# Additively Manufactured Polymers for Electronic Components
## 1 Introduction
However, certain issues are raised at times, such as a lower level of resolution and lower fabrication speeds, which are relevant for industrial purposes, compared to more conventional lithographic technologies. There are several types of AM technologies. Among these, vat photopolymerisation is an AM process in which the curing of a liquid photopolymer selectively occurs by light-based irradiation. Thus, a light source, e.g., a laser, selectively targets the liquid resin, solidifying it layer by layer [22]. There are two main types of vat photopolymerisation technologies, precisely, stereolithography (SL) and digital light processing (DLP) (Figure 1a) [23]. The former uses a laser that selectively irradiates the polymeric resin and cures the material to fabricate the desired object layer by layer. This process works by triggering a chemical reaction, which can be either free radical or cationic polymerisation, within a light-sensitising resin. When exposed to light, small reactive molecules in the resin link together to form a solid, cross-linked structure, allowing the object to take shape one layer at a time. The main advantage of this technology is a relatively higher resolution, defined by the laser spot size, compared to other AM technologies. Limitations include the limited fabrication speed and the rheological characteristics of the material, i.e., the viscosity, which cannot be too high to allow reflow under the printing platform. The latter instead employs a digital mask to irradiate the polymeric resin. The digital mask has the shape of every single layer to be printed. The entire surface of a layer is exposed at once thanks to a dynamic projection system (often based on micromirror arrays). It is thus possible to drastically increase the fabrication speed since the curing of a single layer occurs with a single irradiation. The resin cures at once across the illuminated pattern, following the same basic chemical reaction principles. However, the resolution is much more limited because it depends on the resolution of the digital mask. The powder bed fusion process consists of local melting or sintering of a powder of thermoplastic polymeric materials (Figure 1b) [23]. A roller uniformly distributes the polymer powder across the building platform. Subsequently, a laser selectively heats the powder, facilitating bonding of the particles through melting or sintering processes [24,25]. Here, the laser acts like a precise heat source, melting or partially melting the surfaces of the powder particles. As the particles cool, they fuse to form a solid layer. The exact mechanism, which implies melting versus sintering, depends on the material and temperature, but, in both cases, the key is achieving enough heat to enable inter-particle bonding without degrading the polymer. Inkjet printing (IJP) is an AM technology based on the ejection of ink droplets through a nozzle. It is broadly categorised into two groups, precisely, continuous inkjet (CII) and drop-on-demand (DoD) inkjet. In CIJ systems, droplets are continuously ejected using an electric field and a charging electrode. While DoD printing systems eject droplets using a voltage waveform with piezoelectric or thermal inkjet methods, each is suitable for various printing resolutions, materials, and applications [26]. Piezoelectric inkjet methods use a mechanical structure known as a piezoelectric inkjet printhead (PIP) (Figure 1c) [27]. This device has an ink chamber connected to an ink cartridge through a restrictor. A piezoelectric membrane at the chamber's top pressurises the ink with an electrical pulse, increasing fluid velocity at the nozzle to form droplets. Instead, in a thermal inkjet printing (TIJ) printhead, an electrical pulse heats the fluid in the resistor, creating a vapour bubble that pushes the fluid through the nozzle to form droplets [22,28,29]. In both cases, the goal is to generate small pressure changes inside the printhead that force tiny ink droplets with a volume of a few picolitres out of the nozzle.