Design for Product Build

Unit 3: Components

Components are important because they produce the circuit functionality – everything else we do is involved with interconnecting, protecting and interfacing the components. So in this Unit we are looking at how representative components are made and presented to the assembler. We are also considering how components fail, so that we can design and manufacture our products so as to minimise the failure rate.

This is a unit which could be a complete book in itself, but we have resisted the temptation! To keep it to a reasonable size, we have restricted ourselves to a brief survey of the main types of component. Some of these are described in the main text; other are explained in more detail in subsidiary texts. As with all the units, it’s worth exploring the links.

Unit contents

Introduction

The important considerations about any component are what it is made of, how it is made and how it is presented for assembly. With a manufacturing hat on, our perspective will be different to that of electronic design engineer, who is chiefly interested in the function of a component - a resistor, a capacitor, or other passive device, or an active device such as a transistor - and will initially select components by their function in his/her circuit design, and consult data sheets and books to select the component type (for example, a high power or a low noise device) which best meets his/her requirements.

Ideally, the design engineer should then consider parameters such as physical size, pin configuration and power dissipation, to ensure that the design is acceptable in terms of board layout, manufacturability, and reliability. However, this is often a secondary issue, whereas to others involved in the manufacturing process, the physical size, package style, lead finish and ruggedness are of primary importance in evaluating manufacturability and throughput, and the function of the component is not a major concern.

So the way in which a part is presented for assembly, and the range of acceptable methods available for connecting it to a substrate or system, are critical to the assembler, and these features of the component will depend on its technology. What form does the active element (or elements) take? How are internal connections made? What is the final encapsulation material? Answers to these questions dictate the format of the part, and the format determines the range of processes available and the way in which the part is presented for assembly.

Recommendation

As in the course of your work or travels you come across electronic products, always take a good look at the components. You will find that there are many different types, and frequently styles that you have not seen before, however long you have been in the game! If you come across a new component, why not examine its construction, ask yourself how it was made, and think about how best to use it whilst minimising its chance of failure.

 

The basic materials used within components are many and various, as are the manufacturing methods employed. You will, however, be able to detect a number of the “enabling processes” that you studied in Unit 2. But our concentration in this Unit is on the part produced, and the influence that the manufacturing method will have on the usability and reliability of the component. Whilst most components are adequately reliable, there are usually some aspects of quality and reliability that are intrinsic to the manufacturing method, and peculiar to the device. You will come across a particularly good example in relation to the brittleness of the materials used for chip ceramic capacitors, but most components have some “quirks”.

Supplementary information

Particularly if you are interested in the way components are made, a very user-friendly (if not over-technical) account is given in the first few chapters of Neil Sclater's Electronics Technology Handbook.

 

Because of the variety of function, type, and physical format, it has proved useful to identify and classify electronic components and devices in a number of ways, different classifications being useful to different people. Three particular ways that parts are classified are:

The first of these not only affects circuit function, but impacts on the technology. Most active components are nowadays silicon-based, though other semiconducting materials are also employed for specialist applications. For passive components, on the other hand, there is an enormous range of technologies and structures, which is why much of this unit is devoted to their description.

Whilst the second classification is normally applied to active components, integrating a number of individual parts into a common envelope has distinct advantages to the assembler. And the term “integrated” may describe modules that combine elements made of different technologies. You will see several examples in the final units of this module.

Supplementary information

There is a more extended discussion of this topic in Classes of component.

 

Our final classification of components into through-hole and surface mount types slightly muddies over the fact that component terminations have varied enormously throughout the past century, moving from terminals through wires to thin tabs, with certain types ending up with no lead at all, and only an area of metallisation to which contact can be made. For convenience, however, we generally draw the simple distinction between through-hole and surface mount, because this reflects the nature of the assembly process.

Supplementary information

Take a look at A component review for an insight into the transition into surface mount that has taken place over the past 20 years.

 

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Semiconductors

With semiconductors, there is a great deal to know even about the assembly and encapsulation processes, let alone the many (often several hundred) process steps needed to make a wafer. But it is important that you are familiar with the range of ‘back end’ processes, and understand at least something of the terminology and what is involved in turning a ‘chip’ into a finished package. This is because the processes determine the limitations ofn the finished part, and devising new styles of semiconductor package involves making modifications to the basic processes. For example, making a “thin” package requires changes to the wafer back-grinding process, to the lead frame material and dimensions, and to the bond profile, as well as changing the mould. And making a thin package has knock-on implications for its reliability and moisture sensitivity.

Our aim is to make you conversant with the terminology and some of the issues, which we suggest should start with the review in Semiconductor packages, as this gives an overview of the process and illustrates some of the range of packages. Note particularly that discrete components and integrated circuits may sometimes use different technologies for encapsulation – there are further possibilities for constructions when we look at power devices, but such considerations are beyond the scope of this module.

Figure 1 shows the process sequence followed by a typical semiconductor assembly house. The semiconductors are received from the foundry in the form of a probe-tested wafer, and the first operation is to reduce the wafer thickness by back-grinding.

