Thick Films

Abstract

Thick film technology is an example of one of the earliest forms of microelectronics-enabling technologies and it has its origins in the 1950s. At that time it offered an alternative approach to printed circuit board technology and the ability to produce miniature, integrated, robust circuits. It has largely lived in the shadow of silicon technology since the 1960s. The films are deposited by screen printing (stenciling), a graphic reproduction technique that can be dated back to the great Chinese dynasties of around a thousand years ago. Indeed, there is evidence that even early Palaeolithic cave paintings from circa 15000 BC may have been created using primitive stenciling techniques. With the advent of surface-mounted electronic devices in the 1980s, thick film technology again became popular because it allowed the fabrication of circuits without through-hole components.

This chapter will review the main stages of the thick film fabrication process and discuss some of the commonly used materials and substrates. It will highlight the way in which the technology can be used to manufacture hybrid microelectronic circuits. The latter stages of the chapter will demonstrate how the technology has evolved over the past twenty years or so to become an important method in the production of solid state sensors.

The term thick film is often misinterpreted, and so it is worth noting from the outset that it has little to do with the actual thickness of the film itself. The preferred definition encompasses the fabrication process, namely screen printing, used to deposit the films. The typical range of thicknesses for thick film layers is 0.1–100 μm. Screen printing is one of the oldest forms of graphic art reproduction and involves the deposition of an ink (or paste) onto a base material (or substrate) through the use of a finely-woven screen with an etched pattern of the desired geometry. The process is commonly used for the production of graphics and text onto items such as T-shirts, mugs, pencils, textiles and so on, and is very similar to that used for microelectronic thick films. The degree of sophistication for the latter is, however, significantly greater, resulting in high-quality, reproducible films for use in a variety of electronic systems.

The technology used to manufacture thick film hybrid microelectronic circuits was introduced in the 1950s. Such circuits typically comprised semiconductor devices, monolithic integrated circuits, discrete passive components and the thick films themselves [29.1, 29.2]. In the early days of the technology, the thick films were mainly resistors, conductors or dielectric layers. Evidence of thick film circuits can still be found in many of today’s commercial devices such as televisions, calculators and telephones. The use of thick film technology was overshadowed in the 1960s by the impact of silicon technology. It found popularity again in the 1980s as a result of the advent of surface-mounted devices, which can be attached to circuit boards using screen-printed solder layers. Nowadays, thick films are not only used in hybrid circuits but also in advanced solid state sensors and actuators, as we shall see later. In broad terms, thick films can be classified as either cermets (ceramic∕metallics), requiring high-temperature processing, or polymer thick films, which are cured at significantly lower temperatures. Polymer thick films will be covered in more detail in Sect. 29.3.4.

1 Thick Film Processing

Thick film technology is sometimes referred to as an additive technology in that the layers are built up in sequence without the need to remove (or subtract) parts of the film by techniques such as etching. Compare this with, say, a standard printed circuit board (GlossaryTerm

PCB

), where the conductive tracks are formed by selectively etching away the undesired areas (gaps) from a continuous copper layer. In the early days of the technology, the ability to fabricate together components made with different enabling technologies to produce a hybrid circuit was seen as a way of opening up a whole new field of electronics. This is still one of the most endearing features of the technology today.

1.1 Screen Printing

This is the distinguishing feature of thick film technology and is the method of depositing the desired films. The fundamental aspects of the process, traditionally recognized as an art, have been modernized and updated to provide a scientific tool for microelectronic technology. Figure 29.1 illustrates the main aspects of the screen printing process [29.3].

Fig. 29.1

The basic screen printing process

The screen fabric is permanently attached to the screen frame, which is firmly held within the screen printing machine. The distance between the screen and the substrate is typically around 0.5 mm, although the precise gap size is dependent on the overall screen dimensions. The substrate is held in place in the holder by either a vacuum or a mechanical clamp. The position of the screen can be finely adjusted to ensure good registration between consecutive layers. The paste is applied to the upper surface of the screen and the flexible squeegee is pulled across the screen over the ink, which is forced through the open areas of the screen mesh. At a point immediately behind the squeegee, the screen peels away from the substrate and, due to the surface tension between the ink and the substrate, leaves a deposit of the paste in the desired pattern on the substrate.

The squeegee is a flexible blade whose function is to transfer the ink through the screen and onto the substrate. It is usually made from materials such as polyurethane or neoprene. The squeegee pressure is adjustable and facilitates accurate and repeatable print thicknesses. The pastes generally exhibit pseudoplastic behavior in that the viscosity varies with the applied shear force. This is a necessary property because the ink must have minimum viscosity to ensure good transfer though the screen, but it also must become more viscous after printing to provide a good definition of the film.

The function of the screen is to define the pattern of the printed film and also to control the amount of paste being deposited. The screens used in graphic artwork are generally made of silk and the process is often termed silk screen printing. For microelectronic circuits, however, the screens tend to be made from polyester, nylon or stainless steel. The resolution of the printed line widths is largely determined by the mesh count (in lines per inch or centimeter) and the mesh filament diameter. Nylon has very good elastic properties but relatively poor resilience. Stainless steel screens produce excellent line definition and durability and are particularly well suited to flat substrates. Polyester screens have the highest resilience and offer a long lifetime and low squeegee wear. Figure 29.2 shows a plain weave pattern on a typical thick film screen. An ultraviolet (UV)-sensitive emulsion covers the underside of the screen. The desired pattern is formed onto screen emulsion using a photographic process. The screen printing process can be used to deposit both cermet and polymer thick films; the processing steps that follow are, however, different.

Fig. 29.2

A plain weave screen mesh

1.2 The Drying and Firing Process

The final form of a cermet thick film is a fired, composite layer that is firmly attached to the substrate. Essentially, there are three main stages in the production process: screen printing, drying and firing. At each of these stages, the film is in a slightly different state. Commercial thick film pastes are purchased from the manufacturer in plastic jars and are similar to standard printing inks in many respects. The three main components in a thick film paste are:

• The active material

• A glass frit

• An organic vehicle.

