Thick-film technology

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Thick-film technology is used to produce electronic devices/modules such as surface mount devices modules, hybrid integrated circuits, heating elements, integrated passive devices and sensors. Main manufacturing technique is screen printing (stenciling), which in addition to use in manufacturing electronic devices can also be used for various graphic reproduction targets. It became one of the key manufacturing/miniaturisation techniques of electronic devices/modules during 1950s. Typical film thickness – manufactured with thick film manufacturing processes for electronic devices – is 0.0001 to 0.1 mm. [1]

Contents

Thick-film circuits/modules are widely used in the automotive industry, both in sensors, e.g. mixture of fuel/air, pressure sensors, engine and gearbox controls, sensor for releasing airbags, ignitors to airbags; common is that high reliability is required, often extended temperature range also along massive thermocycling of circuits without failure. [2] Other application areas are space electronics, consumer electronics, and various measurement systems where low cost and/or high reliability is needed.

The simplest form to utilise a thick film technology is a module substrate/board, where wiring is manufactured using thick film process. Additionally resistors and large tolerance capacitors can be manufactured with thick film methods. Thick film wiring can be made compatible with surface-mount technology (SMT), and if needed (due to tolerances and/or size requirements) surface-mountable parts (resistors, capacitors, ICs, etc.) can be assembled on a thick film substrate.

The manufacturing of thick film devices/modules is an additive process involving deposition of several (typically max 6–8) successive layers of conductive, resistive and dielectric layers onto an electrically insulating substrate using a screen-printing process. [3]

Thick Film Resistor Networks Apple 820-0865-A - Vishay SOMC1601111G-4225.jpg
Thick Film Resistor Networks

As a low cost manufacturing method it is applicable to produce large volumes of discrete passive devices like resistors, thermistors, varistors and integrated passive devices.

Thick film technology is also one of the alternatives to be used in hybrid integrated circuits and competes and complements typically in electronics miniaturization (parts or elements/area or volume) with SMT based on PCB (printed circuit board)/PWB (printed wiring board) and thin film technology. [4]

Steps

A typical thick-film process would consist of the following stages:

Lasering of substrates

Typically thick film circuit substrates are Al2O3/alumina, beryllium oxide (BeO), aluminum nitride (AlN), stainless steel, sometimes even some polymers and in rare cases even silicon (Si) coated with silicon dioxide (SiO2)., [5] [6] Commonly used substrates for thick-film processes are 94 or 96% alumina. Alumina is very hard and lasering of the material is the most efficient way to machine it. The thick-film process is also a means of miniaturization, where one substrate normally contains many units (final circuits). With lasering it is possible to scribe, profile and drill holes. Scribing is a process where a line of laser pulses is fired into the material and 30–50% of the material is removed; this weakens the substrate, and after all other processes are completed the substrate can easily be divided into single units. Profiling is, for example, used a lot in sensor fabrication, where a circuit needs to fit round tubes or other different complex shapes. Drilling of holes can provide a "via" (conductive link) between the two sides of the substrate, normally hole sizes are in the range 0.15–0.2 mm.

Lasering before processing the substrates has a cost advantage to lasering or dicing using a diamond saw after processing.

Ink preparation

Inks for electrodes, terminals, resistors, dielectric layers etc. are commonly prepared by mixing the metal or ceramic powders required with a solvent (ceramic thick film pastes) or polymer pastes [7] to produce a paste for screen-printing. To achieve a homogeneous ink the mixed components of the ink may be passed through a three-roll mill. Alternatively, ready-made inks may be obtained from several companies offering products for the thick-film technologist.

Screen-printing and its improvements

Screen-printing is the process of transferring an ink through a patterned woven mesh screen or stencil using a squeegee. [8]

For improving accuracy, increasing integration density and improving line and space accuracy of traditional screen-printing photoimageable thick-film technology has been developed. Use of these materials however changes typically the process flow and needs different manufacturing tools.

Drying/Curing

After allowing time after printing for settling of the ink, each layer of ink that is deposited is usually dried at a moderately high temperature of 50 to 200 °C (122 to 392 °F) to evaporate the liquid component of the ink and fix the layer temporarily in position on the substrate so that it can be handled or stored before final processing. For inks based on polymers and some solder pastes that cure at these temperatures, this may be the final step that is required. Some inks also require curing by exposure to UV light.

Firing

For many of the metal, ceramic and glass inks used in thick film processes a high temperature (usually greater than 300 °C) firing is required to fix the layers in position permanently on the substrate.

Abrasive Trimming of resistors

After firing the resistors can be trimmed using a precision abrasive cutting method first developed by S.S. White. [9] The method involves a fine abrasive media, usually 0.027 mm aluminum oxide. The abrasive cutting is fed through a carbide nozzle tip that can be of different sizes. The nozzle is advanced through the fired resistor while the resistor element is monitored with probe contacts and when final value is reached the abrasive blast is shut off and the nozzle retracts to the zero start position. The abrasive technique can achieve very high tolerances with no heat and no cracking of the glass frit used in the ink formulation.