Figure 1: The semiconductor ‘back-end’ process sequence

The semiconductor ‘back-end’ process sequence

 

For each of the process stages there are at least some options, depending on the application. You may already have had a flavour of this from our paper on Die bonding materials. Each of these processes is described in more detail at these links:

There are of course a number of alternatives to the processes that we describe, and some of these will be covered in Unit 10. Most of these processes are carried out by specialist sub-contract manufacturers, mostly in areas with a low labour cost. You should therefore take advantage of any opportunity given to see semiconductor assembly in real life, even though getting a close-up view of processes may be difficult because of the requirements for tight control of the assembly environment to avoid contaminating the work being processed – full clean-room procedures will be in place, and not just the anti-static precautions familiar to board assemblers.

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Resistors

Under this heading most of our description concerns the ubiquitous chip resistor, but this is just one of the ways in which the function can be performed. After all, any conductor has some resistance, and higher values are obtained by modifying the resistivity of the material, judging the cross-section of the current path, and selecting an appropriate length of resistive path, and frequently all three.

Exercise

Research the types of resistor available other than a chip resistor, referring first to your knowledge of electronic assemblies, and then using reference books or web research, before looking at our answer.

 

Most of the types of resistor that we have indicated in our answer are still available, and may be appropriate for particular applications. But each type needs to be assembled appropriately, and carries with it some limitations on use, being associated not only with a power rating, but also with a temperature coefficient of resistance (which may be highly variable), a voltage coefficient, a maximum voltage rating, and a likely stability on life. It is very important to choose an appropriate resistor for the application, especially when one is dealing with elevated power, tight precision or high reliability, as different technologies differ in their drift over life and their failure mechanisms.

Most chip resistors are of the so-called ‘thick film’ construction, where patterns of inks containing glass frit and a mix of metals and oxides are printed onto a ceramic substrate and converted to adherent, stable films by firing at high temperature (typically 850ºC).

The usual substrate is a high purity alumina3 sheet which is first laser-scribed in two directions at right angles to provide crack lines for breaking out the individual resistors at a later stage in production. The internal metal electrodes, usually of palladium-silver, but sometimes of gold, are printed across the appropriate cracks and fired on.

3 Alumina (Al2O3) is produced from the bauxite ore which is also the source of metallic aluminium. The crushed ore is digested with hot caustic under pressure, the solution filtered, and aluminium hydroxide precipitated by passing carbon dioxide through the solution. The resulting pure hydroxide is dried and calcined at 1100°C where it decomposes to give pure alumina.

 

Overlapping pits on a ceramic substrate form a laser scribe line

Overlapping pits on a ceramic substrate form a laser scribe line

The resistive element, normally based on ruthenium dioxide, is similarly printed between these electrodes and again fired. The resistive value as-fired is always lower than the target so that each resistor can be adjusted upwards to the precise value required. Computer controlled laser trimming is used to vaporise a narrow channel in the resistive element whilst the increasing resistance is being monitored. The central area is then glazed, to protect the resistive element with a glass film (Figure 1).

Early chip resistor shows concept: this example has been
adjusted by air-abrasion

Early chip resistor shows concept: this example has been adjusted by air-abrasion

Figure 1: Chip resistor construction

Figure 1: Chip resistor construction

A completed and coded chip resistor

A completed and coded chip resistor

Such resistors typically range from 1Ω to 10MΩ with a choice of tolerances in the range ±0.5% to ±20%. Because resistors are individually adjusted, the spread of values within a batch will normally be very much closer than the nominal tolerance, ±1% of mean value being typical of what is achieved with ±5% parts.

Power ratings over an operating range of approximately -40ºC to +70ºC depend on the resistor size

0402 and 0603 1/16W (63 mW)
0805 100 mW
1206 1/8W (125 mW) to ¼W (250 mW)

Different sizes and ratings of chip resistor

Different sizes and ratings of chip resistor

The temperature coefficient of this type of resistor is low, typically less than ±0.02%/ºC, making thick film resistors fairly stable components. However, for critical applications, precision chip resistors are available. These are manufactured using a different technology.

Resistor networks

Resistor networks, consisting of several thick-film or thin-film resistors on one substrate, are produced in a variety of package outlines. The earliest of these formats were DIL (Dual-In-Line) and SIL (Single-In-Line), but leadless chip styles have also been developed, and flat-pack styles are now very common. These are similar in concept to the SO-IC but frequently on a smaller pitch.

Leadless chip resistor network

Leadless chip resistor network

Whilst custom networks were once favoured, for most applications the networks contain identical resistors of one of the standard resistance values. The two most common arrays have either separate resistors, or resistors with one side of each connected to a common pin (Figure 2).

Figure 2: Standard configurations for resistor networks

Figure 2: Standard configurations for resistor networks

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Ceramic capacitors

The original style of ceramic capacitor consisted of a disk of barium titanate soldered on opposing faces and with leads soldered to the surface, creating a radial component, and this construction is still found in high-voltage applications. However, the comparatively thick dielectric and the small surface area combine to limit the available capacitance value.

In order to achieve a higher value, component manufacturers helped to devise a way of creating more area and thinner dielectric. Easy enough with a film dielectric that can be rolled, but not so easy with brittle ceramic! The approach taken was to create a number of very thin capacitance elements, and stack these in such a way that they are connected together in parallel. This is done by orientating alternate electrodes so that they make contact with opposite terminations, as shown in Figure 3. A very successful device that was first manufactured in the 1960s, this monolithic multilayer ceramic chip capacitor (MLC or “chip capacitor” for short) is now the predominant capacitor type. This is because of its format and its useful range of values.

