Hybrid microelectronics was the first major application of sintered thick-film materials and is gradually entering military and aerospace applications where reliability is critical. The internal and external electrodes of passive components have been another traditional market for thick-film technology applications. One of the main advantages of thick-film technology is that it is an additive process in which various conductors, resistors, and dielectric pastes are screen-printed, sintered, or continuously cured to form a circuit. This is a circuit manufacturing method with high processing efficiency: patterned screens are relatively easy to manufacture, allowing for flexible circuit design, while screen printing also allows for flexibility in feature sizes, from hundreds of micrometers to 30 micrometers or smaller.
The main drivers of today's electronic products are miniaturization, higher circuit density, adaptation to new forms such as 3D and flexible substrates, and ecological factors such as reduced energy consumption, waste reduction, and reduction of the use of toxic substances. Thick-film technology can address these and future challenges, and due to its reliability, flexibility, and adaptability, it has found new applications.
This article introduces the applications of thick-film technology in 5G communications, smart cars, flexible electronics, and other fields. As major trends continue to drive technological advancements, the versatility, reliability, and adaptability of thick-film technology are expected to further promote these advancements.
| I. Introduction |
By definition, thick-film technology has existed for decades or even centuries. Screen printing is the primary application method for thick-film pastes, with the earliest recorded example being the development of "screen printing" pigments in China 3,000 years ago.
Printing thin metal films onto ceramics has been proven since the 1930s, with sintered thick films developed for electronic circuits in the mid-1950s, gaining commercial appeal in the 1960s. Unsintered polymer-like thick films first appeared in a patent published in 1944, subsequently allowing electronic circuits to be printed onto temperature-sensitive substrates such as polyethylene terephthalate (PET). Since then, thick film technology has proven to be a reliable and cost-effective method for manufacturing highly reliable electronic circuits and components. Hybrid microelectronic circuits were the first major application of sintered thick film materials and have been extensively used in military and aerospace applications where harsh conditions are frequently encountered and reliability is paramount. The internal and external electrodes of passive components have been another traditional application of thick film technology.
The main drivers of today's electronic products are miniaturization, higher circuit density, higher communication frequencies, adaptation to new form factors such as 3D, flexible and stretchable substrates, and ecological factors such as reduced energy consumption, waste reduction, and reduced use of toxic substances. Thick-film technology has the capability to address these challenges, as well as other future challenges. Thick-film technology has been and will continue to be developed to advance advancements in telecommunications, aerospace, automotive, displays, consumer electronics, and medical fields.
| II. Advantages of Thick Films |
One of the main advantages of thick film is that it is an additive process in which various conductors, resistors, and dielectric pastes are continuously screen-printed and fired (or cured) to form a circuit. This provides circuit manufacturers with a cost-effective, easily scalable process and reduces the waste associated with subtractive process methods. Patterned screens are relatively easy to manufacture, allowing for flexible circuit designs, while screen printing also allows for flexibility in feature sizes, from hundreds of micrometers to 30 micrometers or smaller. The combination of screen and paste technologies with advancements in thick film reduces costs and enables miniaturization by reducing linewidth. As shown in Figure 1, Fraunhofer ISE demonstrates screen-printed 19µm silver conductor contact lines for PERC photovoltaic cells.
▲Figure 1: Cross-section of screen-printed silver conductor displayed at Fraunhofer ISE
Besides screen printing, thick-film paste printing technology can be applied to many other methods. For example, spraying, dipping, injection molding, and pad printing are common printing techniques. Gravure, flexographic, and offset printing can be used for printing on electronic products.
Proper paste formulation and process optimization ensure a matched conductor-resistor-dielectric system, allowing substrate or device manufacturers to use the same equipment to deposit continuous layers of material to produce their circuits. Thick film is a versatile technology that can be applied to a wide variety of substrates. High-temperature ceramics such as alumina (alumina) and aluminum nitride are the most common, but matched systems of steel and aluminum can also be used, allowing for the use of newer thick film heaters.
Figure 2 shows a hybrid microelectronic circuit that typically uses conductors, resistors, dielectrics, and ceramic glazes.
▲Figure 2: A typical hybrid power circuit. Image provided by Continental GmbH
Figure 3 shows a comparison of various conductor metallizations. Each metallization has its advantages and disadvantages. Gold is used for applications with the highest reliability requirements. In these applications, the high cost of gold and silver outweighs their low resistivity and excellent chemical resistance, but they exhibit poor solderability and solder leaching resistance. Silver is much cheaper, solderable, and has the highest conductivity of all metals; however, it is subject to silver migration, where silver ions migrate from one conductor to another when exposed to moisture or water, causing a short circuit.
▲Figure 3: Comparison of conductor properties
Migration can be mitigated by adding palladium or platinum, but at the cost of higher cost. While expensive, platinum offers excellent environmental resistance, making it an excellent metal for low-ohmic resistors. Its temperature coefficient of resistance (TCR) of up to 3927 ppm provides a safety margin for over-temperature applications. Copper is currently the lowest-cost conductor, conducting almost as well as silver, but its susceptibility to oxidation at high temperatures necessitates treatment and curing or firing in a nitrogen or reducing atmosphere. Copper is also prone to oxidation and environmental corrosion, requiring glazing to prevent exposure to air and moisture. The diverse properties imparted by various metal platings provide thick-film paste users with a toolbox for their specific applications.