The active material is a finely ground powder with a typical particle size of a few microns. A conductor paste, for example, will contain a precious metal or metal alloy. The glass frit serves as a binder that holds the active particles together and also bonds the film to the substrate. The organic vehicle is necessary to give the paste the correct viscosity for screen printing. It usually contains a resin dissolved in a solvent together with a surfactant that ensures good dispersion of the solid particles.

After screen printing, it is usual to allow the film to stand in air for a few minutes to let the paste level off. The film is then dried in an infrared belt drier or a conventional box oven at temperatures up to 150^∘C. The purpose of drying is to remove the organic solvents so that the film and substrate can be handled during further processing steps. After drying, the films then proceed to the firing stage. In some circumstances, however, they may be overprinted with another thick film layer if the nature of the film allows this.

The high-temperature firing cycle performs three main functions: the remaining organic carrier is removed from the film, the electrical properties are developed, and the film is bonded to the substrate. In order to achieve these, it is necessary to subject the films to temperatures of up to 1000^∘C in a moving belt furnace. Inside the furnace there are several heating zones, which can be independently controlled to a profile specified by the paste manufacturer. Figure 29.3 shows a typical firing profile that is used with commercial thick film resistor pastes. The substrates enter the furnace on a metal chain mesh belt at ambient temperature and slowly (at a rate of a few centimeters per minute) travel through the heating zones, which have their temperatures set to form the desired profile. The substrates remain at the peak temperature for about ten minutes and the electrical properties of the film are formed during this phase. Most thick film materials are fired in a clean, filtered air atmosphere, as this produces high-quality films with repeatable characteristics. Occasionally, however, it is necessary to provide an inert atmosphere such as nitrogen, so that materials such as copper can be processed.

Fig. 29.3

A typical thick film firing profile with a peak temperature of 850^∘C

2 Substrates

The main functions of the substrate are to provide mechanical support and electrical insulation for the thick films and hybrid circuits. Some of the main considerations for selecting substrates are listed below:

• Dielectric constant: This determines the capacitance associated with different elements fabricated onto the substrate. The dielectric strength will also determine the breakdown properties of the substrate.

• Thermal conductivity: Substrates with a high thermal conductivity can be used in applications where the circuit generates significant amounts of heat.

• Thermal coefficient of expansion (GlossaryTerm

  TCE

  ): In general terms, the TCE of the substrate should be closely matched to the thick film materials and other components mounted on it. In some cases this cannot be assured, and in this case the consequences (in terms of thermal strains) must be fully considered.

The main substrate materials used in thick film technology are the ceramic materials alumina (Al₂O₃), beryllia (BeO) and aluminium nitride (AlN). Enamelled or insulated stainless steel substrates are sometimes used in some applications. Silicon has also been used in specialist transducer applications. Alumina, however, is the most common substrate material and it possesses desirable physical and chemical properties in addition to providing an economical solution. Alumina of 96% purity is used in the vast majority of worldwide commercial circuits. The remaining 4% weight fraction of the content is made up of magnesia and silica, which improve the densification and electrical properties. Beryllia has a high thermal conductivity and is used in applications where rapid heat removal is required. It is, however, a very toxic material and is therefore only used in limited application areas. Aluminium nitride is, essentially, an alternative for beryllia, with a high thermal conductivity and also improved mechanical properties such as higher flexural strength. Insulated stainless steel substrates are sometimes used in applications where a high thermal dissipation and mechanical ruggedness are required. They are particularly well suited to mechanical sensor applications.

2.1 Alumina

Alumina substrates are manufactured by blending alumina powder, with an average particle size of around 1 μm, together with small amounts of silica, magnesia and calcia. These are either ball- or roll-milled for about 10 h with lubricants, binders and solvents that ensure thorough mixing. Most thick film substrates are less than one millimeter thick, and the preferred method of fabrication is sheet casting. A slurry is allowed to flow out onto a smooth belt, and it passes under a metal doctor blade which controls the resultant thickness. The material is then dried in air to remove the solvent and, at this stage, it is sometimes referred to as the green state because of its color. The substrates are then fired in a kiln for at least 12 h. A peak temperature of around 1500^∘C ensures that the materials are properly sintered. During firing, the substrates can shrink by up to 20%, and this needs to be taken into consideration for the formation of the final substrate. The surface finish can be improved by coating the surface with a thin, glassy layer (glazing), which is done as an additional step at a lower temperature.

2.2 Stainless Steel

Stainless steel is a strong, elastic material with a relatively high thermal conductivity. Being a good electrical conductor, the steel must be coated with an insulating layer before it can be used as a substrate for thick film circuits. Porcelain enamelled steel substrates are made by coating a stainless steel plate with a glassy layer between 100 and 200 μm in thickness. The steel is enamelled either by dipping, electrostatic spraying or electrophoretic deposition of a low-alkali glass and subsequent firing at several hundred degrees Celsius. Some commercial paste manufacturers produce an insulating dielectric thick film ink that can be screen printed directly onto various types of stainless steel. The substrates are fired at a temperature of around 900^∘C. The paste contains a devitrifying glass that does not recrystallize on further firings and therefore provides compatibility with other standard thick film materials. Insulated stainless steel substrates also offer the advantages of having a built-in ground plane (the steel itself) and excellent electromagnetic and electrostatic shielding properties. It is also possible to machine the substrate, using conventional workshop facilities, prior to the circuit fabrication.

2.3 Polymer Substrates

In some applications it is desirable to fabricate a circuit onto a flexible substrate; evidence of these can be found in mobile telephones, calculators and notebook computers. Cermet thick film materials are not compatible with flexible substrates and hence special polymer thick film materials are used (Sect. 29.3.4). Polymer substrate materials are mainly based on polyesters, polycarbonates and polyimide plastics. The maximum processing temperature is usually limited to around 200^∘C.

