11xx, 31xx & 32xx Pressure Sensor MTBF Reliability Calculations

 

The data was calculated for the following situations:

 

The calculation was performed using FIT which is the number of failures per billion (1,000,000,000) hours. This converts to MTBF using the following equation:

MTBF = 109/FIT

 

The Transducer was broken down into the following sub-assemblies for ease of calculation:

1. Pack.(Sensing Element, Union, and wire bond-out board)

2. ASIC.(Amplifier)

3. Other components.

For details of how the above figures were calculated please see the following pages.

 

Transducer Pack Reliability Calculation:

This data was calculated using the following accelerated test data:

 

The equivalent device time at normal operating temperature was calculated using the Arrhenius equation below:

From the FIT the MTBF can be calculated.

 

Transducer ASIC Reliability Calculation:

This data was calculated using the equation available in MIL-HDBK-217F, section 5.1 for Microprocessors/Linear circuits:

 

The MLP package used by the ASIC has a very low thermal resistance between Junction and case which significantly enhances its reliability.

Using the above coefficients the reliability (FIT) for the ASIC can be calculated as below:

From the FIT the MTBF can be calculated.

 

Other Components Reliability Calculation:

1. The FIT data for each component was gathered from the supplier.

2. The temperature and environment coefficients for each component were obtained from MIL-HDBK-217F.

3. The FIT numbers were corrected for environment and Temperature.

4. The PCBA (laminate and solder) FIT data was calculated using the equations in MIL-HDBK-217F section 16.

For results see below:

 

5. The FIT figures for each component on the BOM were summed to give the results below:

From the FIT the MTBF can be calculated.

 

Notes:

FIT = “Failure unIT” also known as “Failures In Time”.

MTBF = “Mean Time Between Failures”.

 

The figures are for the 1209-500 PCBA which is the worst case (most complicated), other PCBA’s are the same or better.

Other Components includes all the electronic components and the PCB including solder joints (SMT and through-hole). Static mechanical components were disregarded as they do not have a failure rate.

Using FIT allows summing of all failure rates. MIL-HDBK-217F (incorporating amendment Notice 1 & 2) was used to do these calculations. That specification is for guidance only.

It should be recognised that these figures are estimates calculated in good faith based on accurate data gathered for our product. These figures are for information only.

To aid readability many numbers are not shown to their full precision which may lead to slight rounding errors if the calculations above are replicated.

As the Transducer cannot be repaired, MTBF is technically incorrect and MTTF (Mean Time To Failure) should be quoted however for the purpose of brevity we use MTBF.

The confidence Level is calculated using a Chi Square distribution to estimate the accuracy of the number based on the number of fails. The data supplied by the manufacturers of the other components typically had a confidence level of 60%.

Considering the low power consumption of the Transducer, 5oC was allowed as the typical temperature rise due to normal operation.