In a typical process, thick film capacitor electrodes are screen printed onto sheets of doped barium titanate ceramic using an interleaved pattern. These sheets are stacked under pressure, dried, cut to size and sintered at a temperature around 1300°C. The electrodes must be of a metal with a melting point that is higher than the sintering temperature, and platinum (1774°C) or palladium (1552°C) are normally used.

Figure 3: The structure of the multilayer ceramic chip capacitor

Figure 3: The structure of the multilayer ceramic chip capacitor

Different sizes of chip capacitor

Different sizes of chip capacitor]

Contact is then made to the ends of the capacitative layers using metal terminations, commonly silver-palladium, which are applied by screening or dipping followed by firing, each making contact with one set of internal electrodes.

Several solderable termination options are available, for which there is a trade-off between cost and retention of solderability of the metallisation. The most usual final finish is a nickel barrier layer, followed by a solderable outer coating. The nickel prevents leaching of the silver into the molten solder during assembly.

The resultant capacitor is very rugged and the electrode system is totally enclosed and protected from the influence of the ambient atmosphere. Standard versions are able to withstand immersion in 250°C molten solder as well as high humidity, without the need for further encapsulation.

The temperature coefficient of a multilayer chip capacitor is determined by the type of ceramic used. The temperature coefficient of NPO capacitors is close to zero over the relevant temperature range, but for others it can be either positive (capacitance increasing with rising temperature) or negative (capacitance decreasing with rising temperature). The considerable difference between different dielectric types is indicated in Figure 4. Generally, the more capacitance that is crammed into a given volume, the less stable the value will be.

The formulations of ceramic capacitor materials vary very significantly, depending on the dielectric constant and stability required. One can see colour differences – NP0 types are likely to be near-white, X7R often light brown, and Z5U may verge towards purple – but don’t place any faith on this as a definitive indicator of likely value.

Different sizes and dielectrics which may have different
characteristics

Different sizes and dielectrics which may have different  characteristics

Figure 4: MLCs: different dielectrics

Figure 4: MLCs: different dielectrics

Figure 4 and Table 3 give information on the three most commonly used materials. Note that there are differences in detail between the EIA classifications developed in the USA, and the IEC designations, although the specifications are similar.

In using Table 3, also bear in mind that the maximum capacitance value in a given package depends on the voltage rating and on the materials and technology, so that capacitance value ranges will vary between manufacturers. Some makers also truncate the bottom end of general purpose capacitor ranges, preferring to supply the more stable material where there is an overlap between dielectric types.

Table 3: Electrical characteristics of representative chip capacitor dielectrics
EIA classification COG X7R Z5U/Y5V
IEC (BS/CECC) near equivalents
1B
2C1
2F4
Dielectric constant (K)
30-150
500-2000
>4000
Operating temperature range:
-55 to +1250C
-55 to +1250C
-25 to +850C
Capacitance range (50V rating)
0402 body
<1p–150p
100p–3n3
1n–15n
0603 body
<1p–470p
100p–10n
4n7–47n
0805 body
<1p–1n
220p–33n
10n–150n
1206 body
<1p–4n7
470p–100n
22n–470n
1210 body
1210 body
2n2–220n
33n–1µ
Tolerances available
1–10%
5–20%
50–100%
Typical rated voltage
100–200V
50–200V
25–100V

The capacitance value of a multilayer chip capacitor is a function of the number of layers, the area of each electrode, and the permittivity and thickness of the dielectric. All but the first of these are variables which depend on the manufacturing process, and inevitably are not constant. There is therefore a spread of values in any batch, the range depending on the nature of the manufacturing technology and the quality of its process control.

Close tolerance parts are generally selected from the ‘as-fired’ batch, and reflect this in their higher price. It is possible to reduce the value of a capacitor by drilling into the structure whilst measuring its value, and then filling the pit produced with a protective glass, but this is both expensive and of uncertain reliability.

Which ‘selection tolerances’ are appropriate will depend on the dielectric type and its inherent stability: NPO parts are available down to ±1% and X7R types to ±5%. Note however that, for low value capacitors, measurement uncertainties generally restrict the best available accuracy to ±0.25pF.

Self Assessment Questions

To the casual observer, there appears little difference between a chip resistor and a chip capacitor, especially if the parts are really tiny. How would you explain the essential differences from a materials point of view (that is, as distinct from the circuit function)?

go to solution

Explain to your purchasing manager why it is that close-tolerance chip capacitors are much more expensive (relative to loose-tolerance parts) than close-tolerance chip resistors.

go to solution

 

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Other capacitor types

Electrolytic capacitors

Capacitors with values greater than 1µF are frequently required so, since the early days of electronics, when capacitor dielectrics were air or glass, engineers have been trying new materials, in order to cram as much capacitance into the available space. The original round paper capacitors needed to be big to achieve this value; ceramic chip capacitors were typically of much lower values. The problem for any parallel-plate capacitor lies in the formula for its capacitance:

$C = \frac{{\varepsilon _0 \varepsilon _r A}}{d}$

where

A is the area of the two plates
e0 is the permittivity of vacuum
er is the dielectric constant of the material between the plates
d is the distance between the plates

The value is thus proportional to the area of the plates, inversely proportional to their separation, and proportional to the dielectric constant of the material between the plates. When we want to increase the value of a capacitor, we have three approaches: we can reduce the thickness of the dielectric, we can increase its dielectric constant, we can increase the effective area, or we can use a combination of all three.

