp edges Stacked resonances 48Chet Haibel ©2012 Hobbs Engineering Corp.

Some Defects Precipitated by Thermal Cycling Poorly matched expansion coefficients • Boards and components should match • Structures should match Poor solder joints Improperly formed leads Improper crimps PCB shorts, opens Plated through hole defect 50Chet Haibel ©2012 Hobbs Engineering Corp.

Time Compression for Data from the Previous Slide Calculations by G. K. Hobbs At a Ramp Rate of 5⁰C per minute, 400 cycles with a range of 165⁰C (with no dwells) would take 440 hours At a Ramp Rate of 25⁰C per minute, 4 cycles with a range of 165⁰C (with no dwells) would take less than 60 minutes (At a Ramp Rate of 40⁰C per minute, 1 cycle with a range of 165⁰C (with no dwells) would take less than 10 minutes) This is real TIME COMPRESSION ! 53Chet Haibel ©2012 Hobbs Engineering Corp.

Stresses Used in HALT Wide range temperature Humidity High rate temp. cycling Dimensional parameters All axis random vibration Viscosity of a fluid Power cycling Vary pH of a fluid Power voltage and frequency Salinity of a fluid Secondary voltage Add particulates to the fluid Digital clock frequency Back Pressure 54Chet Haibel ©2012 Hobbs Engineering Corp.

More Stresses Used in HALT Vary magnetic tape thickness Inject electrical noise Vary gear diameter Mistune the channel Off axis alignment Radiation (E & M) Mismatch / Overload Nuclear radiation Imbalance Multiple sterilizations Off-track Whatever else makes Higher RPM sense for the particular product 55Chet Haibel ©2012 Hobbs Engineering Corp.

Crossover Effect A flaw may be exposed by a different stress in HALT than the stress which exposes the flaw in the field environment Focus on the failure modes and mechanisms, not the stresses used to expose them or the margin beyond field environment Focusing on margin may lead to missing an opportunity for improvement followed by field failures of the same mode This is a frequent, serious mistake in HALT! 56Chet Haibel ©2012 Hobbs Engineering Corp.

Product Response is of Prime Importance, the Inputs Are Not Vibration • All modes excited • Second modes are very important Thermal • All sites reach the desired temperatures • All sites reach the desired rates of change Voltage Humidity Current density Other stresses or parameters 58Chet Haibel ©2012 Hobbs Engineering Corp.

What Level of Stresses to Use In HALT, one must go beyond customer-specified stress level to compress the time to see the dominant failure modes Stress level has been substituted for sample size! This is one of the MAJOR BENEFITS of HALT We do not need many units to HALT (four is good) We can HALT a few at each stage of development and manufacturing. • Prototype (as early as feasible) • Pre-production (after corrections) • Early production (after design transfer) • Ongoing production (re-HALT) 59Chet Haibel ©2012 Hobbs Engineering Corp.

Understand First Again, the key is to focus on the failure mode, not the stress type used, or the margin beyond the field environment Through failure analysis, gain root cause understanding first and then decide if the weakness would cause field failures or whether the weakness would put limitations on manufacturing screening It’s often easier to fix it than prove it’s not a customer issue! 60Chet Haibel ©2012 Hobbs Engineering Corp.

HALT Attitude Every weakness found represents an opportunity for improvement HALT is proactive, but no action means no improvement We try to break the product in order to find its weak links This is discovery testing compared to qualification (success) testing This is a total paradigm shift! Opportunities not taken will probably lead to field failures much more expensive than the improvement would have been. This fact has been documented in thousands of cases If you find it in HALT, it is probably relevant ! 61Chet Haibel ©2012 Hobbs Engineering Corp.

Example of Success Ed Minor, Boeing, in a presentation at a Hobbs Engineering Seminar Boeing 777 was the first commercial airplane ever certified for Extended Twin-engine Operations (ETOPS) at the outset of service “Dispatch reliability after only two months of service was better than the next best commercial airliner after six years” 63Chet Haibel ©2012 Hobbs Engineering Corp.

Some Product Types Successfully Improved by HALT Accelerometers Magnetic Resonance Scanners Analysis &Test Equipment Medical Products ASICs / Processors / Drives Military / NASA (mixed) Land / Air / Water Craft Monitors / Displays / TVs A/V Products & Systems Ovens Avionics / Aerospace Pneumatic Vibration Compressors/Generators Point of Sale Systems PCs to Mainframes Power Supplies Lipstick Radar / GPS Systems Electronics / Electrical Telecommunications Gears / Transmissions Thermal Controls Instruments / Gauges Jet Engines / Missiles 64Chet Haibel ©2012 Hobbs Engineering Corp.

The Complete HALT Process HALT consists of: • Precipitation – Stresses – Stress Levels • Detection All must be present or no – Detectable State improvement happens ! – Coverage • Failure Analysis • Corrective Action – Corrective Action Verification 65Chet Haibel ©2012 Hobbs Engineering Corp.

