**4. Conclusions**

Here, we examined the properties of films containing copper nitride, copper(II) oxide, and copper(I) oxide for their suitability as wiring inks for IPL sintering-based printed circuit board production. The following was clarified: Among the copper compounds examined, copper nitride had the second highest light absorption and lowest decomposition temperature, suggesting that it is suitable for sintering by IPL irradiation. Liquid ink containing copper nitride had a copper conversion ratio of 0.96 at an irradiation energy of 16.6 J cm−2. The sheet resistance of the film was on the order of 10−<sup>1</sup> Ω sq−1, which was comparable to that of a film made from liquid ink containing copper nanoparticles. To improve the mechanical strength of the film, paste inks containing ethyl cellulose and PGPTMS as a binder and adhesion compound, respectively, were prepared. The optimum amount of PGPTMS was 1 wt %, and a film made from a paste ink containing the optimized vehicle and copper nitride and irradiated at 8.30 J cm−<sup>2</sup> had a sheet resistance of 5.06 × <sup>10</sup>−<sup>1</sup> <sup>Ω</sup> sq<sup>−</sup>1. Together, the present results indicate that copper nitride is a suitable material for the development of wiring inks sintered by means of IPL irradiation.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2079-4991/8/8/617/s1, Figure S1: schematic images for preparing test film of (a) liquid ink and (b) paste ink, Figure S2: XRD patterns of samples for copper nitride, copper(II) oxide, and copper(I) oxide after IPL sintering, Figure S3: appearance of samples prepared from liquid ink including CuO before and after IPL sintering, Figure S4: schematic image for forming hollow particles, Figure S5: XRD patterns of different vehicles including Cu3N films after IPL sintering, Figure S6: appearance of a sample film after IPL sintering front side and back side, Figure S7: XRD patterns of sample films including CuO and Cu2O after IPL sintering, Figure S8: cross-section SEM image of the sample sintered by IPL at 8.30 J cm−<sup>2</sup> of irradiation energy.

**Author Contributions:** Conceptualization, T.N.; Methodology, T.N., S.U. and M.Y.; Validation, T.N. and H.J.C.; Formal Analysis, T.N. and H.J.C.; Investigation, T.N., H.J.C. and S.U.; Resources, T.N., H.J.C., M.T., S.U. and M.Y.; Data Curation, T.N. and H.J.C.; Writing—Original Draft Preparation, T.N.; Writing—Review & Editing, T.N.; Visualization, T.N.; Supervision, T.N.; Project Administration, T.N. and M.T.; Funding Acquisition, T.N. and M.T.

**Funding:** This work was financially supported by a grant from the Adaptable and Seamless Technology Transfer Program (A–STEP) through the Target-Driven R&D Program of the Japan Science and Technology Agency, JST (no. AS2621364M).

**Acknowledgments:** This research is supported by A–STEP of JST.

**Conflicts of Interest:** The authors have no conflicts of interest directly relevant to the content of this article.
