Thick Film Process

TFC’s thick film process mainly base on ceramic material, typically 96% Alumina, (Al203) or aluminum nitride (AIN), is applied as the substrate base upon which thick film circuit is developed. A pattern is formed in an additive process, layer by layer, TFC using successive printing through the screen printing processes (“silk screening”). TFC use thick film pastes (inks) to form insulators, overglazes, conductors, capacitors, resistors, crossover connections, and multilayer-interconnect structure.

TFC do screen-printing by depositing ink through a photographically patterned woven mesh screen with a squeegee (See Figure 2). The thixotropic nature of the ink does not allow it to flow through the screen until the squeegee applies pressure. The inks are deposited on the substrate and as the squeegee moves across the substrate the snap back action of the screen insures the pattern is not smeared.
After allowing time after printing for settling of the ink to occur, TFC dry each layer of ink that is deposited at a moderately high temperature (50C to 200 C) to evaporate the liquid component of the ink and fix the layer temporarily in position on the substrate so that it can be handled before final processing.
As conductor metals (silver, gold, palladium), and glass inks require a high temperature (usually 850C minimum for 10 minutes), TFC do firing in a belt furnace to fuse the layers permanently on the ceramic substrate. Resistor inks are typical fabricated with a ruthenium oxide in a wide range of sheet resistivities (~10 milliohm /square to 10 Mega ohm /square), which gives thick film technology the ability to cover a wide range of resistor values in a given circuit.
After firing, TFC trim the substrate resistors (which are purposely designed ~25% lower in value) to the correct value. A YAG laser is used to vaporize and remove material from the resistor to increase the resistor value. During laser trimming a threshold, typical 1% of value, is obtained and a feedback loop in the laser-trimming machine, monitors the resistor value, and shuts the laser off. (Figure 3)
TFC print large substrates with a step and repeat pattern so that many substrates are fabricated simultaneously to reduce the process cost. The substrate is then separated using a dicing saw or snapped apart using a pre-scored “snapstrates” which look like postage stamps.
Due to improved electrical performance, high reliability, thermal management, durability and low cost, TFC’s thick film circuits are widely used in applications with rugged environments. Some typical applications include downhole and high temperature electronics as well as automotive, Aerospace and Satellite applications (See Figure 4)

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