mtbf (mean time between failures) calculations of an assembly

[murb1]

if I have a board and the weakest link is a fan, can I assume that the mtbf for the board would be the fans mtbf?

 

[Miner]

No. While the fan would be the weakest link and most common failure mode, the MTBF would actually be lower for the assembly because of the additional potential for other failure modes.

 

[Marc]

Also see: MTBF

 

[murb1]

For calculating MTBF for a PCI card, I would be using MIL-217F N1/2 parts count method, using Ground Benign. Worst case part ambient temp will be 40 degrees C.

Is 40 C a good temperature or should I scale back to 30 C?

 

[Miner]

Depends on the application and to what temperatures the PCB is exposed. Not just external ambient, but internal to device ambient, and whether cooling fans or heat sinks are part of the equation.

 

[murb1]

Is there a company out there that if I give them my BOM they will do the MTBF calculations for me without charging major bucks?

 

[Steve Prevette]

Is there a company out there that if I give them my BOM they will do the MTBF calculations for me without charging major bucks?

I'm sure there are folks here (including myself) that could do the calcs. But we would need much more than the Bill of Materials. We'd need to know the application and the specifications, the failure modes and rates that you have identified, the way the components work together in the system and interdependencies between them.

For example, let's take the highest failure rate component that you've referenced in the first email. If it only can fail if another component fails which isn't likely to fail, or if there is another component in parallel it, or perhaps the operating environment doesn't stress the component enough to cause it to fail, then you have a whole 'nother answer.

 

[murb1]

There are companies, (probabilistic software) that offer this service. Cost is 2500. per board. Too much monies, need someone else, know of any?

 

[Jim Wynne]

There are companies, (probabilistic software) that offer this service. Cost is 2500. per board. Too much monies, need someone else, know of any?

There might be companies that are willing to separate you from your money and give you a theoretical or hypothetical MTBF number, but unless there's actual ground-truthing and testing under load in the assembly, you don't have anything that's necessarily meaningful. What you need is a person who is expert in reliability analysis.

 

[Miner]

Let's start at the beginning. What is your goal? Why do you need the MTBF?

Is this a customer request? Internal request? Or is your goal simply to improve the reliability of the product?

If your goal is to improve the reliability, forget MTBF. You already know that the fan is the weakest link, so improve it and you do improve the product. Ford does not worry about MTBF or reliability testing, they focus on DFMEA/PFMEA and improving the demonstrated weakest links.