Depending on the resistive filler and application, the range of emitter thick-film resistors extends from <0.1Ω/□ to >1.0E6Ω/□, and all the fractions in between. The dielectric paste is based on the substrate, the metallization, firing, or curing temperature used, and other key application parameters such as breakdown voltage and leakage current. This broad portfolio of solder pastes provides circuit designers with the flexibility they need to build circuits.
Polymer thick-film slurries do not require firing but are cured at low temperatures, allowing application to temperature-sensitive substrates such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyimide, and FR4. Polymer thick-film conductors are primarily based on silver. With copper beginning to appear in metallized forms, resistors based on carbon and dielectrics allow circuit designers to build complete thick-film systems for their applications.
| III. Thick Films in Current and Future Electronic Products |
A. Automobile
Automotive electronics require reliability across extended temperature ranges. Depending on the application, these ranges can extend from -40°C to 80°C, or from -55°C to 150°C. Furthermore, thick-film technology must withstand harsh conditions, such as environmental factors and vibrations generated by vehicle operation. Therefore, the components and raw materials used must reliably withstand thousands of temperature cycles, aging at 85°C and 85% relative humidity, and other stringent electrical tests. Thick-film technology has proven its reliability under these conditions over the years.
Thick-film technology has historically been used in automotive applications such as fuel/air sensors, airbag deployment sensors, exhaust sensors, ECUs, and fuel level sensors. For fuel level sensors, gasoline is particularly corrosive due to its sulfur compounds, organic acids, water, and alcohol. This necessitates the metallization of thick-film conductors with high tolerance to these compounds. Silver migration can cause reliability issues in fuel level sensors, requiring AgPd-based alloys, or AuPtPd if OEMs require silver-free alloys. With the increasing electrification of automobiles, materials used in fuel level sensors will be phased out, replaced by thick-film materials supporting the electric vehicle market, particularly in thick-film-based heaters and sensors, as well as displays and touchscreens.
Over the years, cars have become computers on wheels, equipped with various driver assistance sensors such as lane departure warning, blind spot monitoring, automatic braking, and regenerative braking. Fully autonomous vehicles are already undergoing beta testing.
More sensors are needed to support safe autonomous driving, such as lidar, radar, fused lidar, and ultrasonic sensors. Both sintered and unsintered thick-film technologies enable these new sensors.
Unburned and, in some cases, burnt thick-film greases are also used in cabin temperature control, rear window defrosters, and seat heaters; for heating sensors in cold environments; electric vehicle battery heaters; rearview mirror defrosters; LED headlight defrosters; steering wheel heaters; other interior heaters; and sensor heaters. Positive temperature coefficient (PTC) carbon resistance grease is best suited for these types of heaters because it self-adjusts to a specific temperature without additional control.
B.Communication
5G communication supports the Internet of Things (IoT) by increasing connectivity and higher data transmission rates. 5G signal filters utilize highly conductive silver coatings to achieve low loss. These are typically thick-film silver conductor pastes that have been sintered. These pastes are expected to see significant use in the future as frequencies increase. Flexible hybrid electronics are anticipated to be increasingly used in IoT applications due to their lightweight nature and potential to replace printed circuit boards.
The ceramics used in low-temperature co-fired ceramics (LTCC) are dielectric powders specifically designed for low loss in high-frequency applications and cast into magnetic tape. A photoimageable paste is emerging in LTCC applications, allowing for the creation of finer structures with high edge resolution, such as 20µm or smaller, through development and etching. These finer features are particularly suitable for high-frequency communications up to 100-300GHz, a vision shared by 6G communications.
Smartphones and other communication devices contain passive components and other circuits that require thick-film materials. A common theme for these components is that as electronics miniaturize, components must also miniaturize, increasing the demand for thick-film pastes. Case sizes as small as 006003 (150x75µm) are now available. Thick-film technology continues to address the challenges of miniaturizing these components through pastes specifically designed for finer lines and defect-free terminations.
C. Power Electronics and Renewable Energy
For decades, thick-film silver conductors have been used as front-side grid contacts in photovoltaic cells, representing the largest market for conductive inks and pastes. Aluminum thick-film pastes are used for back-side metallization in photovoltaics. Thick films have also entered the power electronics field, where not only high conductivity but also high thermal conductivity is required. Another application of thick films in power electronics is in sintered silver and copper pastes for chip bonding. These metal pastes are typically combined with assembly materials, injection-molded or stencil-printed, and then sintered at 200–250°C. This results in metal pastes that closely resemble the integral properties of silver or copper while providing excellent chip shear adhesion to the substrate. Sintered pastes offer a lead-free alternative to lead-containing solders commonly used in chip bonding.
