Plasma-Generated Etchback to Improve the Via-Reliability in High-Tg Substrates Used in Multilayer PWBs for Space Electronic Packaging

Abstract

High-Tg (glass transition temperature, 210 °C) woven fiber-glass reinforced multifunctional-epoxy FR-4 laminates are now becoming main trend in the fabrication of multilayer printed wiring boards (PWBs) for space application due to their inherent properties of better dimensional stability, low coefficient of thermal expansion, and the capability to withstand multiple soldering operations. In PWBs, positive etchback is a key factor to improve the plated through hole (PTH) reliability due to enhanced bond pull strength. However, high-Tg multifunctional-epoxy FR-4 laminates have higher chemical resistance, and hence, it is very difficult to get required levels of positive etchback (10– $13~\mu \text{m}$ ). The absence of positive etchback results in inadequate bond pull strength, which can lead to critical defects, such as inner layer separation, and fails to meet the basic requirements of interconnection reliability tests, such as rework simulation test and thermal stress test. Ability to withstand required number of rework simulations (five times of soldering and desoldering) and multiple thermal stress tests (generally—three times solder dips and enhanced performance—six times solder dips) is essential for all PWBs used in space electronic packaging. The popular method to get the positive etchback in drilled holes of low-Tg Printed Circuit Board (PCB) substrates prior to metallization is by chemical desmearing/etchback process. Based on the success rate of chemical desmear methods on low-Tg PCB laminates, experiments were carried out on high-Tg (210 °C) FR-4 laminates. With the experimental results, it is evident that the chemical desmearing method cannot generate the required levels of positive etchback on high-Tg multifunctional-epoxy FR-4 laminate, and sometimes, it leads to negative etchback, which degrades the reliability of PTH in PWBs. In this paper, the effect of plasma treatment—a dry process to get required level of positive etchback in high-Tg multifunctional-epoxy FR-4 laminates is studied at different vacuum levels. It is observed from the experiment that effective plasma treatment to get 10–13- $\mu \text{m}$ positive etchback is possible only at 30 mtorr and higher vacuum levels at optimized plasma etch parameters. Plasma-treated PTH of high-Tg multifunctional-epoxy FR-4 laminates exhibit higher bond pull strength and readily withstand over six dip thermal stress tests and rework simulation tests without any defect in the PTH barrel. Enhanced bond pull strength of deposited copper in the PTH barrel is attributed to the mechanical interlocking of plated copper inside the microroughened dielectric surface, which results in highly reliable 3-D contacts between the plated copper and the projected inner layer copper.