3 Thick Film Materials

3.1 Conductors

Thick film conductors are the most widely used material in thick film hybrid circuits. Their main function is to provide interconnection between the components in the circuit. For a multilayered circuit, the conductor tracks are separated by dielectric layers and connection between each layer is achieved with metallized vias. Conductors are also used to form attachment pads for surface-mounted components such as integrated circuits or discrete passive components (resistors, capacitors and inductors). They can also be used as bond pads for naked dice, which may be attached directly to the thick film circuit. Another function of conductors is to provide the terminations for thick film resistors. With such a diverse range of applications, it is no surprise that a wide range of conductor materials is available.

The characteristics of thick film conductors are dependent upon the composition of the functional phase of the paste. Typically, these comprise finely divided particles of precious metals such as silver, gold, platinum or palladium. Base metals such as aluminium, copper, nickel, chromium, tungsten or molybdenum are also used. The particle size, distribution and shape also have an effect on the electrical and physical properties of the fired film.

The resistance of a conductor film is given by

$$R=\frac{\rho l}{wt}\;,$$

where ρ is the bulk resistivity (Ω cm), l and w are the length and width of the conductor respectively, and t is the fired thickness of the film. The term sheet resistivity is often used for thick films and is defined as the bulk resistivity at a given thickness ( ρ ∕ t ) , expressed in ohms per square (Ω ∕ □). This is convenient because the resistance of the track can then be calculated by simply multiplying the sheet resistivity by the aspect ratio ( l ∕ w )  of the film. Table 29.1 summarizes some of the most common metals and metal alloys used in thick film conductors together with their sheet resistivities.

Table 29.1 Most of the common metals and metal alloys used in thick film conductors and their sheet resistivities

3.1.1 Silver Conductors

Silver pastes were one of the earliest thick film conductors to be developed. They possess good bond strength and high conductivity. There are, however, several disadvantages to using pure silver which prevent it being widely used in many applications. These include:

• Poor leach resistance to solder

• Oxidation in air over time

• Susceptible to electromigration in the presence of moisture, elevated temperature and bias voltage.

3.1.2 Silver/Palladium Conductors

The alloy of silver and palladium is the most common type of thick film conductor and is the alloy most widely used in the hybrid circuit industry. It can be used for interconnecting tracks, attachment pads and resistor terminations, but is generally not recommended for wiring bonding pads. Silver∕palladium conductors overcome many of the problems associated with pure silver, and low-migration formulations are available from several commercial suppliers.

3.1.3 Gold Conductors

Gold pastes have a high conductivity and are mainly used in applications where high reliability is required. Gold is a particularly good material for wire bonding pads, although it has relatively poor solderability. Gold is a precious material and hence very expensive; it is therefore not used for general purpose applications and is limited to those areas that can justify the higher costs.

3.1.4 Copper Conductors

Copper thick films have to be processed in an inert atmosphere; nitrogen is typically used in this case. Copper is widely used as the conductor material for printed circuit boards owing to its good electrical conductivity and solderability. These features are also applicable to thick film copper conductors, which also offer ability to handle larger currents than most other thick film conductors.

3.1.5 Platinum Conductors

Not too surprisingly, perhaps, these are the most expensive of all the commercial thick film conductor materials. Platinum films have a very high resistance to solder leaching and exhibit similar electrical properties to those of the bulk material: a linear, well-defined temperature coefficient of resistance (GlossaryTerm

TCR

). Platinum films are therefore used in specialist applications such as heaters, temperature sensors and screen-printed chemical sensors.

3.1.6 Gold Alloy Conductors (Au∕Pd, Au∕Pt)

These have good bond strength, good solder leach resistance and are relatively easy to solder. The conductivity of the gold alloys tends to be inferior to that of the other type of thick film conductor, but they can be used with both ultrasonic (aluminium) and thermosonic (gold) wire bonds.

Once the conductor has been fired, a composite structure is formed. Figure 29.4 shows a simplified view of the cross-section of a fired thick film conductor on a substrate. Many of the metallic particles have joined together to make a continuous chain. The glass is mainly evident at the interface between the bulk of the film and the substrate. The presence of voids (gaps), both within the film and on its surface, can also be noted.

Fig. 29.4

Idealized cross-sectional view of a cermet thick film conductor

3.2 Resistors

Resistor inks consist of a mixture of the three main phases listed earlier. The relative proportions of the active material to glass frit can have a dramatic effect on the electrical properties of the fired film. The earliest resistors were made from materials such as carbon, silver and iron oxide and were found to suffer from poor long-term stability, unpredictability of fired resistivity, and unacceptably high temperature coefficients. Modern thick film resistor systems are mainly based on ruthenium dioxide (RuO₂) and have much-improved characteristics. This material has a high conductivity and is extremely stable at high temperatures. Within the resistive paste, the conductive phase comprises submicron particles, and these are mixed with larger glass particles (several microns in diameter). Various additives are added to the formulation to improve the stability and electrical properties of the fired film. The nature of these additives is proprietary knowledge of the paste manufacturer.

Figure 29.5 shows an idealized cross-section of a thick film resistor fabricated onto a substrate. The end terminations are first printed and fired and then the resistor is formed in the same manner, allowing for a slight overlap on the terminations so that small misalignments during the printing of the resistor can be accounted for. The actual surface profile of a real resistor would show a nonuniform thickness across its length.

Fig. 29.5

An idealized cross-section of a thick film resistor

Commercial thick film resistor pastes are available in a range of sheet resistivities from 1–10⁹ Ω ∕ □, and the value of the fired resistor is determined by the selected sheet resistivity and the ratio of the length to the width, as previously described for conductors. For example, a 10 kΩ resistor could be made by printing a square ( l = w )  of 10 kΩ ∕ □ resistor paste. Similarly, a 5 kΩ resistor requires an aspect ratio of 1 : 2 ( l : w )  if printed with the same paste. Note that the absolute value of a thick film resistor is not limited to a preferred value, as is the case for discrete resistors.

