HALT and ALT
HALT and ALT are our two most popular testing services but often times our customers are confused about which to use when. Below is a summary of the presentation we will be giving at the Open House on June 15.
Highly Accelerated Life Testing (HALT) is a great reliability technique to use for finding predominant failure mechanisms in a product. However, in many cases, the predominant failure mechanism is wearout. When this is the situation, we must be able to predict or characterize this wearout mechanism to assure that it occurs outside customer expectations and outside the warranty period. The best technique to use for this is Accelerated Life Testing (ALT).
In many cases, we recommend using both because each technique is good at finding different types of failure mechanisms. The proper use of both techniques together will offer a complete picture of the reliability of the product.
Highly Accelerated Life Test (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 products reliability and the robustness of 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.
Accelerated Life Test (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, Slope of Weibull Distribution (Beta or Wearout factor).
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 existing models are: Arrhenius, Coffin-Manson, and Norris-Lanzberg. Using existing models is much quicker and requires fewer samples than the experimentation method, but it is not nearly as accurate. And assigning values to the variables in the model can be tricky. Note that the transition to Lead Free solder will cause all of these to change (and we dont know yet what they will change to).
When determining the acceleration factor through experimentation, we divide samples into 3 Stress Levels: High Stress, Medium Stress, and Low Stress. We then set up the test to assure that the same failure mechanism occurs at each level. This is a more accurate method but it requires more time and more samples. Then we calculate acceleration factor.
When the Acceleration Factor cannot be determined, often times we are left with only being able to accelerate the duty cycle. For instance, we can increase how often a button is pushed or how often a device is used. However, by increasing the duty cycle, we must make sure that we do not change any other parameters inadvertently we sometimes find that if we increase the duty cycle too much, we also increase the internal temperature of the device, not allowing it time to cool between cycles.
Some examples of good candidate products for ALT are cell phones, fans, hard drives, automotive electronics, robots, and infusion pumps.
Comparing HALT to ALT
In HALT, we dont 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. Our typical HALTs take 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 customers facilities.
Using HALT and ALT Together
Often times we will run a product through HALT and then run the subassemblies through ALT that had predominant wearout mechanisms and thus were not good candidates for HALT.