Although both paper capacitors (still used for some AC applications) and ceramic capacitors have been improved, often by using radically new materials, there is still an unfilled requirement for large values of capacitance. Large values at relatively low voltage rating are used for decoupling purposes; large values at a wide range of voltages are used within power supplies for smoothing. In the latter application, the component carries substantial current, so that a low ESR (Effective Series Resistance) and ability to withstand internal heating are both important.

A number of metals, such as tantalum, aluminium, niobium, zirconium and zinc, can be coated with an oxide film by electrochemical means. By placing the metal in an appropriate solution and passing a current though the circuit, a thin layer of oxide forms on the anode. This oxide film is highly adherent, and its structure is compatible with that of the base metal, so that temperature change neither stresses it nor causes it to flake off – contrast the iron oxide on exposed iron. Just as important, the process is self-limiting, so that the current gradually reduces to zero for a particular forming voltage. What we have done is to produce an oxide that will ‘withstand’ the applied forming voltage, and will not grow and grow as rust does.

This oxide film can be extremely thin1 (typically less than 1µm) as well as having a acceptably high dielectric constant. This combination is a solution to our problem of making large capacitors, and it is the use of the electrolytic process to form the thin oxide dielectric that gives this type of capacitor its name of ‘electrolytic’.

1 BHS Aerovox quote the oxide thickness for a 25 volt working capacitor as being in the order of 45nm. Electrolytic capacitors offer good examples of the very high internal stresses to which components are often subjected, without the user being aware of them. The field strength inside an electrolytic capacitor can be as high as tens of MV.m−1!

 

All we have to do is to implement some way of getting maximum area into minimum volume. The methods are different, depending on which metal is being used as the basis for the capacitor.

The usual materials for electrolytic capacitors are tantalum or aluminium. Their oxides have dielectric constants which are relatively high (k = 10 for aluminium; k = 25 for tantalum) compared with paper and plastic film, but much lower than barium titanate. However, this is more than compensated for by their high dielectric strength and relative freedom from defects, which allows the layer to be very thin, even though the formation (‘forming’) voltage used is typically 3–4 times higher than the rated voltage.

The aluminium electrolytic capacitor is substantially cheaper and the more widely used. However, the tantalum capacitor has ‘higher volumetric efficiency’ (= crams more microfarads into a given volume) and has superior electrical characteristics – you will see many such capacitors in mobile phones. However, you are unlikely to see anything other than aluminium electrolytic capacitors used for power supplies.

The only downside for electrolytic capacitors is that their metal-to-metal oxide interface is a rectifying contact. That is, it is a good insulator only in one direction, and in the other it conducts electricity. That is why electrolytic capacitors are inherently polar, and the only way that non-polar electrolytic capacitors can be created is to use two oxide films back-to-back.

Quote

Effective area

The effective areas of electrolytic capacitors are surprisingly high. Consider for example a 4.7 µF tantalum electrolytic capacitor. The tantalum pentoxide dielectric has a dielectric constant of about 27, and a typical thickness of the oxide film is in the order of 0.2 µm. Therefore,

\[A = \frac{{Cdr}}{{\varepsilon _r \varepsilon _0 }} = \frac{{4.7 \times 10^{ - 6}  \times 0.2 \times 10^{ - 6} }}{{27 \times 8.855 \times 10^{ - 12} }} = {\rm{0}}{\rm{.004m}}^{\rm{2}}  = {\rm{40cm}}^{\rm{2}}\]

Thus a 3.5mm × 2.5mm × 5.5mm, 4.7µF tantalum capacitor is equivalent to a parallel-plate capacitor with a surface area of 40cm2.

Anton Kruger at ChipCenter: http://www.chipcenter.com

As you can see from the quotation from Anton Kruger, we need to cram an enormous amount of surface area into a small volume. Two different styles of construction are used for electrolytic capacitors, using aluminium and tantalum (occasionally niobium). They use different approaches: aluminium electrolytic capacitors start with sheet material, but modify the surface to increase the surface area; tantalum electrolytic capacitors create a sponge of tantalum with a very high internal area.

Supplementary Information

Not only are the approaches different for the two types of electrolytic capacitor, but the resulting formats are very different. Read Electrolytic capacitors for more details on these key components, and a guide to their use.

 

Of course, however much information is given, there will always be other factors to consider. For example, considerable work has been carried out in recent years in order to make surface mount aluminium electrolytic capacitors able to withstand the higher reflow temperatures used for lead-free soldering – some early designs had failed because of the internal over-pressure. Thankfully that problem is in the past, and aluminium electrolytics are deservedly popular because they combine high volume metric efficiency with comparatively low cost.

However, from a technical point of view, tantalum capacitors are often preferred, especially for applications such as mobile phones. The main problem has been a commercial one of lack of supply, particularly during parts of the business cycle where there is rapid expansion. This is not just a matter of manufacturing capacity, but reflects the fact that tantalum is a scarce commodity, with a particularly noticeable supply deficit in 1999–2000, and a corresponding rise in the raw material price. This is why work continues to develop equivalents using alternative materials, predominantly niobium.

Other dielectrics

Early capacitors used cellulose paper, suitably impregnated with oil or wax, and coated in wax; the polystyrene-based capacitors which came along a little later were almost as intolerant of random contact with soldering irons! Occasionally you will still find polystyrene capacitors, and types based on paper are still used in specialist applications.