Laser trimming of resistors

After firing, the substrate resistors are trimmed to the correct value. This process is named laser trimming. Many chip resistors are made using thick-film technology. Large substrates are printed with resistors fired, divided into small chips and these are then terminated, so they can be soldered on the PCB board. With laser trimming two modes are used; either passive trimming, where each resistor is trimmed to a specific value and tolerance, or active trimming, where the feedback is used to adjust to a specific voltage, frequency or response by laser trimming the resistors on the circuit while powered up.

Mounting of capacitors and semiconductors

The development of the SMT process actually evolves from the thick film process. Also mounting of naked dies (the actual silicon chip without encapsulation) and wire bonding is a standard process, this provides the basis for miniaturization of the circuits as all the extra encapsulation is not necessary.

Separation of elements

This step is often necessary because many components are produced on one substrate at the same time. Thus, some means of separating the components from each other is required. This step may be achieved by wafer dicing.

Integration of devices

At this stage, the devices may require integrating with other electronic components, usually in the form of a printed circuit board. This may be achieved by wire bonding or soldering.

Process control of thick film manufacturing

There are numerous steps in the thick film manufacturing, which need careful control like roughness of the substrate, curing temperatures and times of pastes, selected stencil thickness vs. paste type etc., [10] [11] Therefore number of used pastes and process steps define the complexity of the process and cost of the final product.

Designing circuits based on thick-film technology

Same or similar electronic design automation tools which are used for designing printed circuit boards can be used for designing thick film circuits. However, the compatibility of tooling formats with stencil manufacturing/manufacturer needs attention as well as the availability of the geometrical, electrical and thermal design rules for simulation and layout design from the final manufacturer.

See also

Related Research Articles

<span class="mw-page-title-main">Integrated circuit</span> Electronic circuit formed on a small, flat piece of semiconductor material

An integrated circuit or monolithic integrated circuit is a set of electronic circuits on one small flat piece of semiconductor material, usually silicon. Large numbers of miniaturized transistors and other electronic components are integrated together on the chip. This results in circuits that are orders of magnitude smaller, faster, and less expensive than those constructed of discrete components, allowing a large transistor count. The IC's mass production capability, reliability, and building-block approach to integrated circuit design have ensured the rapid adoption of standardized ICs in place of designs using discrete transistors. ICs are now used in virtually all electronic equipment and have revolutionized the world of electronics. Computers, mobile phones and other home appliances are now inextricable parts of the structure of modern societies, made possible by the small size and low cost of ICs such as modern computer processors and microcontrollers.

<span class="mw-page-title-main">Resistor</span> Passive electrical component providing electrical resistance

A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active elements, and terminate transmission lines, among other uses. High-power resistors that can dissipate many watts of electrical power as heat may be used as part of motor controls, in power distribution systems, or as test loads for generators. Fixed resistors have resistances that only change slightly with temperature, time or operating voltage. Variable resistors can be used to adjust circuit elements, or as sensing devices for heat, light, humidity, force, or chemical activity.

<span class="mw-page-title-main">Screen printing</span> Printing technique

Screen printing is a printing technique where a mesh is used to transfer ink onto a substrate, except in areas made impermeable to the ink by a blocking stencil. A blade or squeegee is moved across the screen to fill the open mesh apertures with ink, and a reverse stroke then causes the screen to touch the substrate momentarily along a line of contact. This causes the ink to wet the substrate and be pulled out of the mesh apertures as the screen springs back after the blade has passed. One colour is printed at a time, so several screens can be used to produce a multi-coloured image or design.

<span class="mw-page-title-main">Printed circuit board</span> Board to support and connect electronic components

A printed circuit board is a medium used to connect electronic components to one another in a controlled manner. It takes the form of a laminated sandwich structure of conductive and insulating layers: each of the conductive layers is designed with an artwork pattern of traces, planes and other features etched from one or more sheet layers of copper laminated onto and/or between sheet layers of a non-conductive substrate. Electrical components may be fixed to conductive pads on the outer layers in the shape designed to accept the component's terminals, generally by means of soldering, to both electrically connect and mechanically fasten them to it. Another manufacturing process adds vias: plated-through holes that allow interconnections between layers.

<span class="mw-page-title-main">Surface-mount technology</span> Method for producing electronic circuits

Surface-mount technology (SMT), originally called planar mounting, is a method in which the electrical components are mounted directly onto the surface of a printed circuit board (PCB). An electrical component mounted in this manner is referred to as a surface-mount device (SMD). In industry, this approach has largely replaced the through-hole technology construction method of fitting components, in large part because SMT allows for increased manufacturing automation which reduces cost and improves quality. It also allows for more components to fit on a given area of substrate. Both technologies can be used on the same board, with the through-hole technology often used for components not suitable for surface mounting such as large transformers and heat-sinked power semiconductors.

