Introduction to Materials and Firing Parameters in Thick Film Firing


What is a thick film process? What materials are involved? How to select pastes? What are the firing and airflow parameters.

Introduction

Thick film technology is a type of technology that uses conductive, resistive, and insulating pastes, deposited in patterns defined by screen printing and fused at high temperatures (500-1000 C) onto a ceramic substrate permanently. The thickness typically ranges from 5 to 20 ?m. The substrates are passed through a continuous belt furnace with multiple firing zones to be fired. Inside the tunnel, the parts are going through different phases including organic burnout, softening/melt, resistive compound formation, solidification, and, at last, annealing. The substrates are able to absorb energy from infrared sources because infrared furnaces can reach to high temperatures rapidly and control temperatures precisely. By using an infrared furnace, the fire rates can be increased by three to four times than using a conventional convection furnace.

Material

The most commonly used substrate materials are alumina ceramic with 94-98% alumina content. The particle sizes range from 3 to 5 ?m. Unlike thin-film technology, smooth surface finish is not required for thick-film technology, and "as-fired" ceramics are suitable. Polished ceramics or glass is acceptable. The insulation resistance of the glass is very important, including its behavior at high temperatures (500- 1000 C). Low percentages of low free alkali are required. More and more materials are being used as substrates, such as porcelainized steel, organic materials (epoxies, flexible substrates), and even synthetic diamond. Inks or pastes are added sequentially to produce required conductor patterns and resistor values on the substrate above. With different formulas, the pastes can be used to produce conductors, resistors, and dielectrics which contain binders, carriers, and materials to be deposited (typically pure metals, alloys, and metal oxides). After firing, the metal particles are bound together to the substrate by the glassy phase, and this is particularly important at the substrate-ink interface. On a microscale, fired surfaces are usually not even or homogeneous. This can make the next step-wire bonding-very difficult. The particle sizes are smaller than solder pastes, so the suspensions are more stable. The inks must be at appropriate viscosity for the screen-printing process. It must fall between solid and liquid. If the viscosity is too low, the inks will spread after printing; if the viscosity is too high, there will be mesh marks which is a failure of leveling. No matter what printing method is used, the obvious lower limits on feature sizes will make high-density designs difficult. At the same time, poor edge definition of narrow tracks will have impacts on high-frequency performance. So, some materials are developed for printing over the whole substrate area and subsequent etching.

Conductor Pastes

In hybrid circuit manufacturing, conductor pastes are the largest proportion of pastes that are used. As materials that are used to transfer signals between circuits, resistor terminations, crossover connections, discrete component attach points, wire bond attach points, and so on, conductor pastes must have low resistivity. Some other requirements are good solderability, compatibility with other components, good adhesion to the substrate, good wire bondability, cost/performance trade-off, and so on. A thick film conductor contains three functional materials: metal powders, permanent binders, and the vehicle. As for metal powders, noble metal materials include gold, silver, palladium, and platinum, and/or alloys of them, while copper is the only well-fabricated material to be used in products at this time.

Resistor Pastes

Modern resistor pastes are grounded on oxides of ruthenium, iridium, and rhenium. Compared to the earlier pastes, these materials are stable during the firing process. They also provide a better temperature coefficient of resistance.

Dielectrics Pastes

The lowest possible dielectric constant is to be preferred in order to minimize the capacitance associated with crossovers. A high dielectric constant is preferred for the fabrication of capacitors. A good paste will require high dielectric strength, good insulation resistance, and low power factor.

Paste Selection

It is very important to select a proper paste for circuits so that desirable performance could be achieved. Noble metal pastes seem to be the primary choice at this moment for reliable and excellent results. In the research area, the medium temperature base metal pastes are more popular. However, the low-cost polymer with lower performance is still suitable in many commercial products. Some factors listed below need to be considered when selecting a paste. Conductivity: From low to high sheet resistivity, there are copper, silver-platinum, gold, silver-palladium, and gold-palladium. Firing temperature: The maximum capacity of most furnaces is less than 1000 C. Hengli HSK furnace can heat up to 1050 C. Adhesion: It is the strength required to pull the conductor from the substrate. The solder joint/conductor has to carry the weight of any added chip components. Interconnection: Wire bonding and soldering are commonly used to connect fired circuits with other components.

