As we make things to be more and more reliable, it gets harder and harder to make them fail in reliability tests. On the one hand – that’s great. On the other hand – that sucks if we want to use testing to help measure reliability. So if we are focused on measuring reliability through testing, we need to make our product or system fail in test conditions (without making it less reliable) but make sure these test conditions can somehow be translated back to how customers are going to use it. This is where accelerated testing comes in. [Read more…]
The Eyring Model for Accelerated Testing
Sometimes the reaction rate of a process relies on two stresses. For chemical reactions temperature seems to influence the rate of the reaction. Yet, other stresses such as humidity or voltage may also play a significant role.
H. Eyring suggested a model that assumes the contribution of each stress on the reaction rate is independent thus one could multiple the respective stress contributions to the rate of reaction.
The Erying model provides a means to account for the contributions of temperature and another stress when modeling the time to failure of select failure mechanisms. [Read more…]
4:2:1 Allocation of Test Units
One question that you should consider when planning a multiple stress accelerate life test (ALT) is the allocation of test units to the various stresses.
We want to create a model detailing the relationship between stress and time to failure. We also want to project the time to failure estimates to use conditions. Ideally, we test at nominal conditions only and gather time to failure information. We do not have the luxury of time thus explore using ALT.
One method of allocation is to place an equal number of samples with each stress level. Is that the best approach? [Read more…]
Black’s Equation for Electromigration Accelerated Life Testing
Black’s equation for estimating the time to failure due to electro migration is a classic. James Black explored and wrote about electromigration in aluminum metallization within semiconductors since 1969.
He and others have explored other materials used as conductor prone to electromigration. Thus, there are a number of models and constants available to match your particular system.
Let’s take a look at the general equation for a microcircuit conductor after a brief description of the failure mechanisms called electromigration. [Read more…]
Metal Fatigue Failure Mechanism Accelerated Life Testing
Metal is a wonderful, strong, material. Yet under certain types of stresses metal can fail One in particular is fatigue due to cyclic motion.
Metals in a solid state have an atomic level lattice structure. This provides the strength and flexibility. It is the flexibility part that causes trouble. We don’t get the benefit of flexibility for free. As the metal bends it ‘adjusts’ the lattice to accommodate the motion. In doing so, it changes the metal properties becoming a bit more brittle, for example.
In most cases a very small motion causes imperceptible changes and loss of functionality. In some cases, like bending a wire coat hanger with the intent to break it, just a few cycles of dramatic bending is enough to break the wire.
In metal applications that experience cyclic motion and the risk of metal fatigue failure may occur during the expected duration of product use, we may need to characterize the time to failure behavior. An accelerated life test for a metal fatigue failure mechanism is not difficult, yet does take some planning to get meaningful results. [Read more…]
Peck’s Relationship for Temperature & Humidity Testing
High temperature & humidity is a common test condition. For specific failure mechanisms, there are models available (or you can create a model) to determine the translation from test to use conditions.
These acceleration models generally only apply to one specific failure mechanisms and do not apply to a system level estimate of life. If the failure mechanism is the dominant failure mechanism for the product, then an ALT exploring just that mechanisms would provide a life estimate.
Peck’s relationship is an acceleration model for the effect of humidity on the metallization elements of integrated circuits within plastic enclosures (typically an epoxy over molding). [Read more…]
Time Compression ALT – The Easy One
The easiest ALT is one that you operate an item more often then operated by the customer. Removing spans of time the item is not being bent, moved, heated, etc allows you to use time compression.
For example, a home kitchen toaster may be used for a few cycles during breakfast time in your home. In the lab, we can avoid having to wait the day of idle time and just make toast more often than just at breakfast to accelerate the operation of a toaster.
Time compression ALT is also easy to understand and describe the acceleration factor to cover the ALT results to field use conditions. Let’s explore a simple example, work out the acceleration factor and how to interpret a set of ALT results. [Read more…]
Introduction to Thermal Cycling Life Testing
Materials expand or contract with temperature change. Water expands as it freezes, whereas steel contracts as it cools.
This motion can limit the life of your system.
Materials and mechanical engineers include the expected motion into their designs, well the better engineers do.
Even centuries ago, craftsmen used expansion slots or features when attaching wooden table tops to their frames.
The motion due to temperature change will occur and has the potential to create immense strain within your product. [Read more…]
Reliability or life testing involves estimating the expected durability over time of an item.
This may be an entire system, a product, or an individual component. We may also focus on an element of a component, such has a material property.
At the end of the testing, we want to say something meaningful about the expected performance over time. [Read more…]
Mechanical Systems Accelerated Life or Reliability Testing
Mechanical systems wear out and fail eventually. The ability of a structure to support a load, move through the specified range of motion, or spin degrades with use and time. Even our joints eventually wear out.
Accelerated life testing (ALT) has plenty of literature concerning the failure mechanisms unique to electronic components and materials. This is partially due to the limited number of unique electronic components compared to the often custom mechanical designs. ALT also has value as it provides information about a system’s reliability performance in the future.
Let’s explore an example of mechanical reliability testing (an ALT) in order to outline a basic approach to ALT design and analysis. [Read more…]
Methods to Estimate ALT Value
Here is an example of how to determine the future value of a specific reliability task. Many of us face the challenge of how to justify spending product development resources to provide insights and information to the rest of the team. Accelerated life testing (ALT) is particularly difficult: It is time consuming, expensive, and at times statistically complex. Having a clear method to estimate the value serves your career and the organization well, as both benefit from the right investments.
The other day I got a question about the difference between ALT and HALT. There was some confusion probably because of the similar words in the acronym. ALT is Accelerated Life Test, and HALT is Highly Accelerated Life Test. [Read more…]