HALT and ALT: Two Excellent Reliability Techniquies

Mike Silverman
Certified Reliability Engineer
Ops A La Carte

HALT: Highly Accelerated Life Testing
ALT: Accelerated Life Testing

HALT and ALT are two most of the most effective reliability testing methods but often times people are confused about which to use when. In this paper, we shall examine when to use which technique, and when to use the two techniques together.


About HALT

HALT is the process of improving the reliability of a product in a very short period of time (usually hours or days) by gradually increasing stresses until the product fails. HALT's are good for finding design weaknesses. HALT's are usually performed on entire systems but can be performed on individual assemblies as well. HALT's do not work well when there is a wear-out mechanism involved.

Highly Accelerated Life Testing (HALT) is performed to uncover latent defects in product design and component selection that would not otherwise be found through conventional qualification methods. The process subjects a test product to progressively higher stress levels, incorporating environmental stresses such as temperature and vibration, electrical stresses such as voltage margining and load variation, along with combinations of each of these stresses, to precipitate inherent defects. Moreover, HALT stresses the product to failure in order to assess design robustness and margin above its intended operation.

An essential component of HALT is root cause analysis and the identification and implementation of corrective action to ensure the product integrity, thus increasing the product's reliability and the robustness of the design. Only by finding and fixing these weak areas of a product can we achieve margin improvement.

Root cause analysis is also perhaps the trickiest part of HALT because often times we identify a potential problem area and then need to determine if this is indicative of what will happen in the field or if this was just a matter of taking a product over its specifications and changing the failure mechanism. Making this determination requires up-front planning and product knowledge, experience, and good root cause analysis skills. Often times we determine that the failure is not relevant because we did change the failure mechanism. This is often a good candidate for Accelerated Life Testing.


About ALT

ALT is the process of determining the reliability of a product in a relatively short period of time (usually weeks or months) 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.

In order to set up an ALT, we must know several different parameters, including but not limited to: Length of Test, Number of Samples, Goal of Test, Confidence Desired, Accuracy Desired, Cost, Acceleration Factor, Field Environment, Test Environment, Acceleration Factor Calculation, and Slope of Weibull Distribution or Beta parameter (Beta<1 indicates infant mortality, Beta>1 indicates wear-out).

In order to measure the life of a product with ALT, one key factor we must determine is the Acceleration Factor, and this is sometimes the most difficult to obtain. Two methods we can use are: 1) Existing models (not very accurate without a lot of research) and 2) Determine by experimentation (lots of samples and time).

Some examples of good candidate products for ALT are cell phones, fans, hard drives, automotive electronics, robots, and infusion pumps.


Comparing HALT to ALT

We don't concern ourselves with determining the life of the product because we are more interested in making the product as reliable as we can, and measuring the amount of reliability is not as important. However, with mechanical items that wear over time, it is very important to know the life of the product as accurately as possible.

One key advantage of HALT over ALT is its speed in finding defects that will affect the field population. A typical HALT takes 2 to 4 days to complete and our success rate in finding defects that will ultimately turn into field issues is very high.

One key advantage of ALT over HALT is that we often do not need any environmental equipment. Benchtop testing is usually adequate, and in many cases, this can be performed at the customer's facilities. An additional benefit is that the product's life is determined as well, where that's not typically the case for HALT.

Using HALT and ALT Together

Often times we will run a product through HALT and then run the subassemblies through ALT that had predominant wear-out mechanisms and thus were not good candidates for HALT.

In Figure 1, we performed HALT at the unit level. For most of the assemblies, this was the appropriate level for finding issues, but for the fan assembly, we did not run the stresses long enough to excite the dominant failure mechanisms. Therefore, we followed up the HALT with an ALT on the fan assembly by itself, and from the ALT, we were able to find the dominant failure mechanisms.

HALT on a system in a test facility in Santa Clara, CA		ALT on fans in a traditional chamber at customer site
Figure 1: Fan Assembly run through ALT after Unit Level HALT

At other times, we may develop the ALT based on the HALT limits, using the same stresses but lowering the stress levels so that the acceleration factors are at measurable levels.

In Figure 2, we performed HALT on a power supply assembly, discovered the margins of the product, made several design changes as a result, and then performed a Verification HALT to show improvements in the design margins. Then, we followed up the HALT process with an ALT on the power supply to measure the life of the product.

HALT on a system in a test facility in Santa Clara, CA		ALT on fans in a traditional chamber at customer site
Figure 2 : Power Supply Assembly run through HALT and then ALT

In conclusion, both HALT and ALT are excellent techniques to use as long as you know which to apply when in the process.


About the Author
Mike Silverman has 20 years of reliability, quality, and compliance experience, the majority in start-up companies. He is the founder and managing partner at Ops A La Carte, a Professional Business Operations Company that offers a broad array of expert services in support of new product development and production initiatives. The primary set of services currently being offered are in the area of reliability. He may be contacted at mikes@opsalacarte.com.

About Ops A La Carte and Nuvation
Nuvation and Ops A La Carte are strategic partners. Ops A La Carte uses Nuvation in the area of Product Design and Development, Redesign, and Failure Analysis. Nuvation uses Ops A La Carte in the areas of Reliability and Quality. Ops A La Carte is a Professional Consulting Firm focused on Reliability and Quality Engineering Services, Management, and Education to assist clients in developing and executing any and all elements of Reliability throughout their Organization and their Product's Life Cycle.