# Additively Manufactured Polymers for Electronic Components
## 3 Polymers Used for AM in Electronic Applications
### Insulating Polymers
The study demonstrated that low ejection speeds lead to incomplete droplet ejection, while high speeds produce satellite droplets. Furthermore, it examined the interaction between the substrate and two types of printable inks, which created an array of isolated drops. The prints showed the occurrence of the coffee ring effect (CRE) for inks with enhanced wettability on the substrate. This phenomenon arises from a lower contact angle with the SiO\({}_{2}\) substrate, which leads to CRE and the formation of drops with larger diameters. Furthermore, when printing a square array pattern, CRE results in an accumulation of material, severely compromising the morphological quality. This underscores the need to optimise ink properties to effectively prevent the occurrence of CRE. Material agglomerations and depletions create strong inhomogeneities that negatively impact film quality and should be avoided. Finally, a bonding pattern already obtained with other conventional technologies was successfully printed with BCB ink, demonstrating the ability to print complex shapes down to 130 \(\upmu\)m. Metrology confirmed that the printed features matched the design specification (Figure 10). Figure 10: **(a,b)** Microscopic images of the bonding patterns, measuring **(a)** 5.6 mm \(\times\) 11.5 mm and **(b)** 5.6 mm \(\times\) 45.5 mm, both printed with BCB/mestilbene ink. The scale bars indicate a measurement of 2 mm. **(c)** Schematics of the bonding pattern, along with two detailed comparisons to the actual patterns printed with BCB/mestilbene ink. The Roman numerals denote the numbering of the measured distances. The printed patterns appear to adhere closely to the dimensions of the original design. Adapted and reprinted with permission from ref. [130]. Figure 9: Chemical structure of 1,3-divinyl-1,1,3,3-teramethyldissiloxane-bis-benzocyclobutene (DVS-bis-BCB). The same approach was followed using AJP technology. The effects of various printing parameters on printing times and pattern morphology were studied to achieve high printing speeds of 10 mm/s and to ensure uniform line morphology [131]. Although some overspray was consistently present in all prints, optimal printing parameters were identified, reducing printing time while maintaining quality, and allowing large depositions of 3 cm \(\times\) 3 cm. The study also demonstrated the printing of square patterns with thicknesses ranging from 1 \(\upmu\)m to 43 \(\upmu\)m and intricate geometries, including interdigitated lines and features as small as ~60 \(\upmu\)m, using AJP technology. Another dielectric polymer widely used in the electronics industry is the epoxy-based resin SU-8. Compared to BCB, SU-8 has a relatively higher dielectric constant (k \(\approx\) 3.0-3.2) and a breakdown strength of about 4-6 MV/cm, which ensures electrical insulation in MEMS and biosensor platforms but may present some limitations in high-density devices [132]. It is preferred for protecting electrodes against ionic solutions, which can lead to corrosion, and for ensuring stability for low-cost electronic components, such as electronic biosensors used in direct contact with ionic solutions. However, SU-8 is typically processed using technologies available in cleanrooms, thus increasing device costs and complicating the fabrication process. To reduce fabrication expenses, the AJP of SU-8 employing ultrasonic aerosolisation for electronic components was demonstrated, as shown in Figure 11. This study showed that SU-8 can be printed without reformulation and effectively acts as a passivation layer for conductive silver lines when exposed to ionic solutions. Enlarging the printed SU-8 film by 100 \(\upmu\)m beyond the electrodes, the leakage current was reduced by six orders of magnitude while maintaining stability over 20 voltage sweeps. The optimal post-printing cure time was 15 min at 160 \({}^{\circ}\)C, further minimising leakage current [133].