For high voltage, small value capacitors, you will also find mica capacitors. Made of a natural material with a laminar structure which can be prised apart to form thin tough sheets, plates of mica with silver electrodes can be stacked and adjusted by abrading part of the electrode. Given the hand-crafting involved, it is not surprising that mica capacitors have always been expensive, but at least they produced high yields at close tolerance, a feat difficult to achieve with ceramic parts.

Following the development of improved polymer materials, the wound structure used in polystyrene capacitors was developed into a more reliable structure, usually either cast or potted in a protected box. Capacitors with cellulose paper dielectric were also improved by impregnating with resins. Figure 1 shows one construction which is typical of polymer/paper capacitors. The base is a thin, flexible film of a dielectric such as polyethylene terephthalate, typically with a dielectric constant of around 5. The electrodes may be of a separate foil (as shown), or produced by selectively coating the dielectric with a vacuum-deposited aluminium electrode – this is the cheaper and more common process.

Figure 1: Construction methods for wound polymer/foil capacitors

Figure 1: Construction methods for wound polymer/foil capacitors

Sheets of dielectric are interleaved with alternate contact areas on opposite sides, and the dielectric or dielectric/foil sandwich is wound, and ‘end-sprayed’ with metal to make contact with the entire length of the outside edge of each capacitor plate. Welded connections are then made to both end faces, and the windings cut and encapsulated.

A number of different dielectric materials (polypropylene/polycarbonate/polyester) are made into capacitors by similar methods, and combinations of paper and polypropylene are common for AC applications.

Most of the materials used have severe limitations to their ability to survive high temperatures, and are not often designed for reflow applications. However, given appropriate dielectric materials and encapsulants, capacitors can be wound into a flattened cylindrical shape, and moulded in chip format: capacitance values available are typically 10nF to 0.22µF at 50V, in 1812 packages.

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Review of capacitor types

The vast majority of capacitors used are multilayer ceramic chips or electrolytic types. However, other dielectrics may be chosen, for reasons either of performance or of cost (metallised plastic capacitors can offer a cheaper alternative to ceramic for some non-critical applications).

Figure 2 shows the areas of applicability of some of the major dielectric types – except for highly stable applications, the mica capacitor has been replaced by ceramic. There are evident needs for alternatives to ceramic for high value, high voltage capacitors of ratings such as those used for mains suppression.

Figure 2: Capacitance and value ranges for common dielectric types

Figure 2: Capacitance and value ranges for common dielectric types

Self Assessment Question

You need a reliable 1µF capacitor rated at 100V, ideally surface-mounted. What are the possible materials and constructions of components available, and how do they compare?

Compare your answer with this one

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Magnetic components

So far we have considered the construction of semiconductors, resistors and capacitors, and discovered considerable diversity. However, there is even more diversity in the remaining types of components!

In recent years the use of inductors has reduced, with many radio-frequency applications preferring direct digital synthesis, and power supplies using switching technologies that do not necessarily involve transformers. Nevertheless, such inductive components are still quite common, especially in applications such as filters and power supplies.

Inductive components cover a broad spectrum of values, but all involve a winding of some sort, and all but the lowest values have some type of magnetic core. Making a reliable magnetic component therefore involves both making terminations to the coil, and providing a satisfactory environment for the magnetic core. As we explain in Inductive components, a number of different magnetic materials are used, giving a very wide range of overall formats. Typically inductors need careful mounting, and the larger styles need additional mechanical anchorage.

Activity

Inductors and transformers are obvious uses for magnetic materials, but they are by no means the only ones. Draw up a list of all the electronic uses for magnetic materials that you can think of, and for each of the items put down what you know about the structure and function of the magnetic component involved.

Compare your answer with this one

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Glass components

If you have read the information on discrete semiconductors, you will already have come across one use of glass, as an encapsulant for the MELF diode. This makes innovative use of the larger shrinkage on cooling of molten glass relative to copper to create a permanent hermetic seal with an in-built compression force that ensures good contact between termination and active area.

Glass is a more generally useful sealing medium, because it forms good gas tight bonds to metals such as copper. This was discovered during the 19th century with the development of the electric lamp, and the technology was then adapted for the radio valve. Subsequently glass-to-metal seals have been used for a variety of components, from large oil-filled capacitors down to microcircuits.

Glass-to-metal seals

A glass-to-metal seal has three components, the lead, the housing, and the glass which fuses to form a leak-tight seal between the two. There are, however, two fundamentally different types of seal, with differing characteristics:

Radial-leaded crystal with matched seals

Radial-leaded crystal with matched seals

Matched seals are preferred for larger structures, such as where a row of leads enters a package, each through its individual hole and tubular glass seal. Because no stresses are involved, the housing can be thinner, and the seals closer together.

Both types of seal are made in a similar way, carefully assembling the parts to be joined together, and placing them on a jig ready for firing. Because the temperatures involved are of the order of 600-800°C, depending on the glass used, these jigs have to withstand high temperatures, and are frequently made of graphite.

From the description of the process, you will understand that the manufacture of individual compression seals can be partially automated, but making complex packages with numbers of matched seals is a skilled task, involving yield losses, and consequent expense.

The high cost of hermetic packages made in this way has been one of the driving forces behind the use of polymeric encapsulation. Nevertheless, metal seals of various kinds may still be found on a typical assembly, particularly for encapsulating crystals, where the active element needs to be kept within a carefully controlled environment, since moisture causes crystals to ‘lose activity’ and fail to function.

