HOW TO ESTIMATE RELIABILTY EARLY IN A PROGRAM

In a few discussions about the perils of the mean time between failures (MTBF), individuals have asked about estimating MTBF (reliability) early in a program. They quickly referred to various parts count prediction methods as the only viable means to estimate MTBF.

One motivation to create reliability estimates is to provide feedback to the design team. The reliability goal exists and the early design work is progressing, so estimating the performance of the product’s functions is natural. The mechanical engineers may use finite element analysis to estimate responses of the structure to various loads. Electrical engineers may use SPICE models for circuit analysis.

Customers expect a reliable product. If they are investing in the development of the product (e.g., a military vehicle, custom production equipment, or a solar power plant) they may also want an early estimate of reliability performance. Engineers and scientists estimate reliability during the concept phase as they determine the architecture, materials, and major components. The emphasis is often on creating a concept that will deliver the features in the expected environment. The primary method for reliability estimation relies on engineering judgment.

With the first set of designs, there is more information available on specific material, structures, and components; thus it should be possible to create an improved reliability estimate.

IS TESTING THE TRUE WAY TO ESTIMATE MTBF?

Early in a program means that there are no prototypes available for testing – just bill of materials and drawings. So, what is a reliability engineer to do?

One could argue that, without prototypes or production units available for testing (exercising or aging the system to simulate use conditions), we do not really know how the system will respond to use conditions. While this is true, it is difficult to know what we do not know. However, we often do know quite a bit about the system and the major elements and how they individually will respond to use conditions.

Even with testing, we often use engineering judgment to focus the stresses employed to age a system. We apply prior knowledge of failure mechanism models to design accelerated tests. In addition, we use FMEA tools to define the areas most likely to fail, thus guiding our test development.

CREATING A RELIABILITY ESTIMATE WITHOUT A PROTOTYPE

Engineering judgment is the starting point. One should include the information from FMEA and other risk assessment methods to identify the elements of a product that are most likely to fail and thus limit the system reliability. Then there are a few options available to estimate reliability, even without a prototype.
First, new products are rarely created using all new materials, assembly methods, and components. Often a new product is approximately 80% the same as previous or similar products. The new design may be a new form factor, thus involve mostly a structural change. It may include new electronic elements – often just one or two components. It may involve a new material, reusing known structures and circuits.

We can use the field history of similar products or subsystems and engineering judgment for the new elements to create an estimate. A simple reliability block diagram may be helpful to organize the information from various sources. For the new elements of a design, the engineering judgment should be based on analysis of the potential failure mechanisms, employing any existing reliability models, or based on the use of simulations to compare known similar solutions to the new solution.

Second, for the elements without existing similar solutions and without existing failure mechanism models, we would rely on engineering judgment or component- or test-coupon-level testing. Rather than waiting for the system prototypes, early in a program it is often possible to obtain samples of the materials, structures, or components for evaluation.

The idea is to use our engineering judgment and risk analysis tools to define the most likely failure mechanisms for the elements with unknown reliability performance. Let’s say we are exploring a new surface finish technique. We estimate that exposure to solar radiation may degrade the finish. Therefore, we obtain some small swatches of material, apply the surface finish, and expose the surface to UV radiation. Although this test does not utilize the full product using fully developed production processes, it is an easy way to evaluate the concept.

Another example is a new solder joint attachment technique. Again, use your judgment and risk analysis tools to estimate the primary failure mechanisms, say thermal cycling and power cycling, then obtain test packages with the same physical structures (in which the IC or active elements do not have to be functional) and design appropriate tests for the suspected failure mechanisms.

ESTIMATES BASED ON AVAILABLE KNOWLEDGE

With a little creativity we can provide a range of estimates for elements of a design that have little or no field history. We do not need to rely on a tabulate list of failure rates for dissimilar products created by a wide range of teams for diverse solutions. We can draw from the actual field performance of our team’s prior designs for the bulk of the estimate. Then we fill in the remaining elements of the estimate using engineering judgment, comparative analysis, published reliability models, or coupon or test structure failure mechanism evaluations.

In general, we can understand the bulk of the reliability performance and have rational estimates for the rest. It may just be an estimate, but exercise will help us and the team focus on which areas may require extensive testing.

Bio:

Fred Schenkelberg is an experienced reliability engineering and management consultant with his firm FMS Reliability. His passion is working with teams to create cost-effective reliability programs that solve problems, create durable and reliable products, increase customer satisfaction, and reduce warranty costs. If you enjoyed this articles consider subscribing to the ongoing series at Accendo Reliability.