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Reliability Estimation (MTTF/MTBF)


Does your electrical product meet reliability requirements?

Reliability defines the probability that a device will perform its required function under stated conditions for a specific period. Reliability is a criterion of the equipment’s failures as a function of time. It has an impact on maintenance, repair, and service costs. Performing reliability prediction at the beginning of the design leads to a high-quality product.

Reliability prediction

In a competitive market, having higher reliability products than competitors is one of the keys to success. 

The objective of reliability prediction is not limited to MTBF prediction. It can also:

  • Identifying potential design weaknesses

  • Evaluating the feasibility of a design

  • Comparing different designs and life-cycle costs

  • Providing models for system reliability/availability analysis

  • Establishing goals for reliability tests

  • Shall be used by manufacturers to set reliability test procedures.


Failure rate (F.R.) 

MTBF Bathtub

Figure 1 - Bathtub curve

The failure rate is a function that describes the number of failures expected over a given time. 

F.R. is not constant throughout the life cycle of electronic equipment. This function follows a bathtub curve divided into three periods (Figure 1)

1. Infant Mortality: The F.R. is high but decreases rapidly. The methods assume that this period is the first year hours of operation.

2. Steady-state: This is the period of a constant failure rate.

The MIL-HDBK-217F assumes failure rates as continuous. In contrast, Telcordia SR-332 considers the failure of an electronic component in the period following its first year of operation (8760 hours).

3. Wearout: F.R. increases rapidly. Wearout does not occur during the

the service life of an electronic device, which is about 20 years.

For this reason, Telcordia does not take into account this period.

Factors affecting F.R.

  • Operating requirements 

  • Electric stress

  • Temperature

  • Environment

  • Quality of components

Prediction methods 

  • MIL-HDBK-217 Predictive Method

  • Telcordia SR-332 Predictive Method

  • NPRD (Nonelectronic Part Reliability Data)

Analysis for Prediction

  • Mean Time Between Failure (MTBF) is a term used to provide the average number of failures per million hours for a reparable product. 

  • Mean Time To Failure (MTTF) is the average time expected until the first failure of a piece of a product or equipment. 

  • Mean Time To Repair (MTTR) refers to the average time required to repair a defective product. 

RMA -  Reliability, Maintainability, and Availability

Reliability (R) is the probability that a component or system will perform its intended function without failure during a given period when it is under a specific operating environment.


Maintainability (M) is the possibility that a defective item is replaced or repaired within a given time. Maintainability is an integrated parameter when designing the equipment.


Availability (A) is the likelihood that a repairable system will perform its intended function for a specified time when used in an assigned manner. Therefore, availability is a function of reliability and maintainability.

If "R" is satisfactory, then no need for "M" and "A"; if "R" is not enough, then "R" and "M" are needed. 

Availability is simply the total uptime/total time (uptime+downtime). It is the hours of operation of a device.


Two rules to estimating reliability: 

  1. Calculating the MTBF: is the Total Time in Operation / Number of Failures. 

  2. Computing the Failure Rate: how often a device fails in time. It is the number of failures / total time in operation. 


A high failure rate is indicative of low MTBF

The main achievement of reliability is to assess the success of a product to establish potential failures as early as the product's life cycle and then take appropriate action to make the necessary improvements. 

It is never too late to enhance the reliability of a product. Remember: modifications are less costly at the start of design than later when the product is in production.


Difference Between Quality and Reliability

In manufacturing, quality is a measure of excellence of being free from defects, deficiencies, and significant variations.

A product may have a reliable design, but its reliability is not satisfactory when operating. Perhaps the reason is the result of an inefficient manufacturing process. The failure did not occur due to incorrect design, but its quality is unsuitable due to the assembly method.

How ExpertEMC can help

  • Offer support in the development of your product

  • Review electrical component quality 

  • Estimate reliability for the mechanic assembly

  • Product improvement from a reliability viewpoint

  • Create a reliability test plan 

  • Deliver a complete reliability report

Other domains of expertise:


For Reliability Estimation support, contact ExpertEMC at

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