Highly Accelerated Life Testing (HALT)
HALT is performed to uncover latent defects in product design, component selection, and manufacturing that would not otherwise be found through conventional qualification methods. Unlike conventional testing, the goal of HALT testing is to break the product. By subjecting the product to increasing levels of stress, long term failure modes that would show up under normal operating conditions in months or years can be revealed in just hours or days.
When the product fails, root cause analysis is performed to identify the limiting component(s). After a product has failed, the weak component(s) are upgraded or reinforced. HALT stresses the product to failure using repeatable testing techniques in order to assess design robustness and margin above its intended operation.
Each unit must undergo functional testing while being subjected to the environmental stresses of the HALT process. The purpose of the functional test is to measure the overall performance of the test unit and to detect the occurrence of multiple types of failure modes. The test must exercise the major functions of the product and provide a feedback measurement of the performance of each function. The functional test should achieve as much test coverage as possible.
Before beginning the HALT process, each test unit is subjected to one or more cycles of functional testing to verify the integrity of the test setup and to obtain baseline performance information. Functional testing is to be performed continuously at each applied stress level.
The HALT Test Procedure
Using the following tests, the HALT process subjects the test product to progressively higher stress levels to precipitate inherent defects.
1. Low Temperature Step
This test determines the cold temperature operating limit and destruct limit of the product in order to assess design margin above the product's intended range of operation.
The HALT chamber temperature is set to +30°C and is decreased in 10°C increments. The dwell time at each thermal step is 10 minutes. The dwell time begins when the temperature control thermocouple attached to the UUT is stabilized.
The temperature of the HALT chamber is decreased in 10°C decrements until a failure is observed or the chamber temperature operational limit is achieved. The first point of failure shall be referred to as lower operational limit. Once the lower operational limit (LOL) is found, the temperature of the chamber is increased to allow the product to recover. Following recovery, the units are stressed at temperatures lower than the operation limit to evaluate lower destructive limit (LDL). The units are continuously monitored throughout the test.
2. High Temperature Step
This test determines the hot temperature operating limit and destruct limit of the product in order to assess design margin above the product's intended range of operation.
The HALT chamber temperature is set to +30°C and is increased in 10°C increments. The dwell time at each thermal step is 10 minutes. The dwell time begins when the temperature control thermocouple attached to the UUT is stabilized.
The temperature of the HALT chamber is increased in 10°C decrements until a failure is observed or the chamber temperature operational limit is achieved. The first point of failure is referred to as upper operational limit. Once the upper operational limit (UOL) is found, the temperature of the chamber is decreased to allow the product to recover. Following recovery, the units are stressed at temperatures higher than the operation limit to evaluate upper destructive limit (UDL). The units are continuously monitored throughout the test.
3. Vibration Step
The unit undergoes gradually increasing vibration stress levels to assess the vibration operating and destruct limits. The units are subjected to vibration in six simultaneous degrees of freedom, three translation and three rotational. This provides high mechanical fatigue as the natural frequencies of each component are being excited simultaneously.
Vibration is incremented gradually and the test units are allowed to dwell at each vibration stress for a minimum of ten minutes. The vibration is increased until a failure is observed or the maximum chamber vibration level is achieved. The first point of failure is defined as vibration operating limit (VOL). Thereafter, the units are allowed to recover at lower vibration levels. Following recovery, the vibration stress level is increased to evaluate vibration destructive limits (VDL). The units are continuously monitored throughout the test. The temperature remains at ambient throughout the vibration test.
4. Rapid Thermal Cycling
Rapid thermal cycling subjects the units to rapid freeze and thaw conditions. This test induces significant mechanical fatigue in the product due to the rapid expansion and contraction of materials. Since dissimilar materials experience differing rates of thermal expansion, rapid thermal transitions tend to induce cracks within the product, particularly at solder joints.
Rapid Thermal cycling is conducted between the upper and lower temperature operational limits (UOL, LOL) as determined from the prior tests. The thermal profile is contained to within 10°C of the operating limits. The temperature transition rate is maintained at 60°C/minute. After temperature stabilization occurs, the dwell time at each extreme is 10 minutes. Thermal cycling is repeated a total of five times. The units are continuously monitored for any failures.
5. Combined Environment Test
The unit is subjected to simultaneous cycling of multiple environmental variables; temperature and vibration. This multi-variable testing approach fatigues the product in a manner that provides a close approximation of real-world operating environments in an accelerated manner.
The combined environment stress is a combination of rapid thermal transitions and vibration step tests. The thermal profile is inclusive of both thermal operating limits. A thermal transition rate of 60°C/minute is maintained throughout the test with the units allowed to dwell for 10 minutes at each thermal extreme. The thermal cycle is superimposed with increasing steps of vibration stress levels. This thermal and vibration cycle is repeated for a total of five iterations.
The vibration stress levels for the five cycles are determined by equally dividing the vibration operating limit found during the vibration step stress. The vibration stresses are increased by the same magnitude during each subsequent thermal cycle. The unit is continuously monitored throughout the test.
Fault Recovery Procedure
When a failure occurs, the environmental stress is reduced to determine when the unit recovers. Once the unit recovers, the environmental stress is increased to determine if the failure mode is repeatable. If the unit is still functional after repeating the failure mode, the environmental stress is increased until additional failure modes are encountered or a hard failure occurs. If the unit is not functional after repeating the failure mode, an attempt is made to mask or work around the failure and continue testing in pursuit of additional failure modes.
Summary of Results
Upon completion of the HALT test procedure, the product operating and destruct limits are known. These values are used to assess the design robustness and margin above its intended operating conditions using controlled and repeatable stress factors. Since HALT stresses the product to failure under accelerated life fatigue conditions, the values obtained here are expected to differ from the product operating limits determined in prior design validation testing.
For example, a failure mode that is exposed in ten minutes at 110°C may take several hours to be found at 70°C, and may take several months to be exhibited at 30°C. While it is not reasonable to expect the product will ever encounter these intense conditions, the same failure modes will be encountered at lower stress levels in much longer periods of time.
Root cause analysis is performed on all failure modes encountered during the HALT process. Weak points of the design are determined and improvement recommendations are made. Improving these weak points will have a direct correlation to improving future field failures rates.