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Reliability Goals

RELIABILITY GOALSThe target, objective, mission or goal is the statement that provides a design team with f ocus and direction. Awell stated goal will establish the business connection to the technical decisions, related to product durabilityexpectations. A well stated goal provides clarity across the organization and permits a common language f ordiscussing design, supply chain and manuf acturing decisions.

Let’s explore the def init ion of a ‘well stated reliability goal’. First, is it not simple MTBF, “as good as or betterthan…” or ‘a 5 year product’. These are common ‘goals’ f ound across many industries, yet none permit a cleartechnical understanding of the durability expectations f or the product.The common def init ion f or reliability is

Reliability is … the ability or capability of the product to perf orm the specif ied f unction in the designatedenvironment f or a minimum length of t ime or minimum number of cycles or events.(Ireson, Coombs et al. 1995)

Note this def init ion has f our elements:• Function• Environment• Duration• Probability

Function

The f unction is what the product is to do or perf orm. For example, an emergency room ventilator is to provideassisted breathing f or a person. This requires the ventilator to produce breathable air within a range ofpressures, within a prescribed cycle of respiration. It may include requirements f or f iltering, temperature, andadjustments to pressure and timing of the cycle, etc. Of ten, a product development team either develops or isgiven a detailed set of f unctional requirements.

The f unctional elements of a product are of ten directly measurable. Further, the quality f unction of mostorganizations verif ies that the design and production units meet the f unctional requirements. When theproduct does not met the f unctional requirements it is considered a product f ailure. Within the f unctiondef init ion are the most important f unctions, which must not f ail, as well as the f unctions that upon f ailure maysimply degrade perf ormance, if noticed by the customer at all.

Environment

The environment could be considered the weather around the product when in use. ‘Weather ’ includes suchthings as temperature, humidity, UV radiation intensity, etc. It should also include environmental f actors thatprovide destructive stresses, such as vibration, moisture, corrosive gases, voltage transients, and more.

Another element of the environment is the use of the product. What is the use prof ile? Maybe once a day f or af ew minutes, like a remote control f or the stereo system. Or perhaps its a 24/7 operation, such as f or a serversystem processing transactions f or a major online store. The prof ile may include details concerning humaninteractions, operating modes, shipping, storage, and installation. The environmental conditions need to detailhow the product responds or degrades to the set of stresses the product encounters. The environmentalconditions f ocus on the drivers f or the product’s most likely f ailure mechanisms.

Duration

The duration is the amount of t ime or number of cycles the product is expected to f unction. A computer printermay be expected to print f or f ive years. A washing machine is expected to wash clothes f or 10 years. Animplanted hearing aid is expected to last the lif e of the patient; if the patient is a child this expectation may bemore than 70 years.

The duration expectations may be def ined by contract, market expectations, or by a business decision. Theduration or lif e expectancy most likely is not the warranty period. For example, many personal computers havea 3 month or 1 year warranty period. Yet, the product is expected to last at least two years or more with normaluse.

Many products have multiple durations that are of interest.• Out of box• Warranty• Design Lif e

The init ial, out-of -box, or installation period is that duration when the customer is f irst setting up and usingthe product. Brand visibility is at its highest and the expectation that a new product will f unction as expected isvery high. The types of f ailures that may occur include installation or conf iguration errors, mistaken purchase,shipping or installation damage, or simply buyer error. All of these ‘f ailures’ cost the company producing theproduct resources.

The warranty period is the duration associated with the producer ’s promise to provide a product f ree ofdef ects f or a stated period of t ime. For example a computer may have a 1-year warranty period. During thisone year, if the product f ails (usually limited to normal use and operating environment) the producer will repairor replace the product. Naturally this will cost the producer resources.

The design lif e is the business or market expected product duration of f unction use. Af ter the warranty periodthere isn’t an expectation f or the producer to replace or repair the product, yet the customer may have areasonable expectation that the product will f unction satisf actorily over the design lif e duration. For example,many cell phones have a 3-month warranty, yet as consumers we have an expectation that the phone willf unction f or two years or more.

Marketing or senior management may set the design lif e. They may want to establish a market posit ion f or theproduct related to reliability. One way is to design a very robust product with a long design lif e duration. HPcalculators of ten have only a 3-month or 1-year warranty, yet many have lasted 10 or more years. Thesecalculators are known f or their robustness and of ten cost more to purchase – a reliability premium.

Each of the three durations of ten involves dif f erent risks related to the f ailure mechanisms. It is rare f orbearings to wear out in the f irst 30 days, yet more likely f or a 10-year design lif e. Establishing three or moredurations within the product reliability goal permits the design team to f ocus on and address the f ull range ofproduct reliability risks.

Probability

The probability is the likelihood of the product to survive over a specif ied period of t ime. In the f ormaldef init ion of reliability above, the phrase ‘ability or capability’ ref ers to the probability. This is the statistical partof the reliability goal and without it the goal is f airly meaningless. Furthermore, stating a probability without anassociated duration and distribution is also meaningless in most cases.

What is the chance that a particular product will f unction as expected over the entire expected design lif e? Howmany of the installed units will be f unctional over the warranty period? Since each product and the associatedenvironmental stresses vary, the use of statistics is unavoidable in describing product reliability. Even thedef init ion of a product f ailure may vary by customer.

While there are many common terms to convey the probability of survival, the use of a percentage surviving isthe easiest understood and most easily applied across an organization. Stating that 95% of units are expectedto survive over the 5-year design lif e, means 95 out of 100 units will f unction properly over that 5-year period.A similar statement is that not more than 5% of products f ail over the f ull f ive years. Or, may be stated as notmore than a 1% f ailure rate per year.A common probability statement is the inverse of the f ailure rate, or MTBF. The 95% reliability over 5 years (t)becomes approximately 100 years MTBF (θ). This does not mean the product will last 100 years, it does meanthat 95% of the products are expected to last 5 years.

Finally, stating a separate f ailure probability f or each duration of interest provides a set ofduration/probability couplets that permit dif f erent f ocus f or early or out of box f ailure risks, versus the longerterm f ailure risks.

Sometimes the product has a specif ic mission time, like an aircraf t with an expected 12-hour mission over a20-year serviceable lif e period. The probability of success f or the 12-hour mission time is maybe set relativelyhigh. It may also have a conditional probability, considering the number of missions since the last major service.Some products have availability goals and undergo routine maintenance or repair. These products and manycomplex systems require additional complexity in their goal setting. For the purpose of this discussion, we areconsidering simple products that are not normally repaired, or products where the main interest is in the time tothe f irst f ailure.

The point is that setting the reliability goal f or a product is not as simple as stating a ‘f ive year lif e’ – it requiresa clear statement with suf f icient detail of each of the f our elements: f unction, environment, duration, andprobability. It should also typically include at least three duration/probability couplets. The goal establishes thedirection or target f or the entire design, supply chain and manuf acturing team.