Figure 29.6 shows the temperature dependence of resistance for a typical thick film resistor. The shape of the plot is unusual because there is a point at which the resistance is a minimum ( T_min ) . In general terms, the sensitivity of a resistor to temperature is denoted by the temperature coefficient of resistance (TCR). Mathematically, this can be expressed by

$$\text{TCR}=\frac{\Updelta R/R}{\Updelta T}\;,$$

where ΔR ∕ R is the relative change in resistance and ΔT is a small change in temperature. Metals generally exhibit an increase in resistance with increasing temperature, so the TCR is positive. A thick film resistor appears to have both a positive and negative TCR at different regions of the resistance versus temperature curve [29.4]. The point of minimum resistance occurs around room temperature, which means that the resistance is stable under normal operating conditions. It is common practice to refer to the hot and cold TCR for the regions above and below T_min, respectively.

Fig. 29.6

A plot of relative resistance change against temperature for a thick film resistor

The long-term stability of thick film resistors in air at room temperature is excellent, with a typical change of less than 0.2% over 10⁵ h (about 100 years!). At elevated temperatures the stability is still impressive, with most resistance values changing by only 0.5% over 1000 h at 150^∘C. The tolerance of fired resistors is around ±20% due to a variety of reasons, including variations in resistor thickness, the tolerance on the quoted sheet resistivity and the effects of the firing process. If accurate values of resistance are required, it is therefore necessary to trim the resistor. This is achieved by removing areas of the resistor with either an air-abrasive jet or a laser. Resistor values can only be increased by trimming, so the printed value is always designed to be less than that of the post-trimmed value.

3.3 Dielectrics

Dielectric pastes have four main uses in thick film hybrid circuit applications:

• Cross-over insulators for multilayer circuits

• Thick film capacitor dielectrics

• Passivation layers

• Insulation layers for stainless steel substrates.

Cross-over dielectrics comprise a ceramic material such as alumina, together with the usual glass frit and organic vehicle. These films are required to have a low dielectric constant in order to minimize capacitive coupling between the conducting tracks. They must also have a good insulation resistance and a smooth, pinhole-free surface finish.

There is an occasional requirement to fabricate thick film capacitors, although the wide availability of surface-mounted capacitors has largely precluded their need. It is also difficult to ensure that post-fired values of thick film capacitors closely match those for which they were designed. An expensive trimming process is needed to ensure that the correct valves are obtained.

Overglazes are mainly used to protect resistors from overspray during trimming and also the circuit from environmental attack during operation. The passivation layer is usually the last one to be processed and it is therefore fired at a reduced temperature in order to minimize any adverse refiring effects on other layers. Overglaze materials are therefore almost exclusively made from low-temperature glasses that fire at temperatures of 450–500^∘C.

The use of insulated stainless steel substrates was mentioned earlier. The dielectric materials used for this purpose must provide a high insulation resistance (in excess of 10¹² Ω) and also possess a high breakdown voltage (greater than 2 kV ∕ mm). There is an inherent mismatch between the thermal coefficient of expansion of the metal substrate and that of the insulation layer, which limits the type of steels that can be used for this purpose.

3.4 Polymer Thick Films

The processing temperatures for polymer thick films are significantly less than those needed for cermet materials. Rather than being fired, polymer materials are cured at temperatures below 200^∘C. One advantage of polymer thick films over cermets is a reduction in processing and material costs. As with cermet pastes, the formulation of polymer thick films comprises the active material, a polymer matrix and various solvents. The polymer matrix acts as glue for the active component. Three types of polymer organic composition are used in polymer thick films:

• Thermoplastic

• Thermosetting

• Ultraviolet (UV)-curable.

With thermoplastics, the required viscosity for screen printing is achieved via solvents. The polymer material is typically acrylic, polyester, urethane or vinyl. After printing, the paste is hardened by drying in a belt or box oven. These types of film have relatively poor resistance to environmental conditions and are not resistant to elevated temperatures and solvents. Thermosetting pastes have polymers that are partially cured and are typically epoxy, silicone or phenolic resin. After printing, the polymer is fully cured, providing a strong and stable matrix. Solvents are still needed to provide the correct rheology for printing. The UV curable pastes are generally used for dielectric inks and can be cured at room temperature under an ultraviolet light source.

For conductors, the most commonly used active phases are silver, copper and nickel. Owing to their poor stability at high temperatures, polymer thick film conductors cannot be soldered and alternative forms of attachment must be adopted. Carbon is typically used as the active material in polymer thick film resistors. The performance of these resistors is inferior to that of their cermet counterparts and they are therefore seldom used in critical applications. The dielectric pastes are similar in nature to conductors and resistors except that the conducting phase is omitted from the formulation. Some manufacturers add minerals to improve the electrical and mechanical properties of the dielectric films. Polymer thick films are a popular choice of material for disposable biosensors such as those used in the home testing of levels of glucose in human blood samples.

The process for fabricating polymer thick films is similar to that used with cermet materials. Once they have been screen printed, the layers are left to stand in air for a few minutes to ensure that the surface is level and contains no residual mesh patterns. The curing process is achieved in a box oven or an infrared belt dryer at temperatures in the range 150–200^∘C. For a thermoset polymer, the higher the temperature and longer the curing time, the greater the cross-linking of the polymer chains in the matrix. This can lead to improved film stability and increased shrinkage.

An early and successful application of polymer thick film technology was the fabrication of membrane switches for keyboards. Today, examples of polymer thick film circuits can be found in many consumer products, such as mobile phones, portable computers, personal digital assistants and calculators. Figure 29.7 shows some typical flexible polymer circuits.