A range of crystal assemblies

A range of crystal assemblies

Optical fibres

As well as wetting well to selected metals, glass has other attributes. In fact, the transparency of glass is a much better-known phenomenon than its use for making seals to metal! Given that light has a much higher frequency than even the shortest microwaves, and can thus potentially carry much more information than radio waves, waveguides or copper wire, it is not surprising that much attention has been paid to the optical transmission of information through glass fibres. Unfortunately, standard glasses are less than perfect transmitters of light, with high losses and distortion, and the glasses used in optical fibres are a world removed from window glass.

A typical optical fibre consists of a pure silica tube containing a core of silica which is doped with another oxide such as germanium oxide, GeO2, to give a slightly higher refractive index. As germanium is a member of the same group as carbon and silicon, it can be expected to be accommodated within the structures of glasses based on silicon. The light (or rather laser) pulses are guided along the core by internal reflection at the interface between core and coating. For light to be transmitted any significant distance, the core material must be very transparent, and consequently very pure.

Figure 10: Multiple light paths within a glass fibre

Figure 10: Multiple light paths within a glass fibre

Practical optical fibres are considerably more complex. For example, in a fairly wide fibre, light travels by a variety of different paths, potentially leading to distortion of the signal as the pulses reach their destination at different times (Figure 10).One way in which this is overcome is to use a core material where the refractive index varies from a maximum at the centre to that of the coating at the interface in a near-parabolic manner. Light rays taking longer paths travel faster, so that all the components of the pulse reach their destination simultaneously. An alternative approach, using finer fibres, is described as ‘single-mode’, because there is only one possible path within the fibre.

Practical fibres are much bigger than the 5–15µm of the external cladding. This is because the fibres themselves need to be protected in order to avoid breakage due to surface damage.

A fibre optic component

A fibre optic component

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Mechanical components

If you look at a typical complex board, you will see not just passive and active components but a range of other parts. Some of these have electrical functions; others are purely mechanical; a few even have moving parts, and might be considered electro-mechanical devices. The list below covers most of the generic types:

Surface mount switch and LED

Surface mount switch and LED

Surface mount connector

Surface mount connector

Surface mount RF connector

Surface mount RF connector

Through-hole crystal and oscillator

Through-hole crystal and oscillator

Shielding can

Shielding can

Heat sink

Heat sink

Of these categories, the transducers represent the class with the greatest variety in terms of technology and packaging. They are, however, less important to most designers than connectors and switches. Again, these come in different types, but the technology is broadly similar. However, apparently similar parts are easy to confuse with each other, and it is very important to select the right component for the application.

1 There is an interesting debate about the definition of what is, or is not, a transducer on the Sensors Online web site at http://www.sensorsmag.com/articles/0901/10/debate_main.shtml

 

Connectors

Connectors cover a very wide range of products, and rather than give the details ourselves, we asked Harwin, specialist manufacturers of connectors to write a brief for us. First take a look at Connecting to Printed Circuit Boards, and then tackle the question below.

A representative range of connectors

A representative range of connectors

Exercise

 

Your design team have used through-hole connectors on their computer product for some years and need some convincing that surface mount styles will do the job, and may give some advantages. Try and convince them!

Compare your answer with this one

 

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Variable components

Closely related to switches are variable components. If you look at a piece of hi-fi equipment, you will probably find that the front panel contains not only switches, but also some means of varying the circuit response or adjusting frequency. The real difference between a variable component and a switch is that the variable component is analogue rather than digital: a switch has a discrete number of positions, whereas a high-quality variable component can be set at an almost infinite number of positions between its limits.

The earliest variable components were all passives: resistors, capacitors and inductors:

Whilst it is generally easiest to obtain a linear correlation between movement and electrical function, in certain applications this is not the preferred option. A very common example is a volume control, where the ear responds logarithmically, and the voltage division performed by the potentiometer should behave in the same way, so that equal movements of the knob or slide produce the same effect on the perceived volume. Variable components with this kind of response are referred to as having a logarithmic 'law'.

A trimmer

A trimmer

In the preceding text, you will have noticed a tendency to use the past tense. This is because variable components are now far less common than they were. This is due in part to their mechanical complexity, and hence cost, but there are also issues to do with reliability - have you ever thrown away a transistor radio because its volume control became intermittent or noisy? Reliability and cost issues have therefore favoured electronic means of varying response, either using analogue components with a programmable response, or choosing purely digital methods of achieving the same function.

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The right component in the right place

“Proper words in proper places make the true definition of style” according to Jonathan Swift; for the manufacturer of an electronic product, putting the right component in the intended place is a similar challenge, but demanding accuracy and attention to detail rather than any particular creative flair. Getting the right part is a short brief that reminds the reader of why certain values are ‘preferred’ and then looks at the issues relating to component marking. This is an area where there are many standards, but a trend towards having a reduced amount of marking, especially for very small components. The emphasis of course must be on the sharing of accurate information, preferably a single set of information than multiple copies and translations, and on maintaining good manufacturing discipline.

Putting the right component in the right place is also a question of presenting the correct component at the placement station, whether automatic or manual, and this is a topic that we will study in Unit 6. However, for those who like to browse ahead, the relevant links are in Component feeding for SM parts and in Component insertion for through-hole components. Notice the variety in component presentation, despite the existence of a number of industry standards.

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Reliability issues

What is a failure?

