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Reliability Predictions for Solid State Disks
Reliability of solid state disks is recognized in defense, industrial and commercial electronic systems as an essential part of all applications in the field. Flash solid state disks integrators, in applications ranging from mission critical operations in war theatres to everyday cellular phone communications, depend on reliability prediction models to provide analysis of parts and components to predict and calculate the length of time the solid state disk will be operational. Reliability prediction models are a common form of reliability analysis for calculating failure rates and MTBF (Mean Time Between Failures). This article focuses on two reliability prediction models, MIL-HDBK-217F and Telcordia. For more information on other solid state disk reliability methods; such as, write endurance, wear leveling, ECC and other reliability system modeling methods, refer to the article, "Adtron Flash Disks: Reliability Begins at the Flash Memory Level".
Commonly Used Reliability Predictions Models
Reliability prediction is commonly based on two established models: MIL-HDBK-217F and Telcordia. Both of these models provide procedures for determining failure rates and MTBF for electronic components, including flash solid state disks. Calculations of reliability prediction are based upon information about the components and environments and then use this data to mathematically estimate MTBF. Depending on components used, environments or reliability prediction model, diverse MTBF predictions for a flash solid state disk will be calculated. Other factors, such as various stress parameters, device temperature, operating voltage, rated voltage and power stress ratios, factor into reliability predictions.
The choice of which model would best suit a flash solid state disk is usually based on the requirements of the application. It may be the case that one reliability prediction model is more applicable under some conditions than the other. There are a number of differences between MIL-HDBK-217F and Telcordia, but the primary differences are in the requirements and environments of a vertical market. Because flash solid state disks are regularly used in an array of operating environments, both reliability prediction models are used by Adtron to calculate MTBF and failure rates.
MIL-HDBK-217F
MIL-HDBK-217F is the grandfather of reliability prediction models. Because it is used by the defense industry, this may explain the more pessimistic MTBF calculations when compared to Telcordia calculations. This reliability prediction model may perform a parts count analysis or a part stress test, use any range of temperature and has 14 environmental classifications (3 ground, 8 air, 1 space, 2 sea - refer to the MIL-HDBK-217F table below, for more information). Adtron uses MIL-HDBK-217F reliability prediction method to calculate our flash solid state disk MTBF to 250,000 hours (refer to the Adtron Reliability table below, for additional specifications).
MIL-HDBK-217F - Environments and Descriptions
| Environment |
Description |
| Ground, Benign |
Nonmobile, temperature and humidity controlled environments readily accessible to maintenance; includes laboratory instruments and test equipment, medical electronic equipment, business and scientific computer equipment in ground silos. |
| Ground, Fixed |
Moderately controlled environments such as installation in permanent racks with adequate cooling air and possible installation in unheated buildings; includes permanent installation of air traffic control radar and communications facilities. |
| Ground, Mobile |
Equipment installed on wheeled or tracked vehicles and equipment manually transported; includes tactical missile ground support equipment, mobile communication equipment, tactical fire direction systems, handheld communications equipment, laser designations and range finders. |
| Naval, Sheltered |
Includes sheltered or below deck conditions on surface ships and equipment installed in submarines. |
| Naval, Unsheltered |
Unprotected surface shipborne equipment exposed to weather conditions and equipment immersed in salt water. Includes sonar equipment and equipment installed on hydrofoil vessels. |
| Airborne, Inhabited, Cargo |
Typical conditions in cargo compartments which can be occupied by an aircrew. Environment extremes of pressure, temperature, shock and vibration are minimal. Examples include long mission aircraft such as the C130, C5, B52 and C141. This category also applies to inhabited areas in lower performance smaller aircraft such as the T38. |
| Airborne, Inhabited, Fighter |
Same as Airborne-Inhabited-Cargo, but installed on high performance aircraft such as fighters and interceptors. Examples include the F15, F16, F111, F/A 18, A10, Gripen and Tornado aircraft. |
| Airborne, Uninhabited, Cargo |
Environmentally uncontrolled areas which cannot be inhabited by an aircrew during flight. Environmental extremes of pressure, temperature and shock may be severe. Examples include uninhabited areas of long mission aircraft such as the C130, C5, B52 and C141. This category also applies to uninhabited area of lower performance smaller aircraft such as the T38. |
| Airborne, Uninhabited, Fighter |
Same as Airborne-Uninhabited-Cargo, but installed on high performance aircraft such as fighters and interceptors. Examples include the F15, F16, F111 and A10 aircraft. |
| Airborne, Rotary Winged |
Equipment installed on helicopters. Applies to both internally and externally mounted equipment such as laser designators, fire control systems and communication equipment. |
| Space, Flight |
Earth orbital. Approaches benign ground conditions. Vehicle neither under powered flight nor in atmospheric reentry; includes satellites and shuttles. |
| Missile, Flight |
Conditions related to powered flight of air breathing missiles, cruise missiles and missiles in unpowered free flight. |
| Missile, Launch |
Severe conditions relating to missile launch (air, ground and sea), space vehicle boost into orbit and vehicle reentry and landing by parachute. Also applies to solid rocket motor propulsion powered flight, and torpedo and missile launch from submarines. |
| Cannon, Launch |
Extremely severe conditions related to cannon launching of 155 mm. and 5 inch guided projectiles. Conditions apply to the projectile from launch to target impact. |
Source: MIL-HDBK-217F manual
Telcordia
The Telcordia reliability prediction model is a modified version of MIL-HDBK-217F calculations and often times represents a more optimistic indicator of field reliability. Telcordia has built on MIL-HDBK-217F by including the ability to account for burn-in, field and laboratory testing. These added abilities have made it popular in markets outside of defense, including telecommunications and other commercial organizations. This reliability prediction model uses 5 environmental classifications (3 ground, 1 air, 1 space - refer to the Telcordia table below, for more information) and is quicker and easier to use because it simplifies the calculations by basing analysis on ambient temperature only. Telcordia also implements calculations for infant mortality failure rate in addition to the steady state useful operating life failure rate. Adtron uses Telcordia reliability prediction methods to calculate our flash solid state disk MTBF to 2,900,000 hours and 1,400,000 hours (refer to the Adtron Reliability table below, for additional specifications).
