Development of a via-hole connection process via intense pulsed light sintering with Cu micro/Ag nano-hybrid ink for a multi-layered flexible printed circuit board

Highlights: • Cu micro/Ag nano-hybrid inks were printed and intense pulsed light (IPL) sintered. • Ink contents were optimized for printing of pattern area and via-hole section. • Multi-pulse IPL sintering was introduced to improve sintering characteristics. • Optimally IPL sintered case exhibited superior sintering characteristics. -- Abstract: In this work, the printing and sintering characteristics of pattern area and via-hole section were investigated. To analyze the printing characteristics depending on the rheology of ink, Cu micro/Ag nano-hybrid inks (Cu/Ag hybrid inks) were prepared by varying the amount of epoxy, and the fabricated inks were printed on via-hole formed substrates. The printed Cu/Ag hybrid ink films were subsequently sintered by an intense pulsed light (IPL) sintering method. Various IPL sintering parameters including irradiation energy and pulse number were employed to enhance the sintering characteristics of pattern area and via-hole section. The sintered Cu/Ag hybrid ink films were characterized using scanning electron microscopy and X-ray photoelectron spectroscopy. The detailed sintering mechanism of Cu/Ag hybrid ink inside the via-hole section was monitored in real-time by in situ resistance monitoring. As a result, the Cu/Ag hybrid ink with the optimal ratio of epoxy content (0.56 wt%) showed good printability of pattern area and via-hole section. In addition, the proposed multi-pulse IPL sintering conditions (irradiation energy: 7 J/cm², pulse duration: 1 ms, off-time: 9 ms, pulse number: 20) exhibited low resistivity (pattern area: 5.12 μΩ cm, via-hole section: 7.79 μΩ cm) and a 5B adhesion strength level.