1 Introduction

The calculation of failure rates is an important metric in assessing the reliability performance of a product. JEDEC describes a method for calculating FIT rates in JEDEC JESD85 METHODS FOR CALCULATING FAILURE RATES IN UNITS OF FITS . In this method, a known sample size of the component is placed under environmental stressess such as High Temperature Operating Life (HTOL) test. HTOL tests operate at high temperatures, such as 125 °C, for a certain duration and the number of components that fail during the test are noted and used in the calculation of FIT rates.

Reliability data is expressed either in failure in time (FIT) or mean time between failure (MTBF), or other similar parameters. FIT is equal to one failure occurring in 10×10⁸ device hours (or 1 billion hours). FIT is typically the unit used to express the failure rate and can be defined as 1 parts per million (PPM) per 1000 hours of operation or one failure per 1000 devices run for 1 million hours of operation.

MTBF is the inverse of the FIT value (10×10⁸/FIT). Most of these methods only apply to constant failure rates. These methods assume that a χ2 (chi-square) distribution is a reasonable approximation of the failure distribution over time. The examples provided in this application note use failures that exhibit an Arrhenius behavior. A higher MTBF value (or lower FIT value) corresponds to a more reliable product that experiences fewer problems or failures over the lifetime, resulting in lower repair costs and unplanned downtime.