Plasma Treatment and Copper Metallization for Reliable Plated-Through-Holes in Microwave PCBs for Space Electronic Packaging

Abstract

Radio frequency (RF) or microwave printed circuit board (PCB) is a type of PCB designed to operate on signals in megahertz-to-gigahertz frequency (medium to extremely high frequency) ranges. The materials used to construct these PCBs are advanced composites with very specific characteristics for dielectric constant, loss tangent, and coefficient of thermal expansion (CTE). These microwave materials with critical values of very low dielectric loss and absolute dielectric constant allow high-speed signals to travel through the PCB with more stable impedance characteristics than in standard FR-4 dielectric materials. These microwave substrates are composite dielectric materials produced generally with combination of polytetrafluoroethylene (PTFE), ceramics, hydrocarbons, and/or various forms of glass. PTFE-based microwave PCB laminates, which require vias after drilling, are subjected to an appropriate surface modification for metallization, because of its hydrophobic nature and very low surface energy. Sodium-based wet chemical process followed by various copper deposition process techniques are commonly employed for metallization of microwave PCBs. In our study, plasma treatment process is used to generate microroughened surface in drilled vias, and electroless copper deposition followed by copper electroplating process have been employed for void-free metallization in microwave PCBs with enhanced plated-through-hole (PTH) reliability for space electronics packaging applications, as evidenced from the results of thermal stress and PTH bond strength tests conducted as per IPC-TM-650.