We start by asking the question “what is a failure?”, because this is not always a straightforward issue. Take the high mileage car which you traded in. Was this because it totally failed to function? More likely it was because it had started to exhibit increased fuel consumption, occasionally didn’t start too well, or didn’t look as smart as in its youth. All these represent failures, but at different levels.

Other kinds of equipment may still function as intended, but no longer be compatible with other requirements, standards or legislation – someone, somewhere must still have a working BetaMax VCR! Other kinds of failure in the consumer market relate to a product not being state-of-the-art, or reflecting the most modern styling, considerations which fuel the mobile communications and games industries.

In the same way, at the system level, electronic failures will vary very considerably from total failure to function, through to purely cosmetic issues. Whilst creating a product that looks the part, and is made to an acceptable cosmetic standard, is of major interest to the marketers and manufacturers of equipment, in most of our reliability material we are looking primarily at the failure of individual components, rather than systems, and only at those types of failure that result in system malfunction.

Types of electronic failure

At the component level, electronic failures fall into four categories:

The effect that component failures will have on the system depends on the fault tolerance of the system, and the degree and type of failure. With high-reliability requirements, it is not uncommon for designs to be deliberately made to tolerate degrees of failure, particularly at a system level. An example of this might be a critical server application, where the operation of one system is mirrored by a second fully-functioning machine which takes over automatically if the main system fails.

In considering failure, we also have to bear in mind the possibility that there will be some knock on consequences, particularly of catastrophic failures. For example, failure in protection components can result in the failure of associated circuitry – service personnel will be familiar with cases where a cheap component fails and ‘takes out’ an expensive module elsewhere in the circuit. Sometimes this knock-on failure can be quite dramatic. The writer recalls a very expensive complex assembly becoming a total write-off because of the short-circuit failure of a ceramic capacitor placed across a high current supply, when it literally burnt a hole in the board!

What causes electronic failure?

A range of conditions cause electronic failures. Sometimes the mechanism is simple overload, but often misuse, misapplication, lack of full testing, or defects in manufacture can play a part. At the system level, failures can be caused by:

In considering the potential for failure, it is important to keep in mind that the majority of components used in electronic assemblies do not have mechanisms that would cause the part to degrade sufficiently to fail during storage or normal use, provided that they:

These are important provisos, but remember that the quality of manufacture of modern electronic components is high, with typically fewer than 10 defective parts per million for complex components such as integrated circuits, and even less for simpler components.

Whilst the level of defects is low, a number of factors tend to promote component failure, and often at a much higher level than indicated above, because they represent abuse rather than intended use. In the sections that follow we have differentiated between mechanical causes of failure, electrically-induced failure, failure caused by moisture, and failure in solder joints. These sections are deliberately designed for “dipping into”, and we do not expect you to be familiar with every reliability nuance.

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Mechanical

We have assembled three short papers that discuss various kinds of mechanical failure. Some reflect normal life, but more are related to handling during manufacture and can be influenced by the design of the board. In Failure for mechanical reasons we first focus on the topic of lead damage – What impact does it have on the manufacturing process, if the components have leads that are less than perfectly planar? Then we consider the ways in which mechanical components may wear out, and failure can be induced by shock and vibration during life, and by the effect of thermal cycling. More about these topics in later units.

Then in Failure mechanisms in ceramic capacitors we look at the ways in which MLCs can be made to fail, especially by abuse during the assembly process compounded by poor design. It is particularly important to make sure that the depanelling process using during assembly is compatible with the design, and does not build into the MLCs on the board the potential for failure during life.

Finally, in Hermetic package failure, we consider the case of a package such as might be used on some types of integrated circuit, in crystals and in hybrid microcircuit modules. Testing will weed out only the worst failures, and quality needs to be built into the process of making the hermetic seal in the first instance.

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Electrical

Electrical failures are very common, and we recommend that you at least skim read out note on Failure through electrical stress. This identifies two distinct ways in which components can be damaged, by the circuit in which they are placed, and by electrostatic events from the surroundings.

ESD sensitivity is an issue that is well known within the board assembly fraternity, but perhaps less so in a test or field service environment. The fact is that many components can be damaged by exposure to very small ESD events, well below the threshold at which the operator can experience any kind of shock. As a result of problems experienced, most assembly houses have appropriate ESD management in place, and you may well have received awareness training as part of your induction to your company. From this you will probably be aware than charge does not obey the same rules as current, with the consequence that electrostatics is in many ways counter-intuitive. For many of those involved at the coal face, ESD is more a question of disciplines being enforced, than of real understanding of the issues involved.

Supplementary information

As part of our campaign for ESD awareness, we invite you to look at four links, which form a coherent short course on this topic:

ESD awareness
Electrostatics explained
Modelling ESD waveforms
ESD control during manufacture

 

If you have read Failure through electrical stress, you will know that at least some failures in power systems can be caused to reversed components. Not only is this bad for the component, but it can also be bad for the test gear! There are two ways in which polarity can be reversed; by reversing the supply, by reversing the component. It therefore becomes most important to assemble the part the right way round. And of course polarity can also apply to connectors. . .

‘Polarity’ is a term which has two related meanings:

While resistors and ceramic capacitors are ‘non-polar’, that is it is immaterial which way round they are fitted, electrolytic capacitors are inherently polar. Not only will they function incorrectly when reversed, as would a diode, but the unexpected reverse voltage may do permanent damage, and even result in an internal explosion and rupture of the package.