<span class="mw-page-title-main">Flexible electronics</span> Mounting of electronic devices on flexible plastic substrates

Flexible electronics, also known as flex circuits, is a technology for assembling electronic circuits by mounting electronic devices on flexible plastic substrates, such as polyimide, PEEK or transparent conductive polyester film. Additionally, flex circuits can be screen printed silver circuits on polyester. Flexible electronic assemblies may be manufactured using identical components used for rigid printed circuit boards, allowing the board to conform to a desired shape, or to flex during its use.

<span class="mw-page-title-main">Mixed-signal integrated circuit</span> Integrated circuit

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<span class="mw-page-title-main">Solid Logic Technology</span> IBM hybrid circuit technology introduced in 1964

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<span class="mw-page-title-main">Laser trimming</span> Electronic circuit manufacturing method

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<span class="mw-page-title-main">Electronic component</span> Discrete device in an electronic system

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<span class="mw-page-title-main">Hybrid integrated circuit</span> Type of miniature electronic circuit

A hybrid integrated circuit (HIC), hybrid microcircuit, hybrid circuit or simply hybrid is a miniaturized electronic circuit constructed of individual devices, such as semiconductor devices and passive components, bonded to a substrate or printed circuit board (PCB). A PCB having components on a Printed Wiring Board (PWB) is not considered a true hybrid circuit according to the definition of MIL-PRF-38534.

<span class="mw-page-title-main">System in a package</span>

A system in a package (SiP) or system-in-package is a number of integrated circuits (ICs) enclosed in one chip carrier package or encompassing an IC package substrate that may include passive components and perform the functions of an entire system. The ICs may be stacked using package on package, placed side by side, and/or embedded in the substrate. The SiP performs all or most of the functions of an electronic system, and is typically used when designing components for mobile phones, digital music players, etc. Dies containing integrated circuits may be stacked vertically on a substrate. They are internally connected by fine wires that are bonded to the package. Alternatively, with a flip chip technology, solder bumps are used to join stacked chips together. SiPs are like systems on a chip (SoCs) but less tightly integrated and not on a single semiconductor die.

<span class="mw-page-title-main">Printed electronics</span> Electronic devices created by various printing methods

Printed electronics is a set of printing methods used to create electrical devices on various substrates. Printing typically uses common printing equipment suitable for defining patterns on material, such as screen printing, flexography, gravure, offset lithography, and inkjet. By electronic-industry standards, these are low-cost processes. Electrically functional electronic or optical inks are deposited on the substrate, creating active or passive devices, such as thin film transistors; capacitors; coils; resistors. Some researchers expect printed electronics to facilitate widespread, very low-cost, low-performance electronics for applications such as flexible displays, smart labels, decorative and animated posters, and active clothing that do not require high performance.

<span class="mw-page-title-main">Electronic circuit</span> Electrical circuit with active components

An electronic circuit is composed of individual electronic components, such as resistors, transistors, capacitors, inductors and diodes, connected by conductive wires or traces through which electric current can flow. It is a type of electrical circuit and to be referred to as electronic, rather than electrical, generally at least one active component must be present. The combination of components and wires allows various simple and complex operations to be performed: signals can be amplified, computations can be performed, and data can be moved from one place to another.

<span class="mw-page-title-main">Integrated passive devices</span>

Integrated passive devices (IPDs), also known as integrated passive components (IPCs) or embedded passive components (EPC), are electronic components where resistors (R), capacitors (C), inductors (L)/coils/chokes, microstriplines, impedance matching elements, baluns or any combinations of them are integrated in the same package or on the same substrate. Sometimes integrated passives can also be called as embedded passives, and still the difference between integrated and embedded passives is technically unclear. In both cases passives are realized in between dielectric layers or on the same substrate.

A molded interconnect device (MID) is an injection-molded thermoplastic part with integrated electronic circuit traces. The use of high temperature thermoplastics and their structured metallization opens a new dimension of circuit carrier design to the electronics industry. This technology combines plastic substrate/housing with circuitry into a single part by selective metallization.

<span class="mw-page-title-main">Co-fired ceramic</span> Integrated circuit package made out of fired ceramic material

Co-fired ceramic devices are monolithic, ceramic microelectronic devices where the entire ceramic support structure and any conductive, resistive, and dielectric materials are fired in a kiln at the same time. Typical devices include capacitors, inductors, resistors, transformers, and hybrid circuits. The technology is also used for robust assembly and packaging of electronic components multi-layer packaging in the electronics industry, such as military electronics, MEMS, microprocessor and RF applications.

Stencil printing is the process of depositing solder paste on the printed wiring boards (PWBs) to establish electrical connections. It is immediately followed by the component placement stage. The equipment and materials used in this stage are a stencil, solder paste, and a printer.

Glossary of microelectronics manufacturing terms

References

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  9. Trimming system. S. White Company, Industrial Div.
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