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# AR# 63220

## Description

FIT and MTBF are usually used to express the failure rate for SEU.

PPM
can also be used.

What are the conversions between them?

## Solution

FIT and MTBF

1 FIT = 1 Failure/10^9 Device Hours (114,155 years).

The Mean Time Between Failures (MTBF), represents the average time you can expect a device to operate without failing.

So for example, the conversion between FIT (1489 FIT) and MTBF(?) is as follows:

1,489 FIT or an MTBF of 77 years (114,155 years/1,489 FIT) = 77 years MTBF

(Parts per million) PPM

Since the failure rate is assumed to be a constant, it is acceptable to express Early life failure rate (ELFR) in terms of FIT.

It is often desired to express the ELFR in PPM.

However, PPM is a measure of the cumulative fraction failing per device, whereas FIT is a measure of fraction failing per device-hour.

Thus, when ELFR is expressed in PPM, the early life period must also be specified.

For example:

ELFR(in ppm per tELF) = [tELF x 10^-3 x ELFR in FIT];

ELFR(in FIT) = [1/(tELF x 10^-3) x ppm];

Where early life period (tELF) is the specified early life period as defined by the user or the supplier.

For example 1 year = 356 days 24 h/day = 8,760 hours.

Early life failure rate (ELFR):

Early life failure rate (ELFR) measurement of a product is typically performed during product qualifications or as part of ongoing product reliability monitoring activities.

These tests measure reliability performance over the products first several months in the field.