On 96% alumina substrates for engine-control hybrid modules, the copper-nickel alloy paste, electronic/EL grade, is pre-conditioned at 22 ± 2 °C for 8 h to erase thixotropic shear history before screen printing. The metal fraction consists of gas-atomized Cu-Ni flake with a mass ratio of 80:20, D50 of 1.5–2.0 µm, and tap density of 3.8–4.2 g/cm³. The inorganic binder is a 6.0–7.0 wt% lead-free bismuth silicate glass with softening point 565 °C; the organic vehicle is terpineol, ethyl cellulose, and dibutyl phthalate at 10:3:1 by mass. Once dispersed on a three-roll mill with back pressure 0.4–0.6 MPa, the paste reaches a fineness of grind below 12 µm on a Hegman gauge and a viscosity of 180–220 Pa·s at 10 s⁻¹ and 25 °C. The paste is not specified for pure air firing above 600 °C, because copper oxide enrichment raises sheet resistivity beyond acceptable limits for wire-bonded hybrid layouts.

Screen printing uses 280-mesh stainless steel fabric with 25 µm emulsion thickness, a 45° polyurethane squeegee, and snap-off of 0.8 mm; wet film thickness is 35–40 µm, and line resolution remains within ±15 µm after drying at 150 °C for 10–12 min. The belt furnace profile ramps at 30 °C/min to a peak of 850 ± 5 °C, holds for 10 min, and cools at 40 °C/min; the atmosphere is nitrogen with residual oxygen held between 3 ppm and 10 ppm. Residual oxygen above 20 ppm produces a copper-rich oxide skin that raises sheet resistivity by 12–18% and lowers wire bond pull strength by approximately 15%.

After firing, film thickness is 14–16 µm and sheet resistivity is 25–33 mΩ/square at 25 µm normalized thickness per ASTM B193-20. Pull adhesion on 96% alumina is 18–22 N/mm² when tested per ASTM F2675-19; solderability after 8 h steam aging meets IPC J-STD-002D category 2. The resulting conductor is used as die-attach and wire-bond pads for silicon power transistors in automotive ignition modules. RoHS 2011/65/EU Annex II limits are met for the finished conductor; REACH EC 1907/2006 SVHC screening reports no candidate substances above 0.1 wt%. The paste is not specified for direct immersion soldering above 260 °C for more than 10 s, because glass frit dissolution at the solder interface reduces pull adhesion below 15 N/mm².

What Limits Silver Migration in Screen-Printed Wearable Electrodes?

Replacing silver-loaded paste in single-use electrocardiogram monitoring patches, the Cu-Ni alloy paste suppresses dendritic growth under 85 %RH and 37 °C bias. The alloy flake loading is 55–65 wt% in a solvent-borne vinyl copolymer resin; D50 is 1.2–1.6 µm, and the Ni:Cu ratio is 85:15. Butyl carbitol acetate and cyclohexanone at 4:1 by mass dilute the paste to a viscosity of 22–28 Pa·s at 10 s⁻¹ for 200-mesh polyester screens. Wet film deposits at 25–30 µm on 75 µm thermoplastic polyurethane film. No sintering occurs during cure, so the paste remains compatible with low-glass-transition polymer substrates.

Cure is performed in forced-convection ovens at 125 °C for 25 min; volume resistivity remains 2.5–4.0 × 10⁻⁴ Ω·cm measured by four-point probe per IEC 60468. Sheet resistance is 80–120 mΩ/square/mil. Mandrel bend testing at 2 mm radius per ASTM D522/D522M-17 shows less than 10% resistance increase after 10,000 cycles. Cytotoxicity is evaluated per ISO 10993-5:2009 using L929 cells; extraction from cured electrodes reports cell viability above 90%. Because the alloy is not selectively leached in chloride-containing gel, no dendrite formation is observed during 24 h electrochemical migration testing at 5 V and 85% RH per IPC-TM-650 Method 2.6.14.1. Prolonged immersion in isopropyl alcohol beyond 10 min causes resin swelling and more than 15% resistance drift; the paste is therefore not specified for solvent-cleaning processes without immediate drying. The finished electrode assembly is integrated into flexible printed biosensor patches for continuous cardiac monitoring. REACH EC 1907/2006 and RoHS 2011/65/EU Article 4 apply; the paste contains no elemental mercury, lead, cadmium, hexavalent chromium, PBB, or PBDE.

Diluted to a viscosity of 45–55 Pa·s at 10 s⁻¹, the paste is dispensed with a pneumatic auger valve at 0.25 MPa nozzle pressure and 22 G needle diameter. The trace width is 0.6 mm with 0.3 mm height after two passes; the alloy loading is 70–75 wt% Cu-Ni flake with D50 2.0–2.5 µm in an epoxy-silicone hybrid resin. Thermal cure at 140 °C for 30 min yields volume resistivity 1.8–2.5 × 10⁻⁴ Ω·cm and Shore A hardness 68–72. Shielding effectiveness of the cured trace, measured per ASTM D4935-18 from 100 MHz to 1.5 GHz, is 55–62 dB; at 22–28 GHz using flange-type waveguides, average attenuation is 42–48 dB. The paste is applied over a 5 µm nickel-phosphorus adhesion layer on the LCP surface; without this layer, cross-cut adhesion per ISO 2409:2020 falls from class 0 to class 3. The terminal product is a module-level shield for beamforming antenna arrays, where the Cu-Ni alloy replaces silver-coated copper to reduce galvanic mismatch with nickel-plated aluminium housings. Repeated reflow cycles above 260 °C at module level reduce shielding effectiveness by 6–8 dB due to alloy oxidation at the trace surface. REACH Annex XVII nickel release restrictions are out of scope for this sealed module; UL 94 V-0 flame classification is obtained for the cured resin system.

