3D printing of structural electronics can realize the synchronous forming of dielectric substrates and conductive circuits, and it has a broad application prospect in the manufacturing of, for example, flexible wearable and smart skins. The morphology and electrical properties of conductive circuits after sintering directly influence the in-service performance of structural electronic products. The conventional thermal sintering of conductive silver paste circuits usually requires a long period of high-temperature heating, which has a significant impact on the polymer substrate for conductive circuits and reduces their printing accuracy. Although the emerging electric sintering/laser sintering can reduce the thermal impact on the polymer substrate material, the sintered circuits still suffer from internal pores, poor structural shape consistency, and high resistivity. In this paper, we propose a “hot airflow + laser” composite sintering method, which results in sintered circuits with high densities, high consistency, and low resistivity. We hope that this composite sintering method will contribute to the fabrication of higher precision, lower resistivity conductive circuits and help to understand the relationship between sintering temperature and resistivity of conductive silver paste circuits.

The existing laser sintered conductive silver paste lines suffer from internal pores, circuit expansion, resistivity, and other issues. This study proposes a composite sintering system. A comparison test with single laser sintering is carried out. The surface morphology and internal defects of the sintered circuits are detected. Analyses of conductivity of the silver paste lines, their expansion behavior, and the elemental changes are conducted, showing a clear mechanism of inhibition of the pore defects. A test of the resistance of the circuit is performed to analyze the pattern of change of the resistance value. On this basis, the correlation among pore defects, circuit expansion, and electrical properties is established. This study provides a new method for high density, low resistivity forming of conductive silver paste lines.

In the present study, a “hot airflow + laser” composite sintering process of conductive silver paste is proposed, and a sintering system with hot airflow pre-sintering function is built (Fig. 4). Comparison experiments of laser/composite sintering with variable parameters are carried out (Table 3). The results show that, compared with the laser sintered circuits, the proposed composite sintering leads to lower electrical resistivity (Fig. 6), smaller cross-sectional area (Fig. 9), lower electrical resistance value (Fig. 15), and higher consistency (Fig. 8). The reason for the inhibition of pore defects using the composite sintering process is identified. The hot gas flow promotes the thermal decomposition process of organic substances in the silver paste and volatilizes them from the surface of the nondense silver paste. This avoids the gas volatilization from the dense surface of the laser sintered surface, thus reducing the generation of internal pore defects (Fig. 12). The composite sintering process promotes the reduction in resistance value due to the elevated silver content on the surface of the circuits after hot gas flow sintering, the increase in thickness of the dense surface layer after laser sintering, the increase in the effective transmission area of electrons, and the improvement of the electron flow efficiency (Fig. 17). Finally, the study reveals that the hot airflow promotes the decomposition of organic matter within the silver paste (Table 4), and the laser accelerates the fusion of silver nanoparticles (Fig. 18).

This study proposes a “hot airflow + laser” composite sintering process of conductive silver paste, builds a sintering system with hot airflow pre-sintering function, and carries out a laser sintering and “hot airflow + laser” sintering comparative process test. The low-temperature pre-sintering of the circuit inhibits the internal pore defects, and the electrical properties of the circuit are improved. Based on the characteristics of the organic matter ladder of thermal reaction, it is revealed that the pre-sintering temperature promotes the removal of organic material. A correlation between the organic matter residue and the generation of pore defects is established. This study provides a theoretical basis for the high-precision, low-resistivity molding of conductive silver paste. The specific conclusions are as follows:

1) The reduction in resistivity of the conductive silver paste circuit after the “hot airflow + laser” composite sintering, compared with that obtained with laser sintering, is significant. A maximum resistivity reduction of 67% is achieved.

2) Hot airflow pre-sintering before laser sintering can promote the decomposition of organic solvents within the conductive silver paste. This avoids the violent reaction produced by laser sintering of organic substances, inhibits the generation of pore defects in the conductive silver paste lines, and reduces the expansion of the conductive silver paste circuits. Compared with that obtained with laser sintering, the circuit cross-sectional area is reduced by 50%?60%.

3) “Hot airflow + laser” composite sintering improves the circuit surface layer density, increases the effective cross-section of the circuit current transmission, and reduces the circuit resistance value. Compared with that obtained with laser sintering, the circuit resistance value is reduced by 30%?45%.