Fig. 29.7

A selection of flexible circuits. (Courtesy of Flex Interconnect Technologies, Milpitas, USA)

4 Components and Assembly

4.1 Passive Components

Passive electronic components are those that do not require an external energy source to function. Examples are resistors, capacitors and inductors. As we have seen earlier, thick film technology allows the fabrication of high-quality, stable resistors. It is also possible to add resistors to a thick film circuit in the form of an additional chip component. Such devices are available as surface-mounted devices, which do not require holes to be drilled into the circuit board. Interestingly, chip resistors are often manufactured as multiple parts using thick film techniques on ceramic substrates. These are then diced and the terminations are added. An example of a typical chip component is shown in Fig. 29.8. Resistor values can range 1 Ω to 10 MΩ, with typical tolerances of between ±1 to ±20%.

Fig. 29.8

A typical surface-mounted chip component

Planar screen-printed capacitors are rarely used in thick film hybrid circuits owing to their poor stability and high production costs. They typically comprise at least three layers (two electrodes and one dielectric layer), and trimming is often required to achieve the target value of capacitance. Chip capacitors offer improved performance at a lower cost. Capacitances in the range 1 pF to 100 μF are readily available in a range of sizes from the 0201 series $({\mathrm{0.6}}\,{\mathrm{mm}}\times{\mathrm{0.3}}\,{\mathrm{mm}}\times{\mathrm{0.3}}\,{\mathrm{mm}})$ to the 2220 series $({\mathrm{5.7}}\,{\mathrm{mm}}\times{\mathrm{5.0}}\,{\mathrm{mm}}\times{\mathrm{3.2}}\,{\mathrm{mm}})$. Chip inductors are also available (typical range: 0.1–1000 μH) in a variety of package sizes. There is also a wide choice of variable passive components such as potentiometers and variable capacitors∕inductors that are currently available from many major component suppliers.

4.2 Active Components

Active components are those that require an external energy source to function. Examples are transistors, diodes and semiconductor integrated circuits (GlossaryTerm

IC

s). A wide variety of semiconductor components are available to the thick film circuit designer. Transistors and diodes, being relatively small devices, are obtained in a small plastic package with three terminals. This is known as a SOT-23 package and is shown in Fig. 29.9a. Many standard integrated circuits that are available in dual-in-line (GlossaryTerm

DIL

) packages for through-hole printed circuit boards are also available in small outline (GlossaryTerm

SO

) packages for hybrid circuits. An example of a small outline device is shown in Fig. 29.9b, and these usually have between 8 and 40 pins. For devices with higher pin counts such as microprocessor, gate arrays and so on, it is usual to place the leads on all four sides of the package, as with the flatpack device shown in Fig. 29.9c. Occasionally, ICs may be obtained in the form of a naked die, without any packaging. In such cases, it is necessary to glue the chip to the board and to bond very fine wires from the chip to the board. Care has to be taken to ensure that the naked device is suitably encapsulated for use afterwards.

Fig. 29.9a–c

Examples of surface-mounted packages (a) SOT-23 package (b) Small outline (SO) package (c) Flatpack package

Other forms of IC include flip chips, which are essentially naked chips with raised connection contacts (bumps) made of solder, gold or aluminium. These are mounted by turning over the chip (flipping) and bonding directly to the substrate. Beam lead chips also examples of naked dice with either gold or aluminium leads protruding from the edge. The leads (beams) are an integral part of the chip metallization process. Such devices are usually passivated with a layer of silicon nitride during processing. Tab automated bonding (GlossaryTerm

TAB

) refers to a technique by which the naked chip is attached to metallized fingers on a continuous strip of film. Large quantities of devices can be produced on a single roll and the leads of the devices are welded onto the boards before the carrier film is removed. In this manner, it is possible to achieve good yields on high-density circuit populations.

4.3 Trimming

The tolerance on the printed value of thick film components such as resistors is around ±20% of the desired value. Many applications require a much tighter tolerance, and so the components need to be trimmed. The two most popular techniques used are trimming by laser or by an air-abrasive jet. Both of these are capable of producing resistors with a tolerance of ±0.1%. Laser trimming, however, is more amenable to large-scale component adjustment.

Air-abrasive trimming uses a pressurized jet of air containing a fine abrasive powder to remove a small area of the fired thick film. The diameter of the jet nozzle is around 0.5 to 1 mm. Alumina particles of average diameter 25 μm are often used as the abrasive medium. The substrate containing the component to be trimmed is held underneath the nozzle at a distance of around 0.6 mm and electrical probes are attached to the device. During trimming, the particles in the jet stream remove material from the component. The debris is removed from the substrate by a vacuum exhaust system. In the case of resistor trimming, the value can only increase because the material is being removed. Resistor values cannot be reduced by the trimming process. For this reason, resistors requiring trimming are designed to be between 25 and 30% lower than the post-trimmed value.

Laser trimming has the advantage of offering a fully automated, high-speed way of adjusting component values. The thick film is vaporized with high-energy laser beam pulses. The laser is typically a Q-switched neodymium-doped YAG (yttrium aluminium garnet) type. A single pulse removes a hole of material and a line is achieved by overlapping consecutive pulses.

Resistors are the most common component requiring trimming. Sometimes conductor tracks need to be adjusted in special circumstances. For example, a platinum conductor being used as a classic Pt 100 resistance thermometer must have a resistance of 100 Ω at 0^∘C. In rare circumstances, thick film capacitors can be trimmed by removing an area of one of the plates, although the post-trimmed stability is poor.

Figure 29.10a-e shows some examples of different types of cut that can be used to trim resistors. The straight cut is the fastest way to trim, but it does not provide a reliable way of achieving a high accuracy. The L-cut overcomes this problem; the resistor is first trimmed straight and then the cut runs parallel to its length, providing a finer adjustment of the value. Both the serpentine and double cut require additional cuts perpendicular to an initial straight cut. The top hat structure is used in situations where a large change in resistance is needed.

