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ANSYS-2012-R1

How to Plan and Conduct Highly Accelerated Life Testing (HALT)

A thermal map created by ANSYS Sherlock

An integral part of an electronic design engineer’s job is to assess the robustness of a product to ensure its success and performance. Highly accelerated life testing (HALT) is an important tool for this purpose. Engineers can maximize its effectiveness through careful planning prior to testing and detailed execution.

1. DEVELOPING A HALT TESTING PLAN

Setting clear expectations and directives for HALT testing is a multi-step process. Engineers can come together to start this process by:

  • Developing a test plan based on physics of failure (PoF), clearly defining objectives, expected environments and sample availability
  • Determining the applicable stresses, such as temperature, vibration and/or shock
  • Deciding how many devices (known as samples) are available for testing. Generally, one to five samples are used.
  • Selecting the functional tests to be run during development, such as what the device should be doing, which circuits should be active and what codes/sensors should be gathering data
  • Identifying which parameters need to be monitored based upon the expected environment
  • Defining what constitutes a failure
  • Considering using ANSYS Sherlock design analysis software to simulate the vibration and thermal loads so a model can be created that may reach HALT limits
A thermal map created by ANSYS Sherlock

In conjunction with developing the foundational outline, two key areas must be addressed:

  • Applicable stresses: select the appropriate stresses for testing:
    • Vibration
    • High temperature
    • Low temperature
    • Voltage/frequency margining
    • Power cycling
    • Combined stresses, i.e., temperature and vibration
  • Step stress approach: for each intended stress, clearly delineate:
    • The starting stress point
    • The amount by which to increment the intended stress in each step
    • The duration of each step
    • The device or equipment limit for that stress, thereby ending HALT efficacy

2. SETTING UP A HALT

For accurate results, engineers must pay particular attention to the HALT testing configurations. They can do this by:

  • Designing a vibration fixture to ensure vibrational energy is being transmitted into the product
  • Designing air ducting to ensure thermal energy is being transmitted into the product
  • Using a tuning chamber to test the sample
  • Determining locations for thermocouples to monitor temperature
  • Setting up all functional test equipment and cabling

3. CONDUCTING A HALT

HALT testing is comprehensive and encompasses several phases, each with specific parameters to follow. To complete the HALT, engineers need to assess:

  • Thermal step stress
  • Fast thermal transitions
  • Vibration step stress
  • Combining stress
A chart that depicts the number of cycles to failure (by component)

Thermal step stress tests apply incremental temperature stress levels throughout the product lifecycle to identify product failure modes. Engineers can complete this test by:

  1. Beginning with a cold step stress, followed by hot step stress
  2. Incrementing the temperature by 10 C increments and then by 5 C increments as limits are approached
  3. Setting the dwell time minimum at 10 minutes plus the time needed to run a functional test. Timing should commence once the temperature has reached its set point.
  4. Continuing tests until technology limits are reached
  5. Applying power cycling, load variations and frequency variation during vibration stress test

Fast thermal transitions are exactly as the name implies — changing temperatures as quickly as the testing equipment and chamber allow. Engineers perform this test by:

  1. Keeping the temperature range within 5 C of the operating limits determined during step testing
  2. Decreasing the transition rate by 10 C per minute (if the sample cannot withstand maximum thermal transitions) until the limit is found
  3. Continuing the transitions for 10 minutes, or the time it takes to run a functional test
  4. Applying power cycling, load variations and frequency variation during vibration stress test

Vibration step stress testing applies incremental vibrational stress levels throughout the product lifecycle in order to identify product failure modes. To finish this test, engineers need to:

  1. Determine the gravity root-mean-square (Grms) increments on the product (typically ranging from 3 to 5 Grms)
  2. Set the dwell time minimum at 10 minutes, plus the time needed to run a functional test. Timing should commence once the temperature has reached its set point.
  3. Continue testing until technology limits are reached
  4. Apply power cycling, load variations and frequency variation during vibration stress test

Finally, engineers perform the combined testing by merging the testing results and methodologies to further assess the products. Engineers perform combined testing by:

  1. Developing a thermal profile using thermal operating limits, dwell times and transitions identified in earlier testing
  2. Applying additional product stresses during vibration stress test
  3. Using a constant vibration level of about 5 Grms in the first combined run and stepping up using the same increments in the vibration step stress tests
  4. Adding ticket vibration (about 5 Grms) at higher levels (>20 Grms) to determine if failures were precipitated at high gravitational acceleration (G levels), but only detectable at low G levels

4. POST-HALT Testing

Once HALT testing is completed, the design engineers’ focus becomes determining the root causes of all failures and corrective action. Essentially, a verification HALT needs to be implemented to evaluate if testing adjustments fixed the problems.

Sherlock Automated Design Analysis software shortcuts this process by creating simulations based on testing models before any physical sample modification or the verification HALT takes place — saving time and money. Sherlock can also be applied in the future, as previously tested products are evaluated for engineering changes.

To see how Sherlock can help streamline your HALT process and accurately confirm results, read: ANSYS Sherlock Capabilities.

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