What is MTBF?
MTBF: Mean time between failures
Full English name: Mean Time Between Failure
Test purpose: measure the reliability index of a product (especially electrical products)
Unit: usually in hours, such as: Ec. 5000000 hours

How to test MTBF?

MTBF test method
MTBF prediction method
The MTBF prediction method is to input various influencing factors parameters into the simulation test software PCT, taking a laptop (motherboard) as an example, and input the test conditions: Calculate standard (Calculate standard), Calculate method (Calculate Method), temperature (Temperature), environment (Environment), component quality (Component Quality), etc., the software performs simulation modeling based on the product parameters in the query database and calculates the MTBF value, which is an estimated value.
Reliability prediction method reference standards:
MIL-HDBK-217 "Electronic Equipment Reliability Prediction Manual"
Note: The U.S. Department of Defense Reliability Analysis Center and Rome Laboratory proposed and became an industry standard
GJB/Z 299B "Electronic Equipment Reliability Prediction Manual"
Note: my country’s military standard
Bellcore SR-332 "Electronic Equipment Reliability Prediction Procedure"
Note: AT&TBell Laboratories proposed and became the industry standard for calculating MTBF values for commercial electronic products.

MTBF experimental method
There are three types of MTBF experimental methods:
1. Full life test: Full life test requires that all samples will eventually fail during the test, and the MTBF can be calculated using a simple arithmetic average;
2. Censored sequential test: During the test, the equipment under test is monitored continuously or at short intervals and the accumulated relevant test time and the number of relevant failures are compared with the criteria for determining whether to accept, reject or continue the test. ;
3. Timing or fixed-number censoring test: During the test, the equipment under test is monitored continuously or at short intervals, and the relevant test time is accumulated until or exceeds the predetermined relevant test time (acceptance) or a predetermined number of relevant failures occur (rejection). receive).
Note: In practice, all samples to be tested will fail during the testing process. Especially for products with high reliability levels, the testing time may be several years, ten years or even hundreds of years, so this method can only be used for Products with shorter life span.

MTBF test

Standard reference for reliability test methods:
GB/T 5080.7-1986 "Equipment Reliability Test - Validity Test of Constant Failure Rate Assumption"
GB/T 9813.1-2016 5.9 "General Specifications for Computers Part 1: Desktop Microcomputers Reliability Test"
GB/T 9813.2-2016 5.9 "General Specifications for Computers Part 2: Portable Microcomputers Reliability Test"
GB/T 14081-2010 5.9 "General Specifications for Information Processing Keyboards - Reliability Test"
GB/T 18220-2012 5.17 "General specifications for reliability and life testing of information technology handheld information processing equipment"

MTBF acceleration method
In the production and manufacturing process of enterprises, it is necessary to understand the reliability of the product before leaving the factory and evaluate the after-sales risk of the product. In this case, our traditional MTBF calculation method is obviously not feasible, because by the time the actual MTBF test value of our product comes out, the product has almost arrived. After being eliminated or updated, the test values obtained are no longer meaningful. At this time, we need to use other methods to simulate and calculate the product MTBF value, such as: MTBF acceleration method
The MTBF acceleration method is a method of accelerated life testing in the laboratory. Within an acceptable test time, the MTBF value of the product is evaluated without destroying the original design characteristics of the product. The acceleration factors that affect electronic products mainly include: temperature, Humidity, resistance, voltage, etc.
Accelerated method test standard reference:
GB/T 34986-2017 "Product Accelerated Test Method"

MTBF acceleration method case
Taking a monitor from a medical device company in Shanghai as an example, we will introduce the process of accelerated life testing.
Step1. Determine the evaluation path based on product characteristics
Single function, choose whole machine verification.
Step 2. Determine the number of samples
3 complete machines.
Step 3. Given expected service life and confidence level
The expected service life is 10 years, and the confidence level is 99%. The expected service life of the product is 10 years. It is planned to use the mean time between failures to simulate the expected service life of the product, that is, MTBF=10 years=87600h
Step 4. Determine clinical usage patterns
A certain monitor has three modes during use, namely storage mode, standby mode and working mode.
Step 5. Determine the stress profile for clinical use
Considering that a monitor is used to monitor patients before surgery, the clinical stress is mainly temperature and humidity. The operating temperature and humidity of the product is 25℃, 60%RH;
Step 6. Determine the stress
The maximum temperature and humidity tolerance of the product is:
Maximum temperature and humidity tolerance in storage mode: 60℃ 93%RH
Maximum temperature and humidity tolerance in standby mode: 55℃ 85%RH
Working mode maximum temperature and humidity tolerance: 45℃ 85%RH
In order to avoid product damage at too high temperatures and avoid introducing more variables due to temperature changes, it is planned to use a constant 45°C and 85% RH as the test temperature and humidity.
Step 7. Select failure model
According to the clinical stress profile, refer to the accelerated model of temperature and humidity stress proposed by D.S. Peck: Peck model
Step 8. Determine the coefficient values in the model based on literature data
Check the Telcordia SR-332 2016 version of the electronic product reliability prediction standard. The coefficient of determination Ea is 1.0eV and n is 2.66.
Step 9. Calculate acceleration factor

Acceleration model:

Among them, n is 2.66, Ea is 1.0eV
Calculate the acceleration factor AF=2.525*11.607=29.3
Step 10. Determine the accelerated test time
The confidence level is 99%. Check the normal distribution table and get Z=2.58
Cumulative test time Ttot=MTBF*Z/AF=87600*2.58/29.3=7714h;
Setting a margin of 10%, the total test time is Ttot=8485h.
Three prototypes are provided for testing. Assuming that the samples do not malfunction and each device completes the entire test cycle, each prototype requires testing time T=8485/3=2828h=117.8 days.
Step 11. Stress application
In actual use of the product, it is used 7 days a week, 5 hours a day, 3 hours standby, and the rest of the time is storage time;
In order to simulate the actual use of the product, working stress is applied to the product in a 24-h cycle. In each 24-h cycle, it will first work continuously for 5 hours, then stand by for 3 hours, and finally store for 16 hours.
Step 12.Conclusion
Three prototypes are subjected to working stress in the manner specified in Section 5 under an environment of 45°C and 85% RH. If the test continues for 117.8 days without failure, the lower limit of the MTBF value of the product is considered to be 10 years (with a margin of 10%).

MTBF test

It can be concluded from the above methods that although methods such as timed censoring test and sequential test are very useful, the test time is too long, especially when the product reliability level is particularly high.
In actual production, in order to shorten product time to market, reduce product costs, and meet people's needs for economical and efficient accelerated testing methods, accelerated testing methods solve problems more effectively. This method shortens the test time by increasing the stress level of the product test or increasing the frequency of alternating stress application, and discovers and reduces the failure modes of the product, so as to quickly evaluate the reliability level of the product and increase it.