IC design and reliability: Failure rate and the FIT

The failure rate for ICs is defined by: (Number of devices failed/Total number of devices tested)(1/time). The units can be stated as failures per device hour. This is an important parameter and has to be considered as early as possible in the design of the device. A unit called a FIT can also be defined. In this case a FIT = one failure per 1 billion device hours of operation. A FIT can be used to evaluate devices and distributions. The simplest model of failure is given by the parameter called the mean time to failure or MTTF. MTTF = 1/KF. Here KF is simply the failure rate per unit time. This is used in the exponential model which is very popular. The exponential model is simply expressed as: Fail(t) = 1.0 - exp(-KFt). This model is easy to use and calculate and can be used to assess the design of the device based on a failure rate model. Data is usually available from foundries for the use of this model. Please visit our website at http://www.signalpro.biz for more ASIC and module design and manufacturing information. Please contact SPG for detailed information on more extended types of failure expressions and their use in the design of devices. i.e how these failure parameters mesh with design parameters that allow the design to be more robust in terms of MTTF or failure rate.