D. Thick films in medical applications
One of the earlier applications of thick films was glucose test strips, in which a silver-silver chloride paste for the counter/reference electrode and a carbon-graphite paste for the working electrode were printed onto a polymer substrate. The first electrochemical glucose test strip on the market, manufactured by ExacTech (Figure 4), was introduced in 1988; there are now many suppliers.
▲Figure 4: ExacTech glucose sensor
Silver-silver chloride and carbon paste are also used in continuous glucose sensors. Many continuous sensors are available on the market, used in systems such as the Dexcom G6, Abbott Freestyle Libre, and Eversense CGM. While electrode compositions are proprietary, they may contain a thick film of electrode paste containing silver-silver chloride and a conductive paste for the working electrode.
There are many continuous sensors on the market, such as the Dexcom G6, Abbott Freestyle Libre, and Eversense CGM systems. Although the electrode compositions are proprietary, they may contain a thick-film electrode paste containing silver-silver chloride and a conductor paste for the working electrode.
One area for near- and medium-term growth is expected to be flexible/stretchable electronics for wearable skin patches and smart clothing. These printed electronics must be flexible enough to be stretched countless times without losing their electrical properties. Similar flexible electronic patches and clothing already exist and will find wider applications in medical monitoring and performance monitoring of sports and exercise. Polymer thick-film slurries, already used for different types of disease detection involving blood samples, will also see wider application in the medical field.
E. Printed Electronics – A New Form Factor
In-mold electronics (IME) is a technology that has been around for many years, but it is finally finding increasing application in automobiles, white goods, and consumer electronics. Integrating electronics into molded plastic panels reduces weight, provides a simpler assembly process and lower manufacturing costs, while offering excellent interior aesthetics. The thick-film slurry used has been developed to withstand the thermoforming process, which uses pressure and temperatures above the polymer's softening point.
Beyond medical and motion monitoring, flexible electronics also enable foldable/bendable displays and flexible touchscreens. Curved displays are already on the market and are a feature of IME (Integrated Device Engineering). Flexible curved touchscreens have already appeared in luxury cars and are expected to proliferate in the future.
F. Thick film in smart packaging
According to Transparency Market Research, thick-film-based printed electronics enable smart packaging technologies such as prescription reminders, freshness indicators, temperature indicators showing whether a product has been exposed to abnormally high or low temperatures, and other product information. Temperature-controlled films are used to coordinate the transportation and logistics of temperature-sensitive products.
Since the early 2000s, printed silver antennas for RFID tags have been a focus of attention in printed electronics. However, smart tags with silicon electronics and supporting traditional circuitry are still used for inventory control of goods, as well as RFID passes for high-value items and vehicles. Many industry insiders argue that RFID tags must cost a penny or less to be used in packaging, and the currently used graphic barcodes are practically free. The industry has largely shifted to other applications, but fully printed RFID tags with antennas and organic transistors remain in the background.
G. Thick film heater
More demanding performance requirements and new, unique form factors are expanding the use of thick-film materials in new heater applications. The latest thick-film heaters are thinner, can be designed into any shape, and offer greater reliability, opening up new markets for customers to enhance their competitiveness. Thick-film technology provides excellent heating uniformity for devices with matched material systems on a variety of substrates, including alumina, aluminum nitride, and various grades of aluminum and steel substrates. This gives heater manufacturers the flexibility to design heaters over a wide temperature range.
As reported by Mordor Intelligence, steel thick-film heaters promise to provide new types of heaters for a wide range of applications. Circuits manufactured using this technology can operate at temperatures up to 400°C and are suitable for various automotive applications. These devices are made by bonding a thick ceramic dielectric glaze to stainless steel, with resistive material printed on it, and depending on the application, a glaze layer can also be applied. These heaters can be soldered, and holes can be added for mounting.
H. Passive components
Every electronic circuit requires resistors, capacitors, inductors, and other components. Thick-film solder paste is widely used in passive devices, employing a variety of printing methods: screen printing of internal electrodes, dip-coating of external electrodes, and spraying. The pressure for miniaturization and cost reduction demands that manufacturers possess high-capacity, efficient manufacturing processes to gain a competitive edge, enabling them to print and apply smaller electrodes and handle smaller parts. This necessitates solder paste with sufficient raw adhesive strength to handle customers' manufacturing processes.
Most capacitors are based on base metals, but some high-reliability and power capacitors, as well as other types of passive components, use precious metals to avoid oxidation and potential environmental corrosion in harsher applications. Precious metal pastes based on silver, silver-palladium, and silver-platinum-palladium are used in piezoelectric ceramic components, resonators, filters, capacitors, resistors, rheostats, and inductors. Flexible silver termination materials are particularly suitable for vibration-sensitive applications, such as automotive circuits.
| IV. Conclusion |
Thick-film technology continues to demonstrate its resilience in keeping up with current major trends and anticipating the future development of electronics. From intelligent connected vehicles and other forms of transportation to nascent 6G communications, and the ongoing growth of the Internet of Things and the required connectivity, thick-film remains the preferred technology manufacturing process for achieving high reliability and ease of integration.
Medical sensors and monitoring will continue to be strong markets for thick film materials, as will aerospace, displays, and various thick film heaters serving numerous heating applications.
