■ What is reliability engineering?
What exactly is reliability engineering? Let us start here.
Reliability engineering is also called failure engineering. It is a branch of engineering that involves increasing reliability of products by assessing and analyzing how failure is caused in the product. In other words, it can be considered engineering that creates broken products.
*The difference between failure and defect
・Defective products are defective from the moment they are produced.
・Broken products were conforming products when they were produced, but became defective products over time.
Reliability engineering deals with the process during which a conforming product turns into a defective product.
There are three factors that cause failure:
(1) Latent internal causes that existed in the product from the start (predispositions)
(2) External stressors such as heat and humidity applied from the usage environment (external causes)
(3) Degradation with time
■ What is failure?
In the preceding part, I said that, "Reliability engineering is also called failure engineering." There are actually different types of patterns of failure. The bathtub curve below is a graph that shows the correlation between failure rate and time.
During a product's lifetime, it goes through three successive periods (initial failure, chance failure, wear-out failure) that each has different causes of failure.
Failure occurs soon after starting to use the product, and the failure rate drops gradually over time. The main cause is thought to be latent defects. Improvement of the design and filtering process and screening of products are essential for preventing such products from being leaked to the market.
After the initial failure period eases, a period starts during which failure can occur by chance. These failures are usually caused by unpredictable events such as lightening and dropping the product. This means that such failure occurs at a nearly constant failure rate that is unrelated to how much time has passed. The goal is to reduce accidental defects in the production process and fluctuations in environmental stressors during use to approach a zero failure rate.
After the chance failure period has passed, the failure rate begins to rise gradually with the passage of time. This is mainly thought to be due to wear-and-tear of the product as the product reaches the end of its lifetime.
You can therefore see that there are different types of failures and that each has its own causes. For quality assurance, it is necessary to examine the factors in detail and select the best test method (reliability test).