Fig. 29.10a–e

Examples of trim cuts for resistors (a) Straight cut (b) L-cut (c) Double cut (d) Serpentine cut (e) Top hat

4.4 Wire Bonding

When a naked integrated circuit is needed as part of a hybrid circuit, connection must be made directly to the bond pads on the die. The chip can be attached to the substrate using an epoxy or by a gold∕silicon eutectic bond. Once the chip is firmly held in position, wire bonding can commence. Three main methods are:

• Thermocompression

• Ultrasonic

• Thermosonic.

Thermocompression bonding relies on a combination of heat and pressure. The wire is usually made of gold with a diameter of around 25 μm. Gold or palladium∕gold pads are deposited onto the hybrid substrate prior to bonding. The wire is fed through a ceramic capillary and a ball is formed at the end of the wire by a flame or spark discharge. A temperature of 350^∘C is required for the bond and is achieved by heating either the substrate or the capillary. An epoxy chip bond cannot be used with this technique as it will soften during the bonding process. The first part of the bond is made on the aluminium bond pad on the chip; the capillary is lowered onto the pad and a force is applied to form a ball shape on the pad. The wire feeds out of the capillary and is then positioned over the desired pad on the substrate. As the capillary is lowered, a combination of heat and pressure forms the bond and the wire is then broken so that the process can be repeated for further bonds.

Ultrasonic bonding uses either a gold or aluminium wire and does not require external heat. The ultrasonic energy is supplied from a 40 kHz transducer. The combination of pressure and ultrasonic vibration causes the materials to bond together at the interface of the wire and the bond pad. This is generally a faster technique than the thermocompression method.

The final category, thermosonic bonding, is a combination of the other techniques. The substrate is heated to around 150^∘C and the bond is made using the ultrasonic vibrations. This method is amenable to multilevel and multidirectional bonding and is therefore the preferred method, allowing bonding of up to 100 wires per minute.

4.5 Soldering of Surface-Mounted Components

Several techniques exist for the attachment of surface-mounted components to a thick film hybrid circuit. Surface-mounted components are generally much smaller than their through-hole counterparts. Of course, soldering by hand is also possible, although this is a tricky task requiring good operator skill and is often impractical because of the relatively long length of time needed.

Solder dipping requires the components to be placed on the board, either by hand or by a special pick-and-place machine. The components are fixed in position on the substrate by adding a small dot of glue and elevating the temperature to between 120 and 180^∘C, which is sufficient to cure the adhesive. The board can then be dipped into a bath of molten solder at a temperature of 200^∘C and then withdrawn at a sufficient rate to ensure that an adequate solder coating is obtained.

Wave soldering also requires the components to be fixed in position prior to the soldering process. Wave soldering machines were originally used for soldering through-hole components onto printed circuit boards, but they can also be used effectively with surface-mounted devices. The substrates are placed on a moving belt component side-down and initially pass through a flux bath before entering a solder bath. A wave of molten solder then flows over the substrate and creates a good joint at the desired location. This process can expose the components to a great thermal shock and it is therefore common to have a preheating phase which minimizes such effects. With both of these techniques, it is also necessary to ensure that a solder resist layer is applied to the substrate to cover all the areas that are not required to be soldered. With very densely populated circuits, there can also be a masking effect where some areas are not sufficiently coated with solder.

Reflow soldering is the preferred method of attaching surface-mounted devices. A solder cream is deposited onto the component pads either by screen printing or by a solder dispenser. The flux within the cream is sufficiently tacky to hold the component in place so that handling of the substrate is possible. After all of the components have been positioned on the circuit, the solder cream is dried and then reflowed. This process takes place by belt reflow, vapor phase or infrared belt system. A typical belt reflow system comprises a thermally conducting belt upon which the substrates are placed. The belt then travels through a number of heating stages, which causes the solder cream to melt (reflow). Vapor-phase soldering requires the substrates to be lowered into a vessel containing a boiling, inert fluorocarbon. The vapor condenses onto the substrate and raises the temperature uniformly to that of the liquid below. Infrared belt reflow systems are similar to those used for drying thick film materials. The substrate is placed on a wire-mesh belt, which travels through several infrared radiator zones.

4.6 Packaging and Testing

Thick film hybrid circuits are very versatile and offer advantages over other forms of enabling technologies. Owing to this flexibility, the circuits have a wide range of shapes and sizes and hence there is no standard package type. Selection of a particular form of packaging must therefore involve the consideration of issues such as:

• Protection of the circuit from harsh environmental conditions

• Protection from mechanical damage

• Avoidance of water ingress

• Electrical or mechanical connections to other parts of the system

• Thermal mismatches of different materials.

A simple way of protecting the circuit is to screen print an overglaze layer over the substrate, covering all areas of the substrate except those where components are to be added. A lead frame can then be added to the substrate to allow external connections to be made. An example of a thick film hybrid circuit (without overglaze) is depicted in Fig. 29.11. The two resistors on the left of the circuit have been trimmed and the straight cuts are visible.

Fig. 29.11

A thick film hybrid circuit

Conformal coatings are often used to protect the circuit from environmental attack. These are applied in the form of either a powder or fluid. In the former case, the substrate is heated and immersed into the powder. The temperature is then increased so that the coating dries. For fluids, the substrate is dipped into the coating material and subsequently dried at a temperature of around 70^∘C. Typically, the thickness of a conformal coating is between 300 and 1200 μm.

For circuits requiring operation in harsh environments, a special hermetic packaging is needed. The package can be made from ceramics, metals, ceramic∕metal or glass∕ceramic compositions. The hermetic seal is made by brazing, welding or glass sealing techniques. This form of packaging is often very expensive and is therefore only used in special application areas.