Firing Parameters

After printing the paste on the ceramic substrates with a given circuit pattern, the substrates are leveled at room temperature of about 10 min and then dried at low temperature (150 C) for 15 min in order to remove the organic solvents by evaporation. The drying process can be achieved by passing printed ceramics through the belt furnace, so the printed ink is frozen and ready to be fired. Many firing parameters and conditions such as heating rate, belt speed, atmosphere, and material types will influence circuit properties such as film thickness, sheet resistivity, solder leach resistance, adhesion, and so on. When tuning the thick film furnace parameters, three major factors are very important to control the temperature profile: temperature, airflow, and firing time in each zone. In his study on the manufacturing process of Ag/Pd thick-film conductor circuits, Yen-Chang Tseng analyzed the influence of the three factors on sheet resistivity.

Airflow

The leading thick film material manufacturer DuPont emphasized that proper furnace setup is critical for successful high-yield thick film processing. Per DuPont, the two key parameters that control successful firing are furnace air and the firing profile.

DuPont believes that the airflow is important due to the following six reasons:

    (a) A sufficient quantity of clean, dry air must be present for thick film to fire correctly.
    (b) Intake air should be drawn from a point 6-10 ft above the roofline and as far away as possible from contaminant-containing exhausts.
    (c) Oil-free compressors should be used whenever possible.
    (d) A series of dryers and filters must be used to dry to a dew point of -30 C at 80-100 psi. Standard cooling dryers in conjunction with heated desiccant dryers will achieve this value.
    (e) Activated charcoal filters will help to remove halogen and sulfur-type contaminants.
    (f) Micron and submicron filters will remove particulate matter.

DuPont further recommends a formula to determine how much air is required in the furnace:

    V = PLAWS where:
    . V = The volume of air in liters/minute required in the burnout section of the furnace
    . P = Printed area of the densest circuit expressed as a decimal
    . L = Belt load factor. How much of the burnout section is full when parts are loaded, expressed as a decimal
    . A = Constant of 0.4 l/cm2
    . W = Belt width (cm)
    . S = Belt speed (cm/min)

The amount of air required in the firing section is 10-20% higher than for the burnout section. To set up the airflow properly:

. Turn off all flowmeters.
. Turn up the burnout air to the value calculated above.
. Turn the firing air to its value (1.1-1.2 times PLAWS).
. Check to make sure air is flowing out the entrance and exits of the furnace.
. Turn up the Venturi exhaust until room air is just being drawn into the furnace. This will ensure that all air introduced to the furnace is removed.
. Turn up the entrance and exit air curtains until flow at the exit and entrance is out of the furnace. This will prevent air from being drawn into the furnace.

Overall, DuPont's recommendations work well. Sometimes some customers complained about too high usage of dry compressed. To minimize the usage, one can develop a plan to monitor the fired product quality. Then the airflow into the furnace can be reduced gradually and still maintains the positive pressure inside the furnace, like a clean room design, to keep the furnace inside air clean. In most cases, the airflow to the entrance and exit curtains can be reduced too. The Venturi-assisted exhausts can be kept at a minimum, as long as the furnace chamber has positive pressure.

References

1. Thick film processing. http://www.furnacepros.com/Furnaces/thickfilm.php
2. Thick film technology. http://www.idc-online.com/technical_references/pdfs/electronic_engineering/Thick_film_technology.pdf
Thick_film_technology.pdf 3. Owner's manual of HSK2505-0611, Hengli thick film firing furnace.
4. Yen-Chang, T. (1995). Manufacturing process analysis of Ag/Pd thick film conductor circuits, Lehigh Preserve.
5. Taylor, B. E., Felten, J. J., & Larry, J. R. (1980). Progress in technology of Ag thick film conductors.IEEE Trans, CHMT-3(4), 504-517.
6. Naguib, H. M., & MacLaurin, B. K. (1979). Ag migration and the reliability of Pd/Ag conductors. IEEE Trans, CHMT-2(2), 196-199.
7. Roffey, M., & Metke, N. (1990). Atmospheres and their effects on air firing thick films. Hybrid Circuit Technology-7(11), 45-50.
8. Proper furnace setup in Dupont website. http://www.dupont.com/products-and-services/electronic-electrical-materials/articles/proper-furnace-setup.html