Most products are tested after assembly under factory conditions. In what ways would you expect conditions in the real world to be substantially different? Think about a range of familiar products such as a mobile phone, a personal computer, a car engine management system and the radar in a military aircraft. Which of these environments might be expected to be the most severe?
Compare your answer with the comments that follow.
Factory conditions are deliberately as consistent as possible, not just to make it more comfortable for the workers, but also to enable the products produced to be tested under known conditions, and thus compared accurately against each other. It is unusual for any but the most rugged military products to be fully tested over the range of conditions that they will experience in the real world. The ‘real world’ consists of extreme temperatures and changes in temperature, a range of mechanical hazards, and contamination from the environment.
Taking these in turn:
Typically equipment that is work- or office-based has an ‘easier life’ than portable equipment, and both enjoy a relatively benign experience compared with military and automotive applications. Arguably the hardest life of all is that of a piece of equipment that exists in a car engine compartment – the so-called ‘under-hood’ electronics.
In Table 1, we have tried to compare the environmental conditions experienced by various types of equipment. Each is rated on an arbitrary 1–10 scale, representing a range of impacts from minimal to extreme.
|environmental condition||mobile phone||personal computer||car engine management system||military aircraft radar|
|acceleration; vibration; bump; shock||5||2||9||10|
|dust; mould growth||4||3||9||10|
Classified on a scale of 1= benign to 10 = severe
Obviously the scale is relatively arbitrary, but it is worth noting that some level of hazard can be associated with most items of equipment.
In some cases, the conditions will depend on the user – you may always be careful not to drop your mobile phone, but there are others who do, and in consequence many failed phones! Also, although nothing more is said about this aspect during this module, one has to consider the electromagnetic environment. In other parts of this module, we have mostly considered the EMC aspect, but one of the reasons that a military aircraft radar has a more severe classification is that it needs to withstand the high electromagnetic pulse from a nuclear event!
It becomes very important to know the detail of the ‘real world’ in which your equipment will be operating, in order to make informed choices for technology and construction. Challenges in our real world also include:
In the real world there are also differences between normal and abnormal operation. For example, with industrial computers and motor drives, it is not unusual for equipment to be specified to operate at a high dissipation for restricted periods.
Finally, in the real world, one needs to make provision for likely accidents. You may decide not to have a coffee-proof keyboard for your workstation, but an equivalent function in an industrial environment may need a different input mechanism, in order to guard against operators with gloves dripping unknown fluids!
In this short section we are not attempting to list all the possible ways in which failures can be induced by the affects of the ‘real world’, but just to give some insight into both the likely and unlikely expected failures, and into the way that one problem can lead to another – failure modes rarely exist in isolation.
High temperatures can cause damage or incorrect operation in both electronic and electrical components. Whilst some types of component exhibits higher resistance as the temperature increases, most insulating components show an increase in leakage current: good examples of this are diodes and capacitors. Fortunately, leakage current is rarely a cause of complete device destruction, but malfunctions can occur, particularly in high-impedance circuits.
Given a sufficiently high temperature of course, many of the materials used will degrade: thermoset polymers will char; thermoplastics will distort; solders will creep and then melt. This can be significant in the case of heat-generating components such as inductors, transformers, solenoids and motors. Here high temperature, combined with internal dissipation, can degrade and char the insulation on internal wiring, with insulation failure leading to permanent short-circuit damage.
Low temperatures can also cause components to fail due to parametric changes in electrical characteristics. However, such failures are usually reversible, and correct function is restored when the temperature rises.
One consequential failure of low temperatures that has already been hinted at is the tendency for cold components to attract moisture, frozen out of the surrounding atmosphere. Not only does this have the potential to give rise to corrosion, which only needs some ionic contamination to proceed quite rapidly, but the moisture can dramatically reduce insulation resistance across the tracks.
Low temperature also makes most polymeric materials more rigid, and care must be taken during operation at temperature extremes to ensure that embrittled parts are not shattered.
