Abstract
High frequency printed circuit board (HFPCB), a sequential glass fiber-reinforced resin-based/copper foil multi-layer sheet, is widely used as printed circuit boards in next-generation 5G communication base stations, radar antenna, smart cars, etc. Micro-holes are key components that enable the interconnections of different layers, fixed electronic components, and high-density wiring. However, hole wall debris and micro-drill breakage during the micro-drilling of sequential glass fiber-reinforced resin-based/copper foil multi-layer sheet negatively affect the quality and reliability of signal transmission. Herein, chip removal, chip morphology, drilling temperature, thrust force characteristics, and hole wall morphology of HFPCB micro-drilling are investigated. The effect of chip morphology on chip removal is analyzed. Additionally, the effects of thrust force and micro-drill breakage are analyzed. The formation mechanism of hole wall debris is proposed. Compared with traditional FR4, the fracture mechanism and control method of micro-drill breakage are proposed by analyzing the characteristics and cause of typical thrust force variations. Results show that the drilling temperature and chip removal of HFPCB are primary factors causing hole wall debris. The morphology of HFPCB chips primarily includes the chips of copper foil, agglomerate chips of resin layer, and mixed copper-foil–resin chips. The increase in prepreg, resin content, and number of multi-layers causes a large amount of cutting heat during HFPCB micro-drilling. Meanwhile, the large thickness, large depth ratio, and diameter resulted in poor chip removal. Subsequently, the constant accumulation of heat in the hole resulted in resin melting, followed by adhering to the hole wall, thus generating hole wall debris through a thermo-mechanical coupling extrusion. The long-term fluctuation of thrust force is the main reason for micro-drill breakage during micro-drilling.
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