Thick Film Multilayer: Screen Printing, Firing, and Layer Architecture

Introduction to Thick Film Technology

Thick film is a mature, cost-effective method for manufacturing hybrid microelectronic circuits on ceramic substrates. The technology uses screen printing to deposit metallic and dielectric pastes onto a substrate, followed by a firing step that sinters the paste into a durable, electrically conductive or insulating layer.

Modern multilayer thick film technology enables complex circuits with six or more conductor levels separated by dielectric insulation layers, embedded resistors, and through-substrate vias. The technology is widely used for military hybrids, automotive electronics, RF modules, and consumer electronics -- wherever a combination of reliability, custom circuit design, and moderate-to-high volume manufacturing is required.

Paste Chemistry

Conductor Pastes

Thick film conductor pastes consist of three primary components: metallic powder (the functional phase), glass frit (the binder phase), and organic vehicle (the carrier phase for printing). Metallic powder options include gold (Au), palladium-silver (Pd-Ag), platinum-gold (Pt-Au), and copper (Cu). Gold conductors offer the best oxidation resistance, highest conductivity, and best wire bonding compatibility, but at the highest cost. Pd-Ag conductors are lower cost but subject to silver migration under humid bias conditions. Copper conductors offer the lowest resistivity but require nitrogen atmosphere firing to prevent oxidation.

Resistor Pastes

Thick film resistors use ruthenium-based pyrochlore compounds (RuO2, Bi2Ru2O7) as the resistive phase, combined with glass frit and organic vehicle. After firing, resistor values range from approximately 10 ohms/square to 10 megohms/square, depending on the resistor paste system and the geometry of the printed resistor pattern. Trimming is typically required to achieve precise resistance values.

Dielectric Pastes

Dielectric pastes are used as insulation layers between conductor levels in multilayer thick film circuits. They must have high dielectric strength, low dielectric constant (particularly for RF applications), and be compatible with the conductor firing profile. Common dielectric systems include borosilicate glasses and lead borosilicate glasses fired at temperatures matching the conductor paste.

Screen Fabrication and Mesh Counts

Screens are fabricated by stretching a stainless steel or polyester mesh onto an aluminum frame, then coating the mesh with a photosensitive emulsion. An artwork film (the circuit pattern) is placed on the coated screen and exposed to UV light, hardening the emulsion everywhere except where the artwork is opaque. The unexposed emulsion is washed away, leaving open areas where paste will pass through during printing.

Mesh count (threads per inch) determines the maximum printable feature size and the deposited film thickness. Higher mesh counts (400-500 mesh) produce thinner, finer-resolution deposits for fine-line conductor patterns. Lower mesh counts (200-325 mesh) produce thicker deposits for bulk conductor or resistor layers. Emulsion thickness also affects deposit thickness. For controlled thickness resistors, it is common to use a 280-325 mesh screen with controlled emulsion overmesh (EMO).

Print Parameters

Squeegee Speed and Pressure

The squeegee speed and pressure must be optimised together. Speed determines the fill time of the screen openings; pressure determines the snap-off gap and the force maintaining screen-to-substrate contact. Typical squeegee speeds are 100-200 mm/s for general conductor printing and 50-150 mm/s for fine-line or resistor printing. Squeegee pressure is typically 0.2-0.4 MPa (30-60 psi) for a 200-250 mm squeegee blade.

Snap-Off Distance

The snap-off distance -- the gap between the screen and substrate after the print stroke -- must be set to ensure complete paste release from the screen without deforming the printed pattern. Typical snap-off distances are 1-2 mm for standard prints and 0.5-1 mm for fine-feature prints. Excessive snap-off distance causes uneven paste release and registration errors.

Drying and Firing Profiles

Drying

After printing, the organic vehicle must be removed before firing. Drying is typically performed in a forced-convection oven at 80-150 degrees C for 10-20 minutes. Inadequate drying causes outgassing during the firing cycle, resulting in blisters, pinholes, or cratering in the fired film.

Firing Profile

Thick film firing is performed in a belt furnace with controlled atmosphere (air, nitrogen, or forming gas). The firing profile typically includes: Ramp 1 20-50 degrees C/min to approximately 350-450 degrees C (organic vehicle burn-off zone, requiring oxidising atmosphere); Ramp 2 30-80 degrees C/min to the peak firing temperature (500-850 degrees C depending on paste system); Dwell 5-15 minutes at peak temperature for glass frit softening and metallic particle sintering; and Cool-down 20-60 degrees C/min to below 200 degrees C before removal.

Firing Atmosphere

Gold, Pd-Ag, and Pt-Au conductor pastes are typically fired in air or low-oxygen atmosphere. Copper pastes require nitrogen atmosphere to prevent copper oxidation. Some resistor paste systems require controlled oxygen atmosphere to achieve the correct resistor value by managing the oxidation state of the ruthenium compound during firing.

Layer Registration and Alignment

In multilayer thick film circuits, each subsequent conductor layer must be precisely registered to the previous layers. Misregistration causes circuit shorts or opens and reduces manufacturing yield. Registration is achieved by using fiducial marks -- cross-hair or target patterns printed on the substrate at the same time as the first conductor layer -- as alignment references for subsequent print layers. Vision systems on modern thick film printers automatically locate the fiducial marks and correct the screen-to-substrate alignment before printing. Typical registration accuracy targets are +/- 25-50 micrometres for standard multilayer and +/- 10-25 micrometres for high-density multilayer.

Resistor Trimming

Thick film resistors are trimmed to their final value using laser ablation. The laser removes a portion of the resistor geometry, increasing the effective current path length and raising the resistance. Two-point trim is the most common method: the laser cuts a kerf across the resistor between two terminals. Four-point trim (used for high-precision resistors) adds a second cut parallel to the first, creating a narrow constriction that minimises sensitivity to contact resistance. Typical trim precision is +/- 0.5-1% for two-point trim and +/- 0.1-0.2% for four-point trim.

Via Formation and Conductor Routing

Vias (interlayer connections) are formed in thick film multilayer circuits by either via-fill or via-over-via methods. In via-fill, a separate dielectric paste is printed and fired to form an insulating layer with holes that are later filled with conductor paste. In via-over-via, the conductor paste fills the gap by pressure without a pre-formed via opening. Via diameters in thick film multilayer range from 100-500 micrometres. Via resistance must be characterised and controlled as part of the design validation.

Process Control and Inspection

Thick film process control uses statistical process control (SPC) to monitor key parameters including: fired conductor linewidth, fired conductor thickness, sheet resistance of conductor test patterns, and resistance of resistor test patterns. Lot acceptance testing includes cross-section analysis of multilayer structures to verify layer registration, dielectric coverage, and via fill quality.

Common Defects and Root Causes

Thick Film vs. Thin Film Comparison