Development of a High-Intensity UV Exposure Apparatus under a High-Pressure CO₂ Gas Atmosphere to Manufacture Large-Area Porous Ultralow-k Polyimide Substrates for Flexible Print Circuits

Abstract

Ultralow-k (dielectric constant) films are promising substrates for next-generation flexible print circuits. Introducing numerous pores into the film can effectively reduce the substrate's dielectric constant because the relative dielectric constant of air is smaller than that of any polymer substrate. We recently developed a short-cycle time process employing high-pressure CO 2 and the CO 2 -tertiaryamine zwitterions in polyimide precursor solutions to create 1-3 μm pores of >70% porosity. However, the film size was limited to 30 × 30 mm 2 . A larger film (70 × 150 mm 2 ) was required to measure the signal attenuation of an electrical circuit on a porous PI film as a next-generation flexible cable. In this paper, the developed process was scaled up to obtain 10-fold-larger ultralow-k films of porous polyimide. The process involved a high-intensity UV lamp, thick-quartz window and hydraulically movable sealing plate and produces 70 × 150 mm 2 films, which was a suitable size for high-speed data communication transmission tests. The preliminary results of building up a printed circuit on the porous substrate and signal attenuation measurements at 20 GHz demonstrated that the low-k porous PI substrate reduced the signal attenuation compared to a non-porous substrate with the same cross-sectional line area.

Cover-layering a Porous Polyimide Flexible Print Circuit with a Porous Polyimide Layer

“…The signal attenuation of the high-frequency signal was compared to a solid PI film. With the same length, width, and height of the microstrip line for the porous PI and the solid PI, it was observed that the porous PI substrate could reduce the signal attenuation [14].…”

“…An FPC was prepared using the same procedure as in our previous study [14]. The surface of porous polyimide film was treated by UV light for enhancing the wettability of the plating solution.…”

“…A porous polyimide film was prepared using the high-intensity UV exposure apparatus under a high-pressure CO 2 gas atmosphere reported in previous studies [13,14].…”

Controlling the Pore Structure of Polyimide Films Prepared by Exposure to High-Pressure CO₂ and UV Light

“…To prepare the standard formulation (denoted in previous studies [5,24]), 1.58 g (8.53 mmol) of UV-curable 2-(diethylamino)ethyl methacrylate monomer (DEMA; TCI, Japan) and 0.201 g (0.577 mmol) of diphenyl(2,4,6-trimethylbenzoyl) phosphine oxide (photoinitiator; Irgacure TPO, BASF) were added to 6.56 g of the PAA solution and stirred until a homogeneous solution was obtained. To clarify the effect of monomer concentration on pore structure, the monomer concentration was reduced from that used in the standard formulation.…”

“…However, the increasing demand for mobile devices capable of faster data transfer requires the use of PI FPCs with low dielectric constants to decrease signal attenuation. Therefore, dielectric constant reduction has been investigated by several researchers, as summarized in prior studies [1,2] and reviews [3,4] and in our previous articles [5,6].…”