But polarity, in the wider sense of putting things onto the circuit the right way round, starts with three-pin devices such as transistors. Only in a very few cases (some resistor networks and potentiometers) are multi-pin devices sufficiently symmetric to be placed in different orientations, yet function correctly. Typically, component leads are identified by numbering, and the package will indicate which is pin 1, either explicitly by device marking, or by reference to a data sheet or convention. And of course there need to be corresponding conventions for how the board should indicate the orientation of the component.

Different means of polarity marking on a board

Different means of polarity marking on a board

Be aware that, when numbering connections, there are two possible ways of doing this, even with as simple a component as a small outline integrated circuit. Assuming that you have designated pin 1, then the numbering might go clockwise or anticlockwise. The convention usually applied is that the component is viewed from the top of the package, and the numbering is anticlockwise. This means however that through-hole and surface mount components appear to have different pin sequences when viewed from the copper of the board to which they are attached. This difference in perspective, according to whether the device is viewed from above or below, creates an opportunity for significant error – be warned!

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Humidity

Whilst highly pure water is a reasonable insulator, it is also a polar solvent, and even a small trace of contamination can make a solution that conducts electricity, so components need protection against moisture. The primary requirement of any encapsulation is to prevent the build-up of even a monolayer of water in contact with the active surface.

For this reason, encapsulants are designed to adhere to the active surface, although it is not practicable to prevent any moisture take-up by a polymeric material – a typical polymer will absorb 0.3% to 1.0% of moisture in the steady state. This uptake of moisture, combined with the low purity of the early encapsulant resins, is one reason why high-reliability users are so reluctant to move away from the hermetic package. Yet, as has been pointed out in Hermetic package failure, it is not practicable to test for hermeticity at the very low leak rates that are needed.

Fortunately, the development of high purity resins has led to continued improvement in the performance of moulding compounds, and moisture-related failures are now relatively rare. What happens when moisture and ions meet is shown in our paper on Corrosion; combined with even a modest voltage, the failure becomes different, as in our paper on Electromigration.

But real interfaces break down, and this leads to serious consequences for reliability. With the trend towards ever-reducing PCB dimensions, a failure mechanism known as conductive anodic filamentation (CAF for short) is becoming increasingly important, and we encourage you to follow this link for an explanation.

More serious on an everyday basis is the absorption of moisture into component structures which causes delamination during the severe heat regime involved in reflow soldering. Referred to as “popcorning”, the mechanism is particularly dangerous since many of the defects produced do not result in immediate failure.

Moisture sensitivity has become increasingly important since the adoption of lead-free solders, because of the higher reflow temperatures used. In consequence, many more components are labelled as being moisture-sensitive, and require appropriate handling on the shop floor. In many ways, MSD (= Moisture Sensitive Device) has become an issue, in the same way as ESD did to a previous generation of engineers. More details on both the effect and the precautions to be taken are in our paper on Moisture absorption.

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Joint failure

So far our discussion on failure has concentrated on the component, but all components need to be joined to the circuit, and we should not forget the potential for joints to fail. In fact, given the nature of solders, eventual failure is inevitable. However, the expected life of a good joint is generally well in excess of the design life for the product.

Why do joints fail? The answer is given in our paper Failure in solder joints, which puts the blame for failure on overloading, which causes tensile rupture, on long-lasting permanent loading, which produces creep, and on cyclic loading, which produces fatigue. In most situations, this last is the most important contributor to eventual failure.

Fatigue is caused by the cyclic application of stress, and most of the stress seen in surface mount assembly is caused by thermal mismatch between the component and the board. Read our paper Stress caused by thermal mismatch to understand more of the different stresses on different shapes of solder joints, why compliant gull-wing leads are favoured, especially for larger components, and why some users are prepared to pay the much higher price of column BGAs rather than use devices made with solder balls.

Of course, not all joints are perfect, and it has been proposed that at least some joint failures are accelerated by the presence of voids within the joint. In practice, except for BGAs, where the bonds are highly stressed and their form means that they are prone to internal voiding, there are few failures. For more information, read our paper on Holes and voids.

Many apparent component failures are really failures of joints, which is why we have considered the ways in which joints fail. But other failures are actually failures to make an adequate solder joint in the first place. Both the surface finish of a component and the conditions under which it is stored may affect the wettability of the component and hence its fitness for purpose in the assembly; board and component solderability is extremely variable, and damp and heat combined will do much to reduce the wettability of even the more robust surface.

Ceramic capacitor termination showing poor solderability

Ceramic capacitor termination showing poor solderability

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Final thoughts

Component reliability is something that we take for granted until we come across the consequences of a failure. Component manufacturers are sensitive to this, and the writer has a memory of being shown six very expensive multilayer boards, stuffed full of components and each costing a four-figure sum, which had been destroyed by failure of a ceramic capacitor whose purchase price was only a few pence. Unfortunately there was no current limitation on the track to the component, with the result that the component overheated until failure occurred, by which time it had created a substantial and non-repairable “pit” in the board to which it was mounted. It is only by being aware of the potential for failure that we can devise ways of making electronic products adequately reliable.

Activity

Apart from mechanical components which may wear out, hermetically-sealed components which may leak, and chip ceramic capacitors which may break, how are other passive components used in electronic circuits likely to fail?

Compare your answer with our comments.

 

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