Reliability Services in the Prototype Phase
Accelerated Life Testing (ALT)

DEFINITION
An Accelerated Life Test (ALT) is the process of determining the reliability of a product in a short period of time by accelerating the use environment. ALT's are also good for finding dominant failure mechanisms. ALT's are usually performed on individual assemblies rather than full systems. ALT's are also frequently used when there is a wear-out mechanism involved, thus precluding us from directly substituting sample units for test time as in a Reliability Demonstration Test (RDT).

SITUATION
A company that needs to understand the reliability of key assemblies within a product prior to shipping can use an ALT to determine this. Predictions are valuable early in a design cycle, but later in the design cycle, an ALT can give a much more accurate estimate of reliability, especially in specific use environments.

Also, a company that needs to understand major failure mechanisms within a key assembly can also use an ALT. This is something that a prediction cannot do.

OBJECTIVE
An ALT on an assembly will be able to approximate how long an assembly will last before it fails AND what the failure modes will be when it does fail. We shall run the ALT in an accelerated manner so that we can find out this information before we start shipping the product.

VALUE TO YOUR ORGANIZATION
Accelerated Life Tests have tremendous value in many areas. They can:

• predict the life of the product in the field.
• identify specific failure modes that will occur in the field before the
product is shipped.
• allow you to prioritize product improvements.
• map product risks to various environments.
• determine if the product is ready to ship.

RELIABILITY INTEGRATION
An example of Reliability Integration during an Accelerated Life Test is as follows:

Accelerated Life Tests Can Use Data from the FMECA and HALT
• FMECA reveals dominant failure modes. An ALT can then try to validate these.

• HALT will show the margins of the product at each stress. The ALT can then take advantage of this by running near these stress level for maximum acceleration.

METHODOLOGY
There are 3 stages to an Accelerated Life Test - Planning, Testing/Monitoring, and Final Analysis:

1) Planning Stage
In the planning stage, a reliability goal is established. Then, an accelerated life test model is chosen. We shall then develop a Decision Matrix and outline all of the parameters that we must decide on prior to writing the plan, including types of stresses, number of units, length of test, confidence, etc, along with advantages and disadvantages for each. We must also estimate the Beta factor, or slope of the Weibull distribution. If it is much greater than 1, then there exists a dominant wear-out mechanism. From this matrix, we shall jointly decide on all of the parameters that will go into the ALT Plan.

2) Testing and Monitoring Stage
In the testing and monitoring stage, the ALT is performed and the results are monitored to determine status compared with the goal. The test shall be set up so that the monitoring will only need to be performed a few times a week, once a day at most. As failures occur, Root Cause Failure Analysis is performed to determine how to classify the failures and determine their relevancy, and how to correlate the test results to the life of the product.

3) Final Analysis Stage
In the final analysis stage, the data is compiled and a report is written on the results. The report will compare the achieved results to the goal and will show how the data was calculated. It will contain detailed information about the models used.

CASE STUDIES/OPTIONS
The following case studies and options provide example approaches. We shall tailor our approach to meet your specific situation.

1) ALT using Ballpark Estimates for Major Environments
A Computer Peripheral manufacturer wanted a ballpark reliability estimate in a similar environment. We used existing literature and generic acceleration models for temperature, humidity, and vibration to develop the plan. The test design used existing standards and criteria.

2) ALT using Accurate Estimates for Major Environments
A Networking company was entering the mobile market and needed an accurate reliability estimate for the major environments. Our test design focused on acceleration models specific to their product and environment: temperature, humidity, and vibration. With a reliability life distribution model as the result, based on calculated activation energy, fitting parameters and constants, they had an estimate of reliability performance over time for a wide range of common failure mechanism.

3) ALT using Accurate Estimates for Full Range of Environments
An Aerospace company needed an accurate reliability estimate for a full range of environments. In addition to temperature, humidity, and vibration, the test needed to take into account effects of ESD, power cycling, altitude, and contamination. The ALT completely answered the question "Will the product work as intended in our customers' environment?" No one test can completely answer this question, so we selected the appropriate environmental factors to model, which ones to verify functionally, and which ones to conduct a paper analysis on.

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