Additive Manufacturing 25 (2019) 477–484
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Additive Manufacturing 25 (2019) 477–484 Contents lists available at ScienceDirect Additive Manufacturing journal homepage: www.elsevier.com/locate/addma Reactive material jetting of polyimide insulators for complex circuit board T design Fan Zhang, Ehab Saleh, Jayasheelan Vaithilingam, You Li, Christopher J. Tuck, ⁎ Richard J.M. Hague, Ricky D. Wildman, Yinfeng He Faculty of Engineering, University of Nottingham, University Park, NG7 2RD, Nottingham, United Kingdom ARTICLE INFO ABSTRACT Keywords: Polyimides are a group of high performance thermal stable dielectric materials used in diverse applications. In 3D print this article, we synthesized and developed a high-performance polyimide precursor ink for a Material Jetting Additive manufacturing (MJ) process. The proposed ink formulation was shown to form a uniform and dense polyimide film through Printed electronics reactive MJ utilising real-time thermo-imidisation process. The printed polyimide film showed a permittivity of Polyimide 3.41 and degradation temperature around 500 °C, both of which are comparable to commercially available Inkjet polyimide films. Benefiting from the capability of being able to selectively deposit material throughMJ,we propose the use of such a formulation to produce complex circuit board structures by the co-printing of con- ductive silver tracks and polyimide dielectric layers. By means of selectively depositing 4 μm thick patches at the cross-over points of two circuit patterns, a traditional double-sided printed circuit board (PCB) can be printed on one side, providing the user with higher design freedom to achieve a more compact high performance PCB structure. 1. Introduction allowed to have any intersections in their route. In industrial manu- facturing, double sided PCB circuit is commonly used, the conductive Material Jetting (MJ) is an advanced, high resolution Additive tracks are usually placed on both sides of the board to achieve increased Manufacturing (AM) method, which can be used to produce structures circuit density and a more compact structure. In 2014, Andersson et al. by stacking up material droplets. Benefiting from the capability to in- fabricated workable double-sided PCBs by laminating two separately clude multiple drop-on-demand inkjet print heads in a single machine, printed single-sided PCBs together, presenting one solution to this MJ allows co-printing of various functional materials in picolitre dro- problem [13]. plets by selectively depositing them to the target location to form either In this paper, a simpler solution to preparing double-sided PCBs is 2D or 3D structures [1,2]. In the past decade, there has been growing proposed. Instead of separating the whole circuit using a dielectric interest in manufacturing electronics, especially printed circuit boards plate, as for a double-sided PCB, here in order to achieve a high circuit (PCB) created through the MJ process. The use of MJ for circuit board density, an insulating material is selectively deposited at the cross-over manufacturing can simplify the whole production process compared points, creating an insulating bridge for the circuit. When printing these with conventional PCB manufacturing methods, reducing the material local insulators, it is important to control film topography and to waste and production cost especially for low volume products [3] and eliminate defects such as “coffee stain” drying [4], cracking [4] and also providing great design freedom [3–7]. This will allow the user to delamination [8]. Therefore, a reliable insulating material with su- produce bespoke products such as flexible or disposable elecronics [8]. perior dielectric properties and film qualities is required. Consequently, With the recent advances in MJ of electronic circuits, various types Polyimides (PIs) were selected as the insulating material. PIs are im- of electronics have been printed including, transistors [9], capacitors portant engineering polymers that are well known for their excellent [10], inductors [11], flexible circuit board [12], etc. All of these studies thermal stability, chemical resistance, mechanical and electrical prop- are based on single-sided circuit board designs. Single-sided circuit erties [14,15,12]. It is a particularly attractive material in the micro- boards have limited circuit design freedoms and circuit densities; in electronic industry, and has important applications in interlayer di- order to avoid a short circuit failure, the conductive tracks are not electrics [16] and as protective layers in integrated circuit fabrication ⁎ Corresponding Author. E-mail address: [email protected] (Y. He). https://doi.org/10.1016/j.addma.2018.11.017 Received 20 August 2018; Received in revised form 13 November 2018; Accepted 17 November 2018 Available online 22 November 