The final stage of the process is to test the circuit to see that its performance matches the design specification. Electrical testing can be difficult if the circuit has been coated or hermetically sealed, as physical access to components may be restricted. It is therefore usual to ensure that key test points are brought out to an external pin on the package. Environmental testing over a range of temperature and humidity may also be required in some circumstances. High-reliability circuits are often subject to a so-called burn-in phase, which involves holding the circuits at an elevated temperature for a given time to simulate the ageing process.

5 Sensors

Advances in the field of sensor development are greatly affected by the technologies that are used for their fabrication. The use of thick film processes as an enabling technology for modern-day sensors continues to expand. As we have already seen, the ability to produce miniaturized circuits is clearly one area in which thick film technology excels. The hybrid electronic circuitry can be integrated into the sensor housing to produce the basis of a smart (or intelligent) sensor [29.5]. Thick film technology also offers the advantage that it can provide a supporting structure onto which other materials can be deposited, possibly using other enabling technologies [29.6].

A major contribution of the technology to sensor development, however, results from the fact that the thick film itself can act as a primary sensing element. As an example, the thick film strain gauge, described below, is merely a conventional thick film resistor that is configured in such a way as to exploit one of its physical characteristics. Commercial thick film platinum conductors can be trimmed and used as calibrated temperature sensors. Most standard pastes, however, have not been specifically developed for sensor applications and do not necessarily have optimum sensing properties. The formulation of special-purpose thick film sensor pastes is the subject of intensive research activity [29.7].

For the purpose of this text, a sensor is considered as being a device that translates a signal from one of the common sensing domains (mechanical, thermal, optical, chemical or magnetic) into an electrical signal. An actuator is a device that converts an electrical signal into one of the other domains (mainly mechanical).

5.1 Mechanical

In broad terms, thick film mechanical sensors are mainly based on piezoresistive, piezoelectric or capacitive techniques. Materials that exhibit a change in bulk resistivity when subjected to deformation by an external force are termed piezoresistive. A more common term is the strain gauge, denoting the fact that such devices produce a change in resistance when strained. The effect can be observed in standard cermet thick film resistors [29.6, 29.8]. The sensitivity of a strain gauge is called the gauge factor (GF) and is defined as

$$\text{GF}=\frac{\Updelta R/R}{\varepsilon}\;,$$

where ΔR ∕ R is the relative change in resistance and ε is the applied strain (dimensionless). The gauge factors of metal foil strain gauges and thick film resistors are around 2 and 10 respectively. The former have typical resistance values of either 120 or 350 Ω. As we have already seen, however, it is possible to produce thick film resistors with a wide range of resistance values, and this allows greater flexibility in strain gauge design. It is usual to place the strain gauges in a Wheatstone bridge configuration in order to produce a linear output analog voltage change that is proportional to the mechanical measurand. A wide range of thick film piezoresistive sensors exist, including accelerometers, pressure sensors and load cells.

Piezoelectric materials exhibit the property of producing an electric charge when subjected to an applied mechanical force. They also deform in response to an externally applied electric field. This is an unusual effect, as the material can act as both a sensor and actuator. Certain crystals such as quartz and Rochelle salt are naturally occurring piezoelectrics, whilst others, like the ceramic materials barium titanate, lead zirconate titanate (GlossaryTerm

PZT

) and the polymer material polyvinylidene fluoride (PVDF), are ferroelectric. Ferroelectric materials are those that exhibit spontaneous polarization upon the application of an applied electric field. This means that ferroelectrics must be poled (polarized) prior to use in order to obtain piezoelectric behavior.

Thick film piezoelectrics have been made by mixing together PZT powder, a glass binder and an organic carrier [29.9]. A conducting layer is first screen printed, dried and fired onto a substrate and then several layers of the PZT film are deposited onto this lower electrode. The piezoelectric layer can be processed in a similar manner to conventional thick films. An upper electrode layer is then deposited onto the PZT in order to make a sandwich structure similar to that shown in Fig. 29.12. This is essentially a planar capacitor and the as-fired film must undergo a poling process by applying a direct current (GlossaryTerm

DC

) electric field of around 4 MV ∕ m and elevating the temperature to about 120^∘C.

Fig. 29.12

Cross-section of a thick film piezoelectric sample

Thick film piezoelectrics have been used in a variety of sensor and actuator applications, including accelerometers, pressure sensors [29.10], micromachined pumps [29.11], surface acoustic wave (GlossaryTerm

SAW

) [29.12] and resonant sensors [29.13].

The capacitance C of a parallel plate capacitor is given by

$$\begin{aligned}\displaystyle C=\frac{\varepsilon_{0}\varepsilon_{\mathrm{r}}A}{d}\;,\end{aligned}$$

where ε₀ is the permittivity of free space, ε_r is the relative permittivity of the material between the electrodes, A is the area of overlap, and d is the separation of the electrodes. A mechanical sensor exhibiting a change in capacitance can be made by varying A, d or by displacing the dielectric (changing ε_r). The most popular configuration is to vary d in accordance with the desired measurand. This results in a nonlinear relationship between displacement and capacitance. If the variable plate is positioned between two fixed outer electrodes, then a differential structure with a linear response can be obtained. This is a common arrangement in many types of pressure sensor.

5.2 Thermal

Devices that exhibit a change in resistance in accordance with variations in temperature are termed thermoresistive. For metals, such devices have a linear response to temperature and are known as resistance thermometers. Thermally sensitive semiconductors, typically having a nonlinear response, are termed thermistors. Thick film platinum conductor layers can be used as resistance thermometers and exhibit a linear TCR of around ${\mathrm{3800}}\,{\mathrm{ppm/{}^{\circ}\mathrm{C}}}$ (GlossaryTerm

ppm

: parts per million), slightly lower than that of bulk platinum, which is around ${\mathrm{4000}}\,{\mathrm{ppm/{}^{\circ}\mathrm{C}}}$. Platinum resistance thermometers (GlossaryTerm

PRT

s) are often trimmed so that they have a resistance ( R₀ )  of 100 Ω at 0^∘C. Such sensors are sometimes referred to as Pt 100s and are often made of wound platinum wire. Thick film Pt 100s, fabricated onto alumina substrates, are available commercially and are considerably cheaper than the bulk versions. The following expression applies to a PRT over the linear part of the temperature characteristic (−200 to $+{\mathrm{500}}\,{\mathrm{{}^{\circ}\mathrm{C}}}$)

$$R=R_{0}(1+\alpha T)\;,$$

where R is the resistance at a temperature T and α is the TCR ($\mathrm{ppm/{}^{\circ}\mathrm{C}}$).

