How to test MTBF?

What is MTBF?

MTBF: Mean Time Between Failure

Test purpose: To measure the reliability 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

MTBF prediction method is to input various influencing factor parameters in the simulation test software PCT. Taking a laptop (motherboard) as an example, input the test conditions: Calculate standard, Calculate method, temperature, environment, component quality, etc. The software simulates and models the product parameters in the query database and calculates the MTBF value, which is an estimated value.

Reference standards for reliability prediction methods:
MIL-HDBK-217 "Electronic Equipment Reliability Prediction Handbook"
Description: Proposed by the U.S. Department of Defense Reliability Analysis Center and Rome Laboratory and became an industry standard
GJB/Z 299B "Electronic Equipment Reliability Prediction Handbook"
Description: my country's military standard
Bellcore SR-332 "Electronic Equipment Reliability Prediction Procedure"
Description: AT&T Bell Laboratory proposed and became the industry standard for calculating the MTBF value of commercial electronic products

MTBF test method
There are three types of MTBF test methods:
1. Full life test: Full life test requires that all samples eventually fail in the test, and the MTBF can be calculated by using a simple arithmetic mean;
2. Truncation sequential test: During the test, the tested equipment is continuously or at short intervals monitored 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 truncation test: During the test, the tested equipment is continuously or at short intervals monitored, and the accumulated relevant test time is accumulated until or exceeds the predetermined relevant test time (acceptance) or the predetermined number of relevant failures occurs (rejection).
Note: In practice, all samples to be tested will fail during the test, especially for products with high reliability levels. The test time may be several years, ten years or even hundreds of years, so this method can only be used for products with a short service life.

MTBF test

Reliability test method standard reference:
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 Specification for Reliability and Life Test of Information Technology Handheld Information Processing Equipment"

MTBF Acceleration Method
In the manufacturing process of enterprises, it is necessary to understand the reliability of products and evaluate the after-sales risks of products before leaving the factory. In this case, our traditional MTBF calculation method is obviously not feasible, because when the actual MTBF test value of our product comes out, the product is almost out of date or replaced, and the test value obtained is no longer meaningful. At this time, we need to use other methods to simulate and calculate the MTBF value of the product, such as: MTBF Acceleration Method
MTBF acceleration method is a method of accelerating life test 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.
Acceleration method test standard reference:
GB/T 34986-2017 "Product Acceleration Test Method"

MTBF Acceleration Method Case
Take a monitor of a medical device company in Shanghai as an example to introduce the process of carrying out accelerated life test.
Step 1. Determine the evaluation path according to product characteristics
The function is single, and the whole machine verification is selected.
Step 2. Determine the number of samples
3 complete machines. Step 3. Given the expected service life and confidence
The expected service life is 10 years, with a confidence level of 99%. The expected service life of the product is 10 years, and the mean time between failures is planned to be used to simulate the expected service life of the product, that is, MTBF = 10 years = 87600h
Step 4. Determine the clinical use mode
There are three modes in the use phase of a certain monitor, namely storage mode, standby mode and working mode. Step 5. Determine the clinical use stress profile
Considering that a certain monitor is used to monitor patients before surgery, the clinical use stress is mainly temperature and humidity. The product's operating temperature and humidity are 25℃, 60%RH;
Step 6. Determine the stress magnitude
The product's maximum tolerable temperature and humidity are:
Storage mode maximum tolerable temperature and humidity: 60℃ 93%RH
Standby mode maximum tolerable temperature and humidity: 55℃ 85%RH
Working mode maximum tolerable temperature and humidity: 45℃ 85%RH
In order to avoid damage to the product at too high a temperature and to avoid introducing more variables due to temperature changes, it is planned to use a constant 45℃, 85%RH as the test temperature and humidity.
Step 7. Select failure model
Based on 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
Consult Telcordia SR-332 2016 version of the electronic product reliability prediction standard, and determine that the coefficient Ea is 1.0eV and n is 2.66.
Step 9. Calculate the acceleration factor

Acceleration model:

Where n is 2.66, Ea is 1.0eV
Calculate the acceleration coefficient AF=2.525*11.607=29.3
Step 10. Determine the accelerated test time
The confidence level is 99%, and the normal distribution table is Z=2.58
The cumulative test time Ttot=MTBF*Z/AF=87600*2.58/29.3=7714h;
Set a 10% margin, then the total test time is Ttot=8485h.
Provide 3 prototypes for testing. Assuming that the sample has no failure and each device has completed the entire test cycle, each prototype requires a test time of T=8485/3=2828h=117.8 days.
Step 11. Stress application
In actual use, the product is used 7 days a week, 5 hours a day, 3 hours on standby, and the rest of the time is storage time;
In order to simulate the actual use of the product, the product is subjected to working stress in a 24-hour cycle. In each 24-hour cycle, it first works continuously for 5 hours, then stands by for 3 hours, and finally stores for 16 hours.
Step 12. Conclusion
The three prototypes are subjected to working stress in the manner specified in Section 5 under an environment of 45°C and 85%RH. If there is no failure in the continuous test for 117.8 days, the lower limit of the MTBF value of the product is considered to be 10 years (with a margin of 10%).

MTBF test

From the above methods, it can be concluded that although the timed truncation 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 the time to market, reduce product costs, and meet people's demand for economical and efficient accelerated test methods, the accelerated test method solves the problem more effectively. This method shortens the test time by increasing the stress level of product testing or increasing the frequency of applying alternating stress, and discovers and reduces the failure mode of the product, so as to quickly evaluate the reliability level of the product and increase it.