2018 2214-8604/ © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/). F. Zhang et al. Additive Manufacturing 25 (2019) 477–484 and microelectromechanical system (MEMS) devices [17]. The tradi- (mPa·s). When Z falls between 1 and 10, an ink is considered to be tional method to prepare PIs samples is through a two-step process, in printable [23]. To determine the Z number, the viscosity of the ink was which PI precursor poly(amic) acid (PAA) solution is casted and cured assessed using a parallel plate rheometer (Malvern Kinexus Pro) with a upon heating or chemical treatment [18], followed by being manu- shear rate sweep between 10 and 1000 s-1 at 50 °C, while the surface factured into samples using lithography [20]. In our previous work, we tension was measured at 50 °C using a Kruss DSA100S pendant drop reported a one-step inkjet printing process to prepare ultra-thin PI in- shape analyser. sulating layers during which PAA solutions were jetted and thermo- The printing indicator Z of the ink at 50 °C was calculated to be imidised simultaneously, and applied to the fabrication of parallel plate 3.98, as shown in Table 1. This ink was subsequently demonstrated to capacitors [21]. However, some drawbacks, such as uncontrollable film be printable. thickness and coffee-ring effects, were observed for the previous poly- imide ink, which may lead to electrical failure. In this article, a new method for preparing inkjet printable polyimide is demonstrated. This 2.3. Material jetting method will avoid the drawbacks of previous attempts and form dense films with controllable dimensions and improved topography, which A Dimatix DMP 2800 printer (Fujifilm) was used to carry out the served as great local insulator in the fabrication of circuit equivalent to printing of PI ink. The prepared ink containing 20 wt% PAA was fil- double-sided PCBs. trated (using HPLC Nylon 5.0 μm syringe filters, Cole-Parmer) and 3mL of ink was injected into the cartridge (DMC-11610, 10 pL). The Dimatix 2. Methodology printhead consisting 16 nozzles (21 μm nozzle diameter) was used for jetting. Stable droplets were obtained at a cartridge temperature of 2.1. Ink preparation 50 °C and printing voltage of 26 V. To study the printing quality, in- dividual droplets and squares were printed on glass substrates (micro- All chemicals were purchased from Sigma-Aldrich (UK) and used as scope slides, Cole-Parmer), which were heated and held at different received. To prepare the PI ink, 7.7 mmol (3.17 g) of 4,4′-(4,4′-iso- temperatures (120 °C, 150 °C and 180 °C) using a thin film heater propylidenediphenyl-1,1′-diyldioxy) dianiline (BAPP), 10.2 mmol (KHLV-103/5 Kapton insulated flexible heater, Omega) during printing (1.00 g) of maleic anhydride (MA), 10 g of 1,2-dimethoxyethane (EDM) to enable the solvent evaporation and thermal imidisation. and 10 g of 1-methyl-2-pyrrolidinone (NMP) were mixed and stirred in To demonstrate the capability of producing a single sided circuit a 50 mL round bottom flask at 400 rpm for 4 h at room temperature board with two overlapped circuit patterns through material jetting, a (Fig. 1). After that, 2.55 mmol (0.82 g) of benzophenone-3,3′,4,4′-tet- demonstrator was produced by subsequently printing circuit one fol- racarboxylic dianhydride (BPDA) was added into the mixture and lowed by polyimide insulator deposited on the designed overlap loca- stirred for another 4 h at room temperature. The resultant ink was a tions and then circuit pattern two (as shown in Fig. 2). The printhead dark yellow solution containing 20 wt% of polyamic acid (PAA). Silver height was set to 1000 μm above the glass substrate and elevated for nanoparticle (∼ 38 wt%) ink was purchased from Advanced Nano 10 μm after printing one layer of silver ink or PAA ink. Products Co. Ltd and used for the printing of the conductive tracks. 2.4. Surface morphology 2.2. Printability assessment A Nikon Eclipse LV100ND optical microscope was used to char- To assess the printability of the PAA precursor solution, a printing acterise the droplet sizes and the general surface morphology of printed indicator Z was introduced [22], which is defined by Eq. 1 samples. Surface topography was characterised using a Bruker GT-I 3D r Z = Optical Microscope. The cross-sections of the printed and cast films µ (1) were observed using a Hitachi TM3030 tabletop Scanning Electron Microscope (SEM). where ρ is the density (g·cm−3), r is the orifice diameter (μm), γ is the surface tension of the fluid (mN·m-1) and μ is the viscosity of the ink Fig. 1. Chemical reaction route to prepare polymerise PAA as the precursor for inkjet printable PI ink. (1)10.2 mmol of BAPP containing 20.4 mmol of amine groups, then 10.2 mmol of MA was added which reacted with 10.2 mmol of the amine groups in BAPP; (2) BPDA was then added in to react with remaining amine groups in BAPP and form PAA precursor. 478 F. Zhang et al.