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HALT vs ALT

HALT can be used to find the weak points in the design and the design margins. We can useHALT to improve the robustness of the product and make the product as reliable as we can, but

we cannot measure the reliability of the product

Using ALT, we can measure the reliability of the product. By measuring life time at various

stress levels low stress, medium stress and high stress, we can predict the life time of the

product at normal operating conditions.

HALT

Root Cause Analysis

Corrective Action Identification

Design robustness determination

ALT

Reliability Evaluation Dominant failure mechanisms identification

Advantage of HALT over ALT is that, a typical HALT would take 2 to 4 days to complete and

our success rate in finding defects that would ultimately turn into field issue is very high, while a

typical ALT test might take a few weeks.

One key advantage of ALT over HALT is that we often do not need any environmental

equipment. Bench top testing is usually adequate, and in many cases, this can be performed atthe customers facilities. An additional benefit is that the products life is determined as well,

where thats not typically the case for HALT

Using HALT and ALT together

Its a good practice to run the system through a HALT and find the subcomponents which failpredominantly. Once the failure modes and design margins are identified, we would run the sub

components through ALT and find the failure probabilities of subcomponents.

Once the probability of failure of the subcomponents is known, based on the fault tree we couldfind the probability of failure of the system, from which we know the reliability of the system.

ALT test

Testing requirements

Test length

Number of Samples

Confidence desired

Acceleration factors

Test environment

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4:2:1 allocation procedure

costs

Slope of Weibull distribution (Beta < 1 indicates infant mortality, Beta > 1 indicates

wear-out)

Stress Levels for ALT

Three stress levels are chosen for ALT. Low level, medium level and high level stress. Low

level stress is near to the operating stress levels. High level stress is high enough to accelerate

failure of the product, at the same time the failure modes excited are similar to those thathappen at normal operating stress levels. Medium stress level is in the middle of high level

and low level stresses.

Since the probability of failure is high at high level stress within the test time, lesser number

of samples are allocated to high level stress. The allocation ratio is usually 4:2:1 for low:medium: high level stress.