What is HALT?

THE BASIC PRINCIPLES OF HALT

HALT is performed prior to commencing volume production. It is used to find weaknesses and product stress limits through the techniques of thermal cycling and omni-axial random vibration step stress. Weaknesses in the design and construction of a product are stimulated by the application of progressively higher levels of cyclic mechanical stress. In a relatively short (i.e. accelerated) time, failures can be precipitated that would otherwise have occurred in the field, probably during the warranty period.

HALT is not a compliance test nor is it an attempt to simulate a worst-case operating environment. It is a process that stimulates the UUT to the point of failure in order to precipitate latent defects. Stresses are progressively applied in order to eventually exceed the UUT’s design limits.

HALT should be viewed as an additional tool in the reliability engineer’s “tool kit”. It complements existing reliability test methods and should not be viewed as universally replacing them.

During HALT products must be stimulated and monitored to determine when failures occur. When a failure is detected, root cause analysis is performed to understand the failure mechanism. The defective unit is repaired and strengthened, then returned for further applications of progressively higher stress levels.

Failures precipitated by HALT will be associated with one or more of the following categories:

1. the design of the product
2. the components from which it is made
3. the quality of the assembly process

THE HALT PROCESS

There is no definitive, universally agreed method of implementing HALT. However, most users will carry out most of the following steps on one or more UUTs:

Steady state temperature step stress:

The UUT is tested at progressively lower temperatures, until it ceases to function, thus determining a lower operational limit.

The UUT is tested at progressively higher temperatures, until it ceases to function, thus determining an upper operational limit.

Rapid thermal cycling:

The UUT is subjected to progressively higher levels of cyclic thermal stress by the application of increasing numbers of thermal cycles at high and linear rates of change of temperature. The temperature limits are determined from the levels established during the previous steady state temperature stress tests.

Multi-axis (6 DoF) random vibration :

The UUT is mounted on a vibration table using appropriate test fixturing. Starting at a relatively low energy level the UUT is subjected to multi-axis random vibration in steps of several minutes’ duration. The vibration level is incremented at each step until failures are stimulated.

Combined environment (thermal with vibration) stress:

Depending on the UUT, suitable combinations of vibration and thermal stresses are used to further investigate product robustness.

After completing HALT and analysing the results, suitable modifications may be introduced to the product design to enhance its robustness, prior to commencing volume manufacturing. These changes are comparable to the retrospective engineering changes that are often required once a product has been released to market.

HALT results also provide the manufacturer with a reference ‘robustness benchmark’ for his specific product.

HALT provides data from which a suitable production HASS profile may be determined.

In order to be successful, HALT must be properly managed and integrated into a new product development program. Attempting to add HALT at the last minute in the design validation process is unlikely to be satisfactory.

How can I start to use HALT?

That’s where using Reliability Plus will really help.

Start by thoroughly understanding the principles of accelerated stress screening as applied to HALT. A short training session can be arranged on your own premises.

Analyse the product(s) that you wish to evaluate and develop a tailored strategy.

Design and manufacture the necessary (usually straightforward) mechanical fixturing.

Determine how best to stimulate and monitor your products during HALT. Make sure that you can have access to the necessary equipment when you need it.

Ensure that a suitable number of product samples will be available for HALT.

If you don’t have your own HALT system, arrange to use a properly equipped test house. Typically you will initially need to budget for a week of test house time.

Take expert advice at every stage of the process. That’s where Reliability Plus can make the difference. We offer training, a HALT planning service and hands-on assistance for your first exercise. Working with your own engineers we can make sure that you get the maximum benefit from HALT.

HALT also provides the basic information you will need to develop an effective production screening (or HASS) process. Again, Reliability Plus will help you develop the right solution.