Hi Everyone!

I would like to find MTTF of PCB.
For that I have collected the FIT rates of major components(ICs, Mosfets, etc.) present on PCB by contacting manufactureres.
Now I have all the FIT values.
So MTTF would be just summing the FIT rates of all components and taking its inverse right?

By appplying this formula will I get the MTTF of complete PCB? or I am missing something?
Please help if anyone has done this before.

I also wanted to know if there is any software which directly calculates MTTF of PCB.
Kindly suggest if anyone is using it.

Thanks in advance!

Best Regards
Vishakha

 

How you combine things depends on a number of factors. Just summing up the FIT values relies on the assumption that any one failure equates to a failure of the entire assembly. Is that the case here, or have you designed in redundancy for some of the components?

What about the failure rate of the board itself? Is it sufficiently low that it can be discounted?

What about the non-major components? They may individually have low rates, but if you have a lot of them, you should take them into account to.

 

Hey WBahn!
Thanks for your reply. I have also collected FIT rates of Capacitors and resistors but I wasn't sure if I should take them into consideration. This is new topic for me. 'have you designed in redundancy for some of the components? ' What is designing redundancy for some components? what does it mean? Please guide thanks!

 

Hey WBahn!
Thanks for your reply. I have also collected FIT rates of Capacitors and resistors but I wasn't sure if I should take them into consideration. This is new topic for me. 'have you designed in redundancy for some of the components? ' What is designing redundancy for some components? what does it mean? Please guide thanks!

It means is your design such that if a particular component fails that the circuit will still work. Unless you have deliberately design the circuit to be fault tolerant, this probably doesn't apply

 

First of all: everything I tell now concerns MTBF (Mean Time Between Failures) which is in theory a little bit different from MTTF (Mean Time To Failure) but the concepts are similar and sometimes MTBF and MTTF are mixed up.

Yes, summing up all FIT values and taking the inverse is ONE possibility of calculating the MTBF of a PCBA. Usually the unit of the failure rate (FIT) is defined as failures in 1E9 h. Therefore you can simply calculate (1E9 h )/sum(FIT values). Divide the result by 8760 h/year and you will get the MTBF in years.

You can make your life easy and you can easily get a number for your MTBF. But you can also make some kind of rocket science out of it. The failure rates are usually defined at some reference conditions, e. g. 40 deg C as environmental temperature, working voltage of capacitors is 1/2 of rated voltage and so on. So you can individually calculate the failure rate of each component. For that, you need detailed description what kind of component stress causes how much influence on the failure rate.

But most people don't know what an MTBF of let's say 50 years really means. The MTBF has nothing to do with the lifetime of an individual electronic device. It is only a statistical value. So in this example, assume we have a factory where 500 parts of this device with an MTBF of 50 years are installed. Then you can expect an average of 10 failed devices per year.

 

Years ago I had to submit MTBF calculations for a customer contract. An excel spreadsheet was the simplest way to do it