Telcordia - Environments and Descriptions
| Environment |
Description |
| Ground, Fixed, Controlled |
Nearly zero environmental stress with optimum engineering operation and maintenance. Typical applications are central office, environmentally controlled vaults, environmentally controlled remote shelters and environmentally controlled customer premise areas. |
| Ground, Fixed, Uncontrolled |
Some environmental stress with limited maintenance. Typical applications are manholes, poles, remote terminals, customer premise areas subject to shock, vibration, temperature, or atmospheric variations. |
| Ground, Mobile (both vehicular mounted and portable) |
Conditions more severe than GF-Uncontrolled, mostly for shock and vibration. More maintenance limited and susceptible to operator abuse. Typical applications are mobile telephone, portable operating equipment and test equipment. |
| Airborne, Commercial |
Conditions more severe than GF-Uncontrolled, mostly for pressure, temperature, shock and vibration. In addition, the application is more maintenance limited than GF-Uncontrolled. Typical applications are in the passenger compartment of commercial aircraft. |
| Space-Based, Commercial |
Low earth orbit. Conditions as for Airborne-Commercial, but with no maintenance. Typical applications are commercial communication satellites. |
Source: Telcordia manual
Application Environments and Characteristics
In order to calculate MTBF, knowledge of the environment that the flash solid state disk will be used in, characteristics of the system and acceptable failure rates for the equipment must be known before testing. These are all variables that determine flash solid state disk reliability. Both MIL-HDBK-217F and Telcordia are based on specific environmental conditions, such as temperature control and location. Adtron flash solid state disk product specifications use MIL-HDBK-217F and Telcordia prediction methods to detail MTBF at 30°C in multiple ground conditions. There are varying classifications of ground testing, from ground benign (non-mobile, labs for example) to ground fixed (moderately controlled environments, field permanent devices for example) to ground mobile (wheeled or tracked vehicles). Additional environments may include Naval, Airborne, Space, Missile launch or Cannon launch conditions.
Once all of these critical elements are known individual components can be calculated to determine the overall MTBF of the subsystem. Currently, calculations on
Adtron flash solid state disks have yielded MTBF results shown in the table below:
Adtron Flash Solid State Disk Reliability Predictions
| Prediction Method |
MTBF* |
| MIL-HDBK-217F |
250,000 hours |
| Telcordia |
1,400,000 hours (ground fixed, uncontrolled) |
| Telcordia |
2,900,000 hours (ground fixed, controlled) |
*Tested with 1GB capacity at 30°C
Adtron can calculate reliability predictions for an array of testing environments, conditions and temperature using MIL-HDBK-217F and Telcordia. The above table is just one example.
Last Steps of Reliability Prediction
MTBF can be calculated for flash solid state disks in the field and new flash solid state disks still in the lab or design stage. Once the flash solid state disks have been calculated, accelerated life testing is employed to measure the life span of the device. Accelerated life testing may be used to validate MTBF predictions. In another sense, accelerated life testing is a way to double-check your reliability predictions. Refer the Adtron article, Solid State Data Retention and Beyond, for more information on accelerated life testing.
MIL-HDBK-217 and Telcordia have countless calculation conditions and environments, each with the possibility of predicting a different reliability outcome. It is improbable and a false indicator to compare MIL-HDBK-217F or Telcordia reliability prediction tests, in flash solid state disks, for example, that are not identical. A varying temperature or configuration on a given test method will give entirely different MTBF calculations. This is something that Adtron takes seriously. Adtron flash solid state disks go through extensive reliability testing, qualification and analysis, and numerous reliability prediction models and accelerated life testing.
Only with thorough reliability prediction models like MIL-HDBK-217F and Telcordia can the flash solid state disk reliability be determined and the quality necessary for defense, industrial and commercial applications be ensured.
Contact Adtron at 602.735.0300 to find out more about our flash disks and testing procedures.
