Introduction Introduction to Reliability Historical Perspective Current Devices Trends
The Bathtub Curve (1) Time Failure rate, Constant Useful lifeWear out Infant Mortality
The Bathtub Curve (2) What is the "bathtub" curve? In the 1950’s, a group known as AGREE (Advisory Group for the Reliability of Electronic Equipment) discovered that the failure rate of electronic equipment had a pattern similar to the death rate of people in a closed system. Specifically, they noted that the failure rate of electronic components and systems follow the classical “bathtub” curve. This curve has three distinctive phases: 1. An “infant mortality” early life phase characterized by a decreasing failure rate (Phase 1). Failure occurrence during this period is not random in time but rather the result of substandard components with gross defects and the lack of adequate controls in the manufacturing process. Parts fail at a high but decreasing rate. 2. A “useful life” period where electronics have a relatively constant failure rate caused by randomly occurring defects and stresses (Phase 2). This corresponds to a normal wear and tear period where failures are caused by unexpected and sudden over stress conditions. Most reliability analyses pertaining to electronic systems are concerned with lowering the failure frequency (i.e., const shown in the Figure) during this period. 3. A “wear out” period where the failure rate increases due to critical parts wearing out (Phase 3). As they wear out, it takes less stress to cause failure and the overall system failure rate increases, accordingly failures do not occur randomly in time.
Introduction to Reliability Failure in time (FIT) Failures per 10 9 hours ( ~ 10 4 hours/year ) Acceleration Factors –Temperature –Voltage
Introduction to Reliability (cont'd) Most failure mechanisms can be modeled using the Arrhenius equation. ttf - time to failure (hours) C - constant (hours) E A - activation energy (eV) k - Boltzman's constant (8.616 x 10 -5 eV/°K) T - temperature (ºK) ttf = C e E A /kT
Introduction to Reliability (cont'd) Acceleration Factors ttf L A.F. = ------ ttf H A.F. = acceleration factor ttf L = time to failure, system junction temp (hours) ttf H = time to failure, test junction temp (hours)
Introduction to Reliability (cont'd) Activation Energies Failure Mechanism E A (eV) Oxide/dielectric defects 0.3 Chemical, galvanic, or electrolytic corrosion 0.3 Silicon defects 0.3 Electromigration 0.5 to 0.7 Unknown 0.7 Broken bonds 0.7 Lifted die 0.7 Surface related contamination induced shifts 1.0 Lifted bonds (Au-A1 interface) 1.0 Charge injection 1.3 Note: Different sources have different values - these values just given for examples.
Acceleration Factor - Voltage Oxides and Dielectrics Large acceleration factors from increase in electric field strength A.F. = 10 / (MV / cm) k - Boltzman's constant (8.616 x 10 -5 eV/°K) T - temperature (ºK) = 0.4 e 0.07/kT
Acceleration Factor: Voltage Median-time-to-fail of unprogrammed antifuse vs. 1/V for different failure criteria with positive stress voltage on top electrode and T a = 25 °C.
Device and Computer Reliability 1960's Hi-Rel Application Apollo Guidance Computer –Failure rate of IC gates: < 0.001% / 1,000 hours ( < 10 FITS ) –Field Mean-Time-To-Failure ~ 13,000 hours One gate type used with large effort on screening, failure analysis, and implementation.
Device Reliability:1971 Reliability Level of Representative Parts and Practices MTBF (hr) Commercial 500 Military 2,000 High Reliability 10,000 (10 4 hours)
Actel FPGAs Technology FITS # Failures Device-Hours 2.0/1.2 33 2 9.4 x 10 7 1.0 9.0 6 6.1 x 10 8 0.8 10.9 1 1.9 x 10 8 0.6 4.9 0 1.9 x 10 8 0.45 12.6 0 7.3 x 10 7 0.35 19.3 0 4.8 x 10 7 RTSX 0.6 33.7 0 2.7 x 10 7 0.25 88.9 0 1.0 x 10 7 0.22 78.6 0 1.2 x 10 7
RAMTRON FRAMs Technology FITS # Failures # Devices Hours Device-Hours 1608 (64K) 128 1 1 100 10 3 10 5 4k & 16K Serial 37 15 2 4257 10 3 4.3 x 10 6 Note: Applied stress, HTOL, 125ºC, Dynamic, V CC =5.5V. 1 The one failure occurred in less then 48 hours. The manufacturer feels that this was an infant mortality failure. 2 12 failures detected at 168 hours, 3 failures at 500 hours, and no failures detected after that point.
Skylab Lessons Learned 58. Lesson: New Electronic Components Avoid the use of new electronic techniques and components in critical subsystems unless their use is absolutely mandatory. Background: New electronic components (resistors, diodes, transistors, switches, etc.) are developed each year. Most push the state-of- the-art and contain new fabrication processes. Designers of systems are eager to use them since they each have advantages over more conventional components. However, being new, they are untried and generally have unknown characteristics and idiosynchracies. Let some other program discover the problems. Do not use components which have not been previously used in a similar application if it can be avoided, even at the expense of size and weight.
Reliability - Summary Covered device reliability basics Design reliability is another set of topics –Advanced Design: Designing for Reliability –Fundamental Logic Design: Clocking, Timing Analysis, and Design Verification –Fundamental Logic Design: VHDL for High- Reliability Applications - Coding and Synthesis –Fundamental Logic Design: Verification of HDL-Based Logic Designs for High-Reliability Applications