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Reliability of electronic components

Written by: H. U., Component Business Headquarters

http://www.murata.com/products/emicon_fun/2012/04/

Chapter 1: What is a reliability test?

■ 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

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■ 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.

<Initial 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.

<Chance failure>

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.

<Wear-out failure>

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.

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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).

■ What is a reliability test?

Next, I will explain reliability tests. Reliability tests are tests for predicting quality during the time a product

will be used, from factory shipment to the end of mechanical lifetime in the market. The aim is to select

stress factors that correlate strongly with the market environment, set the size of the stress and duration of

application and accurately assess product reliability in as little time as possible.

Tests have various test items. Some tests go beyond looking at simple stressors and test the impact of

multiple stressors acting simultaneously, and yet other tests have been developed to examine failure

mechanisms.

The following chart shows some of the most common reliability tests used on electronic components.

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These tests are performed so that only items that have been determined useable in market environments are

brought to the market as a product.

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Chapter 2: How do you estimate the lifetime of components?

In Chapter 1, we explained the basic concept of reliability and failure and various reliability test methods

actually used in engineering. From here, we will discuss accelerated tests performed to estimate the service

life of electronic components, using the example of monolithic ceramic capacitors.

Electronic components are built into many different kinds of electronic devices. When actually used in the

market, they are exposed to all types of external stress. For example, there is the physical stress of the

electronic device being dropped, the thermal stress of temperature differences and the electrical stress

applied when the device is powered up. These types of external stress become factors that may cause failure

of electronic components during use of the product in which they are embedded. To address this, we

investigate the mechanisms of external stress and failure occurrence in each type of electronic component

from the design stage and use the results as feedback for reliability design of electronic components.

Moreover, by assessing the relationship between the degree of external stress and the onset and probability

of failure occurrence, we can build an "external stress and failure occurrence acceleration model" that lets us

assess service life of electronic components more quickly.

To give a specific example of an acceleration model, I will talk about temperature and voltage acceleration

aspects of service life in monolithic ceramic capacitors. In general, monolithic ceramic capacitors are made

of an electrical insulator (dielectric) and are known to be extremely highly reliable even when continuously

energized.

For example, the ambient temperature around the control module installed near the automobile engine room

becomes very hot during use.

Figure 1 shows what happens inside ceramic material used in capacitors when energized under a high-

temperature environment.

The atomic level electrical defects contained in minute quantities in ceramic material are thought to move

from the anode (+) to the cathode (-).

Figure 1

In barium titanate and other electric ceramics, a minute number of atomic level defects (called oxygen

defects) are encapsulated in the crystal structure during the firing process. They gradually shift when voltage

is applied externally and eventually accumulate in the vicinity of the cathode, at some point leading to

breakdown of the ceramics.

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The service life (lifetime) of monolithic ceramic capacitors is thus thought to be determined by how fast

oxygen defects move through the ceramic material and the number of defects present, and a model has been

created with ambient temperature and applied voltage during product use as parameters. The most common

acceleration model uses the Arrhenius theory, but the following empirical formula can also be used as a

simple method for estimation.

From this relational expression, you can conduct accelerated tests under relatively harsh conditions (higher

temperature and higher voltage) to estimate service life under the actual conditions in which the product will

be used.

Let us consider a comparison between accelerated tests on monolithic ceramic capacitors and the estimated

conditions of practical product usage. To do so, we will use the above formula with the endurance test time

of the accelerated test on the capacitor as LA and lifetime in standard condition under the actual usage

conditions as LN.

A 1000-hour-long endurance test conducted at 85ºC with an applied voltage of 20V is estimated to be

equivalent to 1,448,155 hours at 65ºC with an applied voltage of 5V. This is about 165 years! The voltage

acceleration constant and temperature acceleration constant used in the formula vary with the type and

structure of the ceramic material. However, we can use the acceleration model to verify the service life

under certain usage conditions over the long term from the results of a relatively short test.

This is an example using monolithic ceramic capacitors, but the types of commonly used electronic

components and estimated usage conditions vary widely. It is therefore important to establish an

acceleration model related to the stress that affects each type of electronic component.