MTBF (Mean Time Between Failures-MTBF)
The life expectancy predicted by accelerated life test is used over the years. The most representative model is the temperature accelerated Arrhenius Model (fig. 1). Coffin-Manson Model (fig. 2) is used for Thermal- metal fatigue life test. For lead-free life expectancy, most large foriegn companies use the Coffin-Manson Model to test life of PCBA's solder joint.
For systems product life expectancy, three models are listed below with its strengths and weakness:
Common method: Usually uses component parameter or the measured temperature as general information and entered into a software program to calculate the product's MTBF (Parts com. or Parts stress)
Strengths: Can obtain a satisfied MTBF immediately
Weakness: Calculation is doubtful due to the constant changing of component specifications causing prediction unable to progress as components advances, and therefore, when components assembled into a system, the impedence, current, and voltage are unable to simulate for computing.
Reliability Demonstration Test
Common method: Arrhenius model- Equation 1
Strengths: Test report+ statistics-Has a long history so customers are less doubtful.
Weakness: Cycle time of product is shorter, so more and more customers are now demanding IT products to last at least five years. Therefore, testing takes longer time. To shorten this time, a proper temperature stress analysis is conducted.
Strengths: Genuine and true
Weakness: Needs a long period of time to collect enough field data; in addition, if product fails, a massive number of products are in the market already.
||Fig. B：Coffin-Manson Model|| |