Thermistors are usually available in the form of discs, rods or beads comprising a sintered composite of a ceramic and a metallic oxide (typically manganese, copper or iron). Thermistor pastes are commercially available and screen-printed sensors are fabricated in a similar manner to conventional thick film resistors. The electrodes are first printed and fired onto an alumina substrate. The thermistor is then deposited across the electrodes and can be trimmed to a specific value if desired. Most thermistors have a negative temperature coefficient (GlossaryTerm

NTC

) of resistance; in other words their resistance decreases as the temperature increases. The resistance versus temperature relationship is of the form

$$R=R_{0}\exp\left[{\beta\left({\frac{1}{T}-\frac{1}{T_{0}}}\right)}\right],$$

where R₀ is the resistance at a reference temperature T₀ (usually 25^∘C).

Positive temperature coefficient (GlossaryTerm

PTC

) thermistors are also available, although they are generally not as stable or repeatable as NTCs and are therefore used as simple thermal detectors rather than calibrated devices.

Another type of temperature sensor can be made by joining together two dissimilar metals (or semiconductors). If a temperature difference exists between the joined and open ends, then an open-circuit voltage can be measured between the open ends. This is known as the Seebeck effect and is the basis of a thermocouple. A thick film version can be made by overlapping different conductor materials on a substrate, although the thermal sensitivity is much less than traditional wire-based devices.

5.3 Optical

Screen-printed photosensors are probably one of the earliest examples of thick film sensors, and their use dates back to the mid 1950s. Materials that exhibit a change in electrical conductivity due to absorbed electromagnetic radiation are known as photoconductors. Cadmium sulfide (CdS) is an example of such a material and is notable because of its highly sensitive response in the visible range (450–700 nm). The resistances of such devices can drop from several tens of MΩ in the dark to a few tens of ohms in bright sunlight.

Thick film photoconductor pastes are not widely available commercially, but have been the subject of some research activity [29.14]. Such pastes, based on cadmium sulfide and selenide, are prepared by sintering powdered CdS or CdSe with a small amount of cadmium chloride (which acts as a flux) at a temperature of around 600^∘C. This is then ground into a powder and mixed with an organic carrier to make a screen-printable paste. This can then be printed over metal electrodes and fired at a temperature of around 600^∘C in air.

5.4 Chemical

Thick film materials have been used in a variety of chemical sensor applications for the measurement of gas and liquid composition, acidity and humidity [29.15]. The two main techniques are impedance-based sensors and electrochemical sensors. With the former method, the measurand causes a variation in resistance or capacitance, whilst the latter relies on the sensed quantity changing an electrochemical potential or current.

Impedance-based gas sensor pastes usually comprise a semiconducting metal oxide powder, inorganic additives and organic binders [29.16]. The paste is printed over metal electrodes and a back-heated resistor on an alumina substrate. The heating element is necessary to promote the reaction between the gas being measured and the sensing layer. Figure 29.13 shows an example of a thick film sensor, without a heating element, that can be used to measure humidity. The interlocking finger electrodes are often referred to as interdigitated electrodes and are screen printed and fired onto an alumina substrate. A porous dielectric layer is then screen printed onto the electrodes. As the humidity increases, moisture will penetrate the surface of the dielectric layer causing a change in dielectric constant within the sensitive layer. This results in a change in capacitance between the electrodes.

Fig. 29.13

A thick film humidity sensor

Electrochemical techniques can be used to realise pH sensors. These are often used in biomedical, fermentation, process control and environmental applications. These devices often make use of a solid electrolyte which generates an electrochemical potential between two electrodes in response to the measurand.

Perhaps the most common example of a thick film chemical sensor is the disposable, polymer-based glucose sensor used in many home testing kits for diabetic patients. This illustrates how thick film sensors can offer robust, compact and cost-effective solutions to many modern-day requirements.

5.5 Magnetic

Some screen-printable conductors, particularly those containing nickel, exhibit a change in resistivity in response to an applied magnetic field. Such devices are referred to as magnetoresistive sensors. Air-fireable nickel-based conductors have been shown to exhibit a nonlinear change in resistance for a linear increase in applied magnetic field [29.17]. A peak change in resistance of around 1% can occur at an applied field of 0.1 T. Researchers have made linear and rotary displacement sensors based on thick film nickel pastes, although it should be noted that such devices are also thermoresistive and therefore the magnetic measurements need to be taken in a temperature-controlled environment.

5.6 Actuators

We have previously defined an actuator as a device that converts a signal from the electrical domain into one of the other signal domains. It was noted earlier that piezoelectric materials produce a mechanical stress in response to an electrical charge. Such materials can therefore be used as actuators. Thick film piezoelectric layers have been screen printed onto thin silicon diaphragms in order to form the basis of a micropump [29.11].

Photovoltaic devices convert incident optical radiation into electric current and are often termed solar cells. They are used to power devices such as calculators, clocks, pumps and lighting. In general terms, the output power level is proportional to the physical size of the photovoltaic cell. The device is essentially a heterojunction between n-type and p-type semiconductors. Thick film solar cells have been made comprising CdS (n-type) and CdTe (p-type) as the junction materials [29.18]. Such thick film actuators have been shown to have relatively low conversion efficiencies (between 1 and 10%).