Thick Film Process
A ceramic material, typically 96% Alumina,
(Al203) or aluminum nitride (AIN)), is utilized as the
underlying substrate upon which circuitry is developed. A pattern is formed in
an additive process, layer by layer, using successive printing through the
screen printing processes ("silk screening"). Thick film pastes (inks) form
insulators, overglazes, conductors, capacitors, resistors, crossover
connections, and multilayer-interconnect structure.
Figure 1 Flow Chart for thick film printing
Solid, or plated, through holes or wraparounds connect the top and bottom surface to high frequency ground planes, interconnect or for thermal management requirements.
A thick film is defined by MIL-STD-883 as greater than 5u thick and typical thickness, for thick film conductors is 12-15u thick.
Screen-printing is the process of depositing ink through a photographically patterned woven mesh screen or stencil using 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, each layer of ink that is deposited is usually dried 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.
Conductor metals (silver, gold, palladium), and glass inks require a high temperature (usually 850C minimum for 10 minutes) 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, the substrate resistors (which are purposely designed ~25% lower in value) are trimmed 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)
Large substrates are printed 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, ruggedness and low cost, thick film circuits are used primarily in applications characterized by harsh environments. Some applications, include downhole and high temperature electronics as well as automotive, Aerospace and Satellite applications (See Figure 4)
Figure 2: Screen Printing Dynamics
Figure 3: Laser trim of thick film resistors
Figure 4: Typical military thick film hermetic hybrid (~1.25 x ~ 2.0 inches)