Zinc Oxide Varistor Silver-Palladium Replacement in Nitrogen-Fired Terminations

For zinc oxide multilayer varistor bodies sintered at 1,150–1,200 °C, the termination paste consists of 68–72 wt% Cu-Ni alloy powder with Ni:Cu ratio 75:25, D50 1.0–1.5 µm, and 4–6 wt% zinc-bismuth-borosilicate frit. The vehicle is pine oil and polyvinyl butyral at 8:1 mass ratio. Screen printing with 250-mesh stainless steel screens across the ceramic end faces deposits 20–25 µm dry film. The parts are fired at 800 ± 10 °C for 15 min in a nitrogen tunnel with 2–8 ppm oxygen. The fused termination forms ohmic contact with ZnO grains; contact resistance is below 5 mΩ·cm² per IEC 61051-1:2019.

After solder dip at 260 °C for 5 s, adhesion force exceeds 25 N on 1206 package varistors. The replacement of silver-palladium terminations removes Ag⁺ migration paths under 85 °C/85% RH load; no insulation resistance loss is measured after 1,000 h, whereas Ag-Pd terminations typically lose two decades of insulation resistance under the same bias. Oxygen above 10 ppm during firing leads to copper-oxide segregation at the ZnO interface and increases contact resistance to 12–18 mΩ·cm². The terminal varistor assemblies are used in automotive 12 V and 24 V load-dump suppression modules. Compliance with AEC-Q200 Table 9 and RoHS 2011/65/EU is specified on the product data sheet.

When Co-Firing Temperature Exceeds 900 °C in LTCC Constraint Sintering

To achieve hermetic inner conductor continuity in Ca-Al-B-Si-O glass-ceramic tapes, the paste is blended at 70–74 wt% alloy solids with an Ni:Cu ratio of 70:30, D50 1.5–2.0 µm, and 3–5 wt% crystalline quartz filler. The organic system is terpineol and butyl stearate at 9:1 mass ratio. Vias of 150 µm diameter are filled by stencil printing, and traces are patterned by 325-mesh screens. Co-firing with the tape is performed at 900 ± 5 °C for 20 min in nitrogen with 5–15 ppm oxygen; constraint plates maintain x-y shrinkage at 0.2–0.4%, while z shrinkage is 36–40%. The fired conductor exhibits sheet resistivity 12–18 mΩ/square at 10 µm thickness and tape adhesion of 12–16 N/mm² by peel test per IPC-TM-650 Method 2.4.8.

After co-firing, the conductor is electrically continuous across 300 µm via strands with resistance below 30 mΩ per via stack. The low copper oxidation in the constrained nitrogen atmosphere avoids blistering observed in pure copper paste when oxygen surpasses 20 ppm. Residual oxygen above 15 ppm causes copper oxidation at the via wall and increases via resistance by 25–40%. The resulting multilayer packages are used in RF filters and baluns for automotive keyless entry modules at 434 MHz. The conductor meets the inner-layer resistance requirements of IPC-6012F Class 3 after co-firing. Published data for exact Cu-Ni alloy behaviour in other glass-ceramic tape chemistries is limited.

After a 70 µm dielectric isolation layer is screen-printed and cured on 0.8 mm 430 stainless steel plate, the Cu-Ni alloy paste is screen-printed as serpentine traces with 0.5 mm line width and 0.3 mm spacing. The paste contains 62–68 wt% alloy flake with Ni:Cu ratio 80:20, D50 1.0–1.5 µm, and 8–10 wt% alkali-resistant barium-zinc-borosilicate frit. The vehicle is a rosin-free ester alcohol system. Wet film thickness is 40–45 µm. Firing on the insulated steel substrate is performed in a nitrogen tunnel at 820 ± 5 °C for 12 min, with oxygen limited to 4–10 ppm.

The heater trace develops sheet resistivity 18–24 mΩ/square at 15 µm fired thickness. Temperature coefficient of resistance is 180–220 ppm/K over -40 °C to 150 °C. Thermal cycling from -40 °C to 150 °C for 2,000 cycles per IEC 60068-2-14 Test Na changes resistance by less than 5%. The completed thick-film heater is used in electric vehicle battery thermal management plates for cold-start preheating. Dielectric withstand is tested at 1,500 VAC for 60 s per IEC 60664-1. Operation beyond 150 °C under 85% RH leads to alkali-assisted glass frit leaching and heater resistance drift exceeding 10%. The printed heater assembly is supplied as a UL 94 V-0 recognized component with operating temperature class 150 °C.