Arcing may occur whenever current-carrying components are separated, as when switches or relays are operated, and is often seen between the brushes and commutators of motors and generators. Apart from generating electromagnetic noise, arcing also progressively damages the contact surfaces, leading to eventual failure. Arcing becomes more likely, and is more difficult to suppress1, at low air pressures, since the dielectric strength of air is proportional to the pressure. That is one reason why aircraft and spacecraft electrical systems operate at relatively low voltage levels.
1 Attempts are usually made to reduce arcing by using voltage suppression components, such as capacitors and diodes, across relay or switch contacts.
For most electronic products, the most severe environmental challenges result from high humidity. Not only is humidity associated with a number of failure mechanisms, it is also essentially uncontrolled. Whilst warm air usually has low humidity (less than 50% RH), cold air may have a very wide range of humidities, from very dry (as in Arctic conditions) to very damp (“precipitation in sight” as the shipping forecast puts it!). Most severe of all is the hot and damp combination found in the tropical jungle.
Keys to the effect of high humidity are:
Laboratory tests use pure water, but the real environment often contains contaminants. For this reason, the battery of tests available includes exposure to salt spray and a range of lubricants and other fluids associated with vehicles.
We have seen already that the temperature cycling of joints leads to eventual failure through stress. For complete assemblies, there are a number of other possible failure modes associated with the incompatibility of materials. Sometimes you will just hear creaks and groans; at other times fracture and failure may occur. In the laboratory, temperature cycling is always carried out so that the final half-cycle takes the product from hot to room temperature. This is to avoid the situation where a product allowed to regain room temperature from cold might attract condensation, which could adversely impact on performance. Of course, real life is not so kind!
And there are many other hazards. We tend to forget insulators, although they are important in cables, connectors, and coils, as well as on a circuit board. Insulators can degrade and fail, usually in the long term, due to mechanical damage, absorption of humidity or excessive temperature. The last of these will cause embrittlement and then fracture, but the same effect is produced by sunlight or chemicals.
Most of these aspects are covered by standardised tests, but the cost of assessing every aspect of a product is substantial. And, however careful you are, there will be failure causes that are difficult to predict – some insulation materials are enjoyed by rodents which may infest agricultural machinery or military equipment which provides snug winter quarters!
We have seen that the real world contains many hazards for the poorly-designed circuit, and a range of tests has been devised to emulate the life experiences of a product. The key question which a designer has to face is which tests to use, and at what ‘severities’ – typically specifications describe how the test is to be carried out, and suggest a number of standard conditions (severities) which should be chosen. This standardisation helps control the proliferation of different standards.
First define the requirement! Traditional specifications described the worst case ‘envelope’ of conditions which embraced all the possible conditions under which a product might be expected either to be stored or work. The designer was then left to determine an appropriate test regime. An attempt has, however, been made to classify environmental conditions, and communicate these in ways that are meaningful to the designer, and also to generate guidance documents which relate the environmental requirements to standard environmental tests.
One classification of environmental conditions is given in BS EN 60721. This identifies many classes of application that are given a three-character code (Table 2), with a digit defining the application, a letter indicating the environmental challenge, and a final digit indicating the ‘severity’, where a higher value normally indicates more stringent conditions. For example, class 7K3 is for portable and non-stationary use under Climatic Conditions, Severity 3. The various parts of the publication contain tables with the severity of each environmental parameter for each class.
|1st (digit) – Application||2nd (letter) – Environment|
|3||Stationary use, weather-protected||C||chemically active substances|
|4||Stationary use, not weather-protected||S||mechanically active substances|
|5||Ground vehicle installations||M||mechanically active conditions|
|7||Portable and non-stationary use|
The severities given are those which are exceeded either for an insignificant part of the continuous exposure time (for example, temperature conditions), or for an insignificant fraction of the total number of events (for example, shocks). Thus the classes define the maximum short term environmental stresses, but do not give information on the long term stress for a component. This is, however, illustrated in Figure 1. The severities given in the classification are represented by the one value x1, whilst the totality of environmental stresses during the product lifetime includes the integral of the curve for all values of x.
