Phase 2: Nickel Screen Printing Study
Since the most common practical approach to formulating is to modify an existing mixture, a viscosity adjustment was made to an existing HPN-DEV nickel flake paste following the procedure published by Faddoul et al. in their paper, “Optimization of Silver Paste for Flexography Printing on LTCC Substrate,” in Vol. 52, Issue 7 of Microelectronics Reliability, to achieve a rheological profile similar to the known inks that print well on the Harper QD flexography proofer (Figure 6).
A 200 cpi/14 bcm anilox was used to print 100 percent and 90 percent solid patches on 144-lb. International Paper 10-pt. C2S cover paper. Butyl carbitol (Sigma Aldrich, 579963) was used as a diluent and a Cowless Model CM-100 high shear lab dissolver was used for dispersing the dilution. UV-vis spectroscopy measurement was conducted to see the light absorption behavior of the nickel particles on a SpectraMax Plus spectroscopy.
The sample was examined in ethanol solution. Particle size reductions were conducted by Fujifilm on both a sonic disrupter and a microfluidizer. Particle sizes were measured before and after the reduction process with an FE-SEM imaging system. The sample for FE-SEM was prepared by placing it in about 25-ml. of nanopure water and placing it in a sonicator for 15 minutes to form a suspension. Then, a drop of the sample was placed on acetate film, dried and analyzed using the FE-SEM imaging system.
For comparison purposes, NovaCentrix HPN-DEV nickel flake paste was screen printed. A screen frame with 165 mesh size was prepared, including 1.5-in. by 1.5-in. squares (Figure 7) and printed on the both aforementioned cover paper and DuPont Melinex ST506 PET substrate.
The PulseForge was used for sintering printed samples at 450-v., once through mode, at a 20 fpm web speed with two overlap (Figure 8). Pulse length was varied starting from 1,000-μ. and increased proportionally to be able to find optimum sintering condition. A Keithley 2400 four point probe sourcemeter was used to measure sheet resistivity of the samples.
Phase 3: Sythesis & Analysis of Ni Nanoparticles
The synthesis reaction was carried out using a Turkevich based method to obtain monodisperse nanoparticles using nickel nitrate in ethanol solution. Hexyl sulfide was used as a ligand at room temperature. The targeted particle size was 8-nm. and minimum solid content level was 15 percent to 20 percent.
Nanoparticles were characterized by size on a Nicomp Submicron Particle Sizer Model 370, light absorption behavior on the aforementioned UV vis spectroscopy, and solid content levels on a TGA Q500 thermogravimetry analyzer.
Phase 4: Formulation
Both 20-nm. and 40-nm. nano nickel particles were purchased from US Research Nanomaterials, Inc. An FTA200 (First Ten Angstroms) system with a syringe size of 0.9-mm. was used to measure surface tensions of both commercial graphical and conductive silver ink, as well as surface energy of the substrate. Three different surfactants (Triton X-100, T-Det N4 and Dylon 604) were used to study the effect of surfactant addition to the CXT-0346 conductive silver ink. A Computrac LX-10 moisture analyzer was used to check the solid level of the inks.
Before any addition of the surfactants, the ink was diluted down to 62 percent with deionized water (DI) to reach the original solid content level reported on the MSDS form. The dynamic contact angle measurement of the substrate was measured by using a polar liquid (DI water)
and a non polar liquid (methylene iodide, or MI) to calculate its surface energy with the Owens-Wendt formula. Ink formulation ingredients and their function can be seen in Table 1. Four different binders (titled B#1, B#2, B#3 and B#4) were used for the ink formulation and their effect on sheet resistivity was measured on the aforementioned four point probe sourcemeter.
A #12 Meyer rod was used for drawdown of the ink formulations. Samples were calendered at 500 pli with no heating against soft roll after the sintering. Sheet resistivity measurements were collected before and after calendering. Print trials were made on Harper’s QD flexography proofer with the aforementioned settings. Images of the printed surface were taken with Bruker non contact three dimensional (3-D) optical metrology.
Before formulating any kind of graphical or conductive inks, it is essential to understand how they flow or deform, and how they respond to changes in both shear rate and temperature.
Rheology helps generate a curve to analyze and characterize these changes, as well as changes in the concentration and type of pigments, binders, vehicles and additives.
Rheology curves of known commercial inks can be used as references for new formulations. Once a formulated ink displays a similar curve, it can be called successful. Similar to having a proof of a job before the real print production, a rheology trial can simulate how the ink will run in the press and if there is a need for alterations. These trials can help to save materials, time and money.
To recap the first half of the study, nano nickel pigments were used in the formulation of a conductive ink as an alternative to silver, gold and copper for certain printed electronics applications. The rheology of known flexo and screen inks was studied to compare with the formulation. A micron sized commercial nickel ink was screen printed on a paper and a PET to use as a reference for electrical characterization and its particle size and viscosity were optimized for flexo printing trials. Before purchasing the commercial nano nickel powder pigments for the formulation trials, a Turkevich based synthesis method was used to experience producing the nano nickel solution.