Your Reliability Engineering Professional Development Site
Let’s take a graphical view of reliability improvement that occurs during product development or improvement projects.
If we are making improvements the system reliability should increase. We can use the build, test, fix approach to measure improvements, find failures, design improvements, and repeat. [Read more…]
The operating characteristic curve is useful to understand the capability of a lot sampling plan. It depicts a graphical relationship between the unknown lot’s defect rate and the probability of the specific sampling plan to accept the lot. Ideally, we want a sampling plan the correctly accepts good lots and rejects bad lots. [Read more…]
Reliability testing to determine what will fail or when will failures occur is expensive.
Organizations invest in the development of a product and attempt through the design process to create a product that is reliable. [Read more…]
One approach for reliability improvement is to find the weaknesses or faults, then fix them.
This is best done before shipping to the customer.
You may hear this called the Test, Analyze, and Fix Method (TAAF). Or, you may have heard of it called a Reliability Growth Test program (RGT). Either way, the essence is evaluating prototypes to find specific faults. [Read more…]
Testing a new design is expensive and takes time to accomplish. During the design, process engineers make assumptions across many decisions. Unknown interactions and unanticipated events may lead to product failure.
Finding design weaknesses early in the design process permits the team to address and correct problems. [Read more…]
Before Norris-Landzberg’s study there was the work of Coffin and Mason.
One way to approach accelerated life testing is to use a model for the expected dominant failure mechanism. One such model is for solder joint low-cycle fatigue originally published by Coffin (1954) and Manson (1953), independently.
Norris and Landzberg proposed the plastic strain range is proportional to the thermal range of the cyclic loading (ΔT). They also modified the equation to account for effects of thermal cycling frequency (f) and the maximum temperature( T). They and other than empirically fit the parameters for the equation. [Read more…]
A form of testing that reduces the time till results are known, ALT provides a means to estimate the failure rate over time of a product without resorting to normal use conditions and the associated duration.
For example, solar photovoltaic cells should operate for 25 years without failure. The product development time is less than a year for a new panel and the team wants to estimate the reliability of the cells over the 25-year duration. [Read more…]
The operating characteristic curve is used to understand lot sampling plan. It graphically provides a relationship between the unknown lot’s defect rate (or total) and the probability of the specific sampling plan to accept the lot. Very good plans discriminate between good and bad lots. Poor plans may accept bad lots or reject good lots to easily. [Read more…]
I recently received a question concerning what sample size to use when assessing call center calls. Not a lot of information in the request, so my answer was rather general. And, thought it might provide some insight to others facing sample size questions of their own. [Read more…]
Two short questions to evaluate your knowledge of failure mechanisms (root causes) and common reliability models. The answers will be posted in a comment, later.
Which of the following failure root causes is most likely NOT due to power line variation (electronic-based product)?
A. Circuit design margin exceeded
B. Power dissipation
C. In-rush current response
D. Mechanical fatigue [Read more…]
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…]
There a few different ways to sample a lot (or group) of material to determine if it has an acceptably low failure rate (or proportion that are considered ‘bad’). The following is an example of the sequential sampling method, which happens to be rather efficient by generally using the fewest samples for the same risk protection. [Read more…]
Hi Fred,
I would take this opportunity to ask the reliability guru about bathtub curve for hardware reliability. I am running 27 units for life test for a million cycles around 555 hours. I have one failure at 300,000 cycles, and the rest of the units are running fine. Would this be classified as an early life failure? Also, how do I make a determination of when the early life failure time interval ends and constant failure rate starts in this example based on failure rate of remaining units? Thanks. [Read more…]
Reaching for a goal may include taking some risks. In reliability testing, we are often limited by the number of samples available for testing. And, in the case where time is available or the acceleration factor is high we can take advantage of testing longer. [Read more…]