The probability of exceeding the classification severity is low, and the probability of exceeding both maxima simultaneously, correspondingly lower. However, parameters such as sun radiation and high temperature are statistically dependent, unlike vibration and temperature.
BS EN 60721 points out that these extreme environmental conditions may occur at any time in the product life, and a product which survives when new may fail when subjected to the same conditions after it has undergone slow degradation with life.
The standard also explains that the conditions may affect the product when it is either operating or non-operating, or perhaps both. It is important therefore to define whether the product needs to be capable of operating under the extreme conditions, or is only required to survive without permanent damage.
These environmental classes are used as a basis for the choice of design and test levels, but that does not mean that the limits should be used for test or design, or that zero failure should be required or expected at the limits. Care must be taken to choose appropriate conditions having regard to the acceptable risk of failure – a higher or lower severity may be chosen depending on the expected consequences of failure.
“It should be noted that an over-design of a product, in order to withstand environmental conditions more extreme than necessary, does not necessarily result in higher reliability. An over-design, or unnecessary built-in protection, may lead to a more complex product with an increased number of failure modes.”
BS EN 60721
Related to the severity classifications in BS EN 60721 is a set of guidance documents with titles of the form “IEC TR 60721: Guidance for the correlation and transformation of environmental condition classes of IEC 60721 to the environmental tests of IEC 60068”.
While the documents make it clear that they have no mandatory status, and are intended only for guidance, in practice they are extremely helpful in suggesting levels of test that are appropriate for assessing products for conformance. They also contain the useful idea of a ‘climatogram’. This shows temperature against relative humidity and plots the area under which a product may be expected to operate.
Two examples for different environmental classifications are given in Figure 2. Note that the second of these (Class 7K3) has a very complete definition of the locations at which it applies, and be aware that, if you use these documents, you will need to familiarise yourselves with its detail and jargon.
Class 1K2: use at temperature-controlled enclosed locations, humidity not controlled
Class 7K3: use at totally or partially weather protected locations in areas with Warm Temperate, Warm Dry, Mild Warm Dry, Extremely Warm Dry, Warm Damp types of climates and at non-weather protected locations covered by the Restricted group of open air climates
For testing products against the conditions of the climatogram, only three tests are normally used:
These tests are indicated on the climatogram. Other boundary conditions of the climatogram are not required to be tested and there are no IEC 60068 tests available.
Obviously more than just these three tests may be needed, and the
tables in the guidance document indicate suitable tests. More helpfully,
they suggest whether or not tests are required, and whether different
severities might be appropriate.
It is clear from the information that, whilst guidance is available, the responsibility lies with the designer to ensure that the selection of test severities is appropriate for the application, critically depending on the effects of failure.
“Selection of test severities depends upon the failure consequences of the product. Two types of product may be placed at locations covered by the same environmental class. However, one type of product may be tested under significantly more severe conditions than the other because of its different failure consequences. This report only addresses normal failure consequences; for higher failure consequences, the test severity may need to be increased on the basis of specialist knowledge of the product.”
IEC TR 60721
The tests themselves have a long history within the British Standards systems as BS2011, and as IEC Publication 68. Over the years they have been adapted, extended and harmonised, and contain a battery of environmental test procedures and appropriate test severities that make available to the specifier standard tests covering most eventualities.
The generic document is now called “BS EN 60068-1:1995, IEC 60068-1:1988 Environmental testing. General and guidance”. This document is an introduction, which explains how the system works. The detailed tests themselves comprise Part 2 of what has become a large, loose-leaf publication. The contents of this are indicated in Figure 3 – for each of the families of tests, there are one or more detailed specifications and guidance documents.
The families of tests in BS EN 60068 are designated by the following upper case letters:
|A||Cold||L||Dust and sand|
|B||Dry heat||M||Air pressure (high or low)|
|C||Damp heat (steady state)||N||Change of temperature|
|D||Damp heat (cyclic)||Q||Sealing (including panel sealing, container sealing and protection against ingress and leakage of fluid)|
|E||Impact (for example shock and bump)|
|F||Vibration||R||Water (for example rain, dripping water)|
|G||Acceleration (steady state)||S||Radiation (for example solar, but excluding electromagnetic)|
|J||Mould growth||T||Soldering (including resistance to heat from soldering)|
|K||Corrosive atmospheres (for example salt mist)||U||Robustness of terminations (of components)|
The letter X is used as a prefix together with a second upper case letter to allow the list of test families to be extended, for example, Test XA: Immersion in cleaning solvents.
