What is HALT HASS? [Definitive Guide]

Chapter 1

What is HALT / HASS?

HALT and HASS testing are performed to identify design weaknesses, improve product reliability and reduce life cycle costs. The purpose of HALT and HASS testing is to determine whether a product will remain reliable in its intended environment over a required duration of time.

Although both methods test the physical integrity of the products being manufactured - (which in turn earns trust and builds customer confidence), there are differences between them.

What's the difference between HALT and HASS?

What is HALT? Highly Accelerated Life Testing 

HALT is a stress testing methodology used to improve the robustness and reliability of the product design. During HALT, a product is stressed beyond the product specifications to identify design weaknesses early in the development stage. It focuses on exposures such as temperature and vibration to determine its operational and destruct limits.

The testing method allows redesign as needed at a lower product development cost, improves time-to-market and discovers the physical limitations of a design.

The product is stimulated and monitored to identify limits and any failure points. TT Electronics visually identifies the nature of the failure and advises customers on the action needed. (ie. poor resistance or poor positioning)

Results of HALT testing allow visibility of:

• Thermal and mechanical design weakness
• Operational component limitations
• The gap between the product’s required operating limits and maximum limits

What is HASS? Highly Accelerated Stress Screening

HASS is also a stress testing methodology performed to improve the reliability of the finished product. While HALT is performed during the development stage, HASS is performed during the manufacturing stage and otherwise prior to shipment and sale. It guarantees ongoing reliability of the product and adheres to the design changes built during HALT.

Taking the information from HALT testing, thermal and vibration limits are set between the extreme failure points of HALT and the normal operating limits. The new goal is to quickly apply limited stress to the product so that you avoid customers receiving products that are  DOA (Dead on Arrival) or experiencing early failure.

The testing method allows for failure analysis with corrective action. For example, an action may require an alternative component or soldering improvement. The HASS tests are used to drive the products failure. This allows the manufacturer to identify any product issues, fix them and repeat the process until confident with the design and manufacturing.

HASS can be applied to all manufactured products and is typically used for high-reliability applications, new product introduction or when a production process is changed.

Chapter 2

Stresses Used In HALT and HASS

HALT HASS testing process can effectively impact product reliability. The idea behind the tests is to emulate or duplicate stress levels that could take months or years to occur in a real-life environment by stimulating failures. These stresses reveal design defects or weaknesses. All failures will be addressed by corrective or preventive action to eliminate the chance of failures happening in the field.

Weaknesses can be identified by the common stress test exposures to temperature and vibration, which allow the defining of upper and lower operating and destruction limits to these tolerances.

1. Temperature

Changes in temperature can affect electronic circuits. The test temperatures consist of cold and hot step stressing.

Cold thermal step stressing reveals the Lower Operational Limit (LOL) and hot thermal step stressing can reveal Upper Operational Limit (UOL).

Cold thermal step stressing will also reveal the Lower Destruct Limit (LDL) and hot thermal step stressing will reveal the Upper Destruct Limit (UDL).

Electronics can be exposed to rapid temperature changes as in the case with space and aerospace materials. Thermal cycling test can be used for screening components under harsh thermal conditions.

HALT HASS is a step-by-step process which induces vulnerability. This will eventually cause product failure (soft or hard), which will be analyzed and then corrected.

"Successful engineering is all about understanding how things break or fail."

- Henry Petroski

 

A soft failure occurs when a product under test ceases to operate correctly but resumes correct operation when the stressing environment is eased - for example, failing at extreme temperatures but resuming normal operation once the product is returned to within its normal operating temperature range. A hard failure occurs when the normal operation cannot be achieved even under normal operating conditions.

HALT testing is used to find issues that will only appear once stresses are applied. HASS is performed on completed assemblies to verify no additional or new weaknesses are found during the manufacturing process.

2. Vibration

Vibration step testing reveals the vibration operational limit (VOL). It is the vibration stress level where a product begins to malfunction but can be returned to normal once the stress is decreased or removed. Vibration testing is performed to determine a product's resilience to forces encountered in its intended environment. 

Progressively higher levels of thermal and mechanical stress are applied to the product in a cyclic procedure, beyond normal operating levels the product would be expected to experience. In an accelerated timeframe, failures are encouraged that highlight weakness in the design. Typical failures would be cracking or loss of mechanical strength due to fatigue.

The stress level where failure is identified and UUT (unit under test) functionality is not recovered when the stress is reduced is an example of a hard failure. Vibration step stressing will reveal the Vibration Destruct Limit (VDL). When reaching VDL, a product will no longer operate unless there is some type of repair. 

Chapter 3

Why HALT Works - Performing HALT

During the HALT process, operational and destruct limits are identified for a product. The limits are used as a foundation for the HASS profile. 

However...

Before moving to HASS, the design will be improved.  After eliminating one weakness, the next design weakness is discovered and the design is again improved. The cycle continues until no design defects are found.

Weaknesses found in HALT offer manufacturers the opportunity to fix and make continuous improvements. The HALT process will maximise the operating and destruct margins of a product.

The HALT Test Sequence (Sample)

HALT consists of progressively higher step stress increases.

1. HALT testing (Cold Step) - The test will start with a rapid low-temperature step-stress. (Initial temperature starting at +25°C, and continue to lower the temperature in -10°C steps)

2. HALT testing (Hot Step) - With a rapid high-temperature step-stress, the testing begins at + 25°C, with +10°C steps) 

3. Stepped Vibration - Vibration step-stresses start small at 3-5 G_rms, with small steps of 2-3 G_rms, run with 10-minute intervals.

4. Temperature cycle step-stresses

5. Combined stresses (vibration and temperature cycle) Cool down or heat up as rapidly as possible.

6. At some point a decision will be made, to fix or not.

7. After all HALT based design modifications are complete, a final test will be run.

Basic Steps in HALT/HASS Process:

Precipitation - usually accomplished in HALT. The process of precipitation will transform the imperceptible flaw to one that is detectable.

Detection - determining a fault exists

Failure Analysis - determining why a failure occurred

Corrective Action - changing the design or processes that caused the fault

Verification - determining the product has been fixed or improved by performing tests before product release.

Collective data - entering learned knowledge into the database to use for similar events