Board-Folding Method for Fabrication of 3-D System in Package Devices

Abstract

This paper presents a new method, based on slotting and folding of rigid printed circuit board (PCB), for creation of 3-D cuboid system in package (SIP) devices. The method, which can create folds at any angle and uses standard manufacturing and design processes and commercial off-the-shelf components, is suitable for making SIPs down to at least 1 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> in volume. It offers significant advantages over current approaches to 3-D SIP fabrication, particularly in terms of simplified manufacturing and increased packing density, as it eliminates the need for connectors or flex tapes to form folds. The method involves removing substrate material, using a dicing saw to cut a slot at the area where the fold is to be formed, and then applying heat to the area and forming the fold. Once cooled the fold remains rigid and the electrical tracks remain intact. The optimum parameters for creating a 90° fold in a PCB using this method were identified in a designed experiment with variables including substrate material, temperature, cut depth, and conductor width. Having identified the optimum process parameters, 25-mm × 10-mm × 10-mm × 10-mm cuboid test modules were fabricated and reliability screening involving temperature storage, temperature cycling, temperature-humidity, and shock/vibration was carried out. All of the modules passed reliability screening test. Finally, a functional system-in-cube module was fabricated using the folding method. The module, which combines environmental stress sensors and a microcontroller in a 1 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> package, was intended for deployment as a prognostic reliability monitor. The monitor was evaluated over a range of temperature and humidity and underwent shock/vibration testing without failure. This is believed to be the first time that a board-folding approach has been used to fabricate 3-D SIP modules.