Some families of tests are subdivided, by adding a (lower case) second letter, for example:
U: Robustness of terminations and integral mounting devices
Test Ua: Subdivided as Test Ua1: Tensile and Test Ua2: Thrust
Test Ub: Bending
Test Uc: Torsion
Test Ud: Torque
The letter Z followed by a solidus and a group of upper case letters denotes combined and composite tests. For example, Test Z/AM combines cold and low air pressure.
The basic document itself deals with some important concepts:
The specification also suggests a standard sequence of climatic tests, where the test components are independent. This ‘climatic sequence’ is carried out in a defined order:
This sequence is just one of the ways in which attempts have been made to reduce the very large number of possible combinations of tests and severities. Another is the use of a ‘component climatic category’ which indicates generally the conditions for which components2 are suitable. The category is indicated by a series of three groups of digits separated by strokes, corresponding respectively to the temperatures of the cold test and the dry heat test, and the number of days of exposure to steady state damp heat that the components will withstand. As an example, a component classified 25/085/04 would meet the requirements after at least a –25°C cold storage test, a +85°C dry heat test, and 4 days damp heat (steady state) storage.
2 The climatic categories referred to are used primarily for components, but BS EN 60068 applies to the complete range of what it calls ‘electrotechnical products’, and suggests it can also be applied in other fields.
How well do the tests in BS EN 60068 emulate the real world?
Compare your answer with the comments that follow.
But does BS EN 60068 cover everything you need to test a product? You will already be familiar with the UL test for flammability of materials, but BS EN 60068 specifically excludes fire hazards. The guidance document referred to in BS 6221-3, the guide for design and use of printed wiring boards, actually makes reference to guidance in BS EN 60695. However, this guidance is expressed in very general terms, although supported by test methods 3.
3 Interestingly, the IEC standard includes a ‘glowing wire’ test as well as the familiar needle flame test, and in many ways this covers more fully the possible causes of ignition within a system. However, technically superior though it may be, the fact is that UL are the main drivers in this area.
Another omission from the specification is that of ingress protection, assessing the ability of an electronic enclosure to resist people, dust and water. This is covered later in this document.
You also may have noticed that the damp heat test suggested for Class 7K3 is less severe than the maximum relative humidity to which the assembly may be exposed. This is because the tests have not only been standardised, they have been aimed at what is achievable rather than fully attempting to simulate the real world. With moisture, this is due to the fact that real-life climatic changes include temperature fluctuations, and these will result in precipitation of moisture unless the humidity is maintained slightly below 100% RH.
Be aware that not all the tests which you will come across are intentionally realistic; many of them have extreme severities in order to impart stresses to the component that will accelerate failure. Two examples are with constant acceleration, where often semiconductor packages tested at values of g which are impracticable for equipment – if you apply 20,000g, the aim is to try to pull off the wire bonds without having to resort to pull test. Similar, but less dramatic, is the replacement of slow natural cycling by the temperature rapid change test, where a product is transferred between cold and hot chambers, and sometimes even plunged alternatively into hot and cold liquid.
Probably the key criticisms of any approach involving standard tests is that:
There will be more on these topics in Reliability in Screening. It is worth making the point, however, that having standardised tests available is very helpful when it comes to being able to make comparisons between different studies.
Observers elsewhere have noted a general trend from company and national specification towards industry and international specifications, and nowhere is this seen more than in the automotive industry. In fact, BS EN 60721-3-5, whose object is to classify “the environmental parameters and severities to which a product will be exposed when installed in ground vehicles”, specifically excludes products which form part of the vehicle.