The First Part of Detection Achieve a Detectable State, the “Magic Level” or the “Sweet Spot” where the intermittent is detectable • Detection Screens are a well established technique commonly practiced by the experts • Requires equipment designed for HALT and HASS for best results • Modulated ExcitationTM frequently improves detection by two orders of magnitude, sometimes even more 66Chet Haibel ©2012 Hobbs Engineering Corp.

Detection Excellence Some damage from the HALT stresses may not be immediately discernable – it may be LATENT ! HAST (Highly Accelerated Stress Test -- Pressure Cooker) may precipitate latent damage, making it patent -- discernable • Cracked component bodies (e.g. MLCC) • Other long term failure modes not yet completed If feasible, expose all HALT units to HAST Or perform a biased (power on with signals toggling) exposure to 60°C and 90% RH for one week 67Chet Haibel ©2012 Hobbs Engineering Corp.

Equipment Required Combined all-axis, broad-band vibration and high-rate thermal cycling. Low frequencies must be present in sufficient amplitude to precipitate the defects. Electrical stressing (power supply, clock frequency, loads) Monitoring with high coverage is absolutely essential Temperature, pressure, and humidity (HAST) equipment Traditional 85/85 takes 1,000 to 5,000 hours HAST takes only 48 hours! Other stressors (such as corrosive atmosphere or radiation) as appropriate for the product and its environments 69Chet Haibel ©2012 Hobbs Engineering Corp.

Reasonable Example Suppose an R&D project has a product reliability goal to have less than 5% Annual Failure Rate. (this is not a lofty goal) How many prototype units would have to be put on test to have 70% probability of seeing all the problems that must be resolved to be successful? 71Chet Haibel ©2012 Hobbs Engineering Corp.

Infinite, Decreasing, Geometric Series Mathematical Model for a Pareto 2 3 F1 , F1R , F1R , F1R , ... Sum = F1 / (1 - R) 0<R<1 Example: If sum = 5%, R = 0.8, solve for F1 Answer: (Sum)(1 - R) = F1 = (5%)(0.2) = 1% 72Chet Haibel ©2012 Hobbs Engineering Corp.

70% Chance of Seeing Failures for 5% Annual Failure Rate 1000 Number of units on test 0.50 0.70 0.90 0.99 100 O 10 0.001 0.01 0.1 Failure modes failure probability 74Chet Haibel ©2012 Hobbs Engineering Corp.

Minimum Prototypes and Time To see the failure modes that must be eliminated for even mediocre reliability (5% AFR), Test 120 units for a year at normal mission (customer, field) conditions, or HALT 4 units for a week 75Chet Haibel ©2012 Hobbs Engineering Corp.

How to Prove that HALT Works There are “Accelerated Reliability Test Centers” where you can take some products to try a HALT chamber The persons at the ARTC will run the chamber, but you have to run your product using diagnostic software Take an existing (currently shipping) product for which you know the failure modes experienced by your customers This is an excellent way to prove that HALT will find the relevant failure modes in YOUR product 76Chet Haibel ©2012 Hobbs Engineering Corp.

Preparing to HALT a Product In any test we have to stimulate the product and look for a response from it. HALT is no different, we need inputs and outputs which we can control and observe from outside the HALT chamber. Ideally, we want to check all functions of the product so we can see any (soft) failures. We often figure out a “quick test” which we can run at each condition of voltage, temperature, vibration, etc. This might be the power-on self-test (POST), so we power cycle the product at each condition. Then occasionally, we will take the time to do a thorough checkout. 78Chet Haibel ©2012 Hobbs Engineering Corp.

Preparing to HALT a Product Many products (especially software driven products) detect power supply voltage and will shut down outside an upper and lower limit. Some products detect temperature and will shut down outside an upper and lower limit. These protections must be disabled, either with special HALT software (firmware) or by modifying the hardware (supplying a stable voltage to the temperature and / or voltage comparators). We want to see the underlying (raw) performance of the circuits. These voltage and temperature limits will improve design margin. 79Chet Haibel ©2012 Hobbs Engineering Corp.

Preparing to HALT a Product Some products have rubber feet on them to reduce skidding and scratching, and take out minor irregularities in the support surface. These will tend to dampen the vibration we are trying to drive into the product. We must overcome this dampening by removing the feet or supporting the product next to the feet on the chassis. Similarly, inside the product there may be elastomer material to dampen vibration. These dampeners must be defeated to transmit vibration. 80Chet Haibel ©2012 Hobbs Engineering Corp.

Preparing to HALT a Product Most products have covers to protect the electronics from foreign (conductive) material and protect the user from coming in contact with live voltages. Some products have fans to circulate air to cool the hot components (and heat the cool components). These covers and fans will get in the way of the turbulent airflow in the HALT chamber, which is trying to impose a temperature on the components. It makes a convection oven look tame! Unless these covers are structural, they should be removed. If they are structural, they must have holes drilled in them to let the airflow in. 81Chet Haibel ©2012 Hobbs Engineering Corp.

Overview of highly accelerated life test (halt)

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