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I am currently working on a hardware FMEDA as per IEC 61508 and I am using thick film resistors such as AC0402FR-071K5L and I want to calculate/find the FIT rate of the thick film resistors according to SN29500. According to the SN29500 document there are resistor types such as carbon film, metal film etc. with different FIT values. Which type of resistors should I consider for the thick film resistors?

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See Panasonic's surface mount resistor technical guide, section 6 (as one example)...

https://industrial.panasonic.com/cdbs/www-data/pdf/RDM0000/DMM0000COL11.pdf

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"Thick film" says nothing decisive about the composition. And it is the composition and construction which determine the safety.

So either ask the manufacturers of all your resistors about specific failure data or at least check all their datasheet for their respective composition and then assume standard values for this composition.

For "thick film" resistors nowadays, both carbon and metal composition are unusual. I think most use some oxide composition.

Moreover, unless you have resistors with guaranteed failure behavior, assume that they can fail open or short and that they can catch fire when overpowered.

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