If you are interested in learning more about automotive issues, visit the sites of the Society of Automotive Engineers at http://www.sae.org/ and the Automotive Industry Action Group at http://www.aiag.org/
As well as having independent specifications on environmental and other tests, the automotive industry also has its own (tighter) quality management standards. For example, ISO/TS 16949, which combines ISO 9001:2000 with automotive-specific requirements from the American QS-9000 and German, French and Italian quality standards. For a useful introduction to this topic visit the Perry Johnson site at http://www.pji.com/iso_standards.htm.
We have already mentioned that aspects of enclosure protection are dealt with in BS EN 60529 (formerly IEC 629), and you will come across ‘IP’ numbers when you buy enclosures. In this context, IP now stands for ‘International Protection’ rather than ‘Ingress Protection’ and the number 4 usually consists of IP plus two digits. The first of these digits refers to the protection given against intrusion by solid objects (and of course by people), the second to the protection given against liquids. Generally, the higher the number, the better the protection given. A summary of the codes is contained in Table 3 – occasionally supplementary letters will also appear.
4 A third number that is commonly omitted refers to protection against mechanical impacts.
|First number||Second number|
|0||No protection||No protection|
|1||Protected against solid foreign objects up to 50 mm diameter (back of hand)||Protected against vertically dripping water e.g. condensation|
|2||Protected against solid foreign objects up to 12.5 mm diameter (finger)||Protected against dripping water up to 15° from vertical|
|3||Protected against solid foreign objects up to 2.5 mm diameter (tool)||Protected against direct spray of water (0.07 l/m) up to 60° from vertical|
|4||Protected against solid foreign objects up to 1 mm diameter (wire)||Protected against direct spray of water (0.07 l/m) from all directions|
|5||Dust-protected (limited ingress allowed, with no harmful deposit)||Protected against low pressure jet (12.5 l/s) of water from all directions|
|6||Dust-tight (totally protected against dust)||Protected against high pressure jet (100 l/s) of water|
|7||Protected against the effect of temporary immersion between 15cm and 1m|
|8||Protected against long periods of immersion under pressure|
In terms of affording protection against water, some penetration of water is generally permitted, although this may be varied by the appropriate technical committee. In general, if any water has entered the enclosure, it shall not:
It is not our purpose in this document to discuss the various ways in which system level performance can be enhanced by strategies such as redundancy. However, we do have to consider the board level issue, which concentrates on the assembled components. When we carry out a design, our basic philosophy should be to:
Likely failure modes
Table 4 and Table 5 summarise the main failure modes for the most common component types. The failure modes listed are not exhaustive and the device types listed are only a summary of the range. The failure mode proportions can vary considerably depending on type within the generic headings, application, rating and source 5. Devices used within a particular design should also be individually assessed: for example a resistor rated very conservatively is likely to have a reduced relative chance of failing open-circuit.
5 This information is identical to that published in the previous edition in 1993, and must therefore be regarded as indicative, rather than presenting current data. The only problem is that no later information is readily available in the public domain. Part of the reason for this is the enormous reduction in the amount of qualification testing carried out in recent years, with the trend for military users to purchase commercial parts, rather than specially-qualified types.
|Type||Main failure modes||Typical approximate proportions (per cent)|
|Digital logic||Output stuck at high or low||80|
|Hard over output||10|
|High leakage collector-base||30|
|Rectifier and general purpose diodes||Short circuit||10|
|High reverse current||70|
|Type||Main failure modes||Typical approximate proportions (per cent)|
Circuit design should take account of the likely failure modes whenever practicable. As examples:
In all cases, the extent to which protection against failure can be afforded and is implemented will depend on the likelihood of and the consequences of failure.
Where components are likely to fail, and a simple Pareto analysis will quickly identify the most vulnerable components, we recommend taking specific steps to reduce the failure rate of these components. The possibilities for this, which will be discussed in the next sections, include the derating of components, and the screening of any high risk devices to remove components that are potential defects.
Explain to one of your colleagues how he/she might approach the design of a high-reliability communication product and the subsequent verification that it will survive its intended hand-carried military application.
Compare your answer with this one.