Newly Developed Real Time Monitoring System for Ionic Migration by Means of Quartz Crystal Microbalance (QCM) and Electrochemical Impedance Spectroscopy (EIS)

Abstract

Micro-fabricated or Nano-fabricated Printed Circuit Boards (PCB) using environmental friendly materials and processes will be desirable candidates for future electronic packaging technologies. To investigate the ionic migration for thus developed PCB, quite new measurement method has been developed by the authors, which enables real time monitoring of the growth process of ionic migration using QCM (Quartz Crystal Microbalance) and Electrochemical Impedance Spectroscopy (EIS). First part of this research has focused on the QCM method and EIS to study the process of ion migration (hereinafter we call it simply “migration”) in various types of lead-free solder plating and the effects of the reflow (Heat treatments) processing and flux residue of soldering processes. In addition, we investigated the anodic dissolution characteristics of the elements in each type of solder alloy by measuring the current-potential curves in 0.1 kmol m−3 KNO3 solution. When using Sn-3.5 mass%Ag solder plating, reflow processing segregate the stable compound Ag3Sn layer and Sn layer. The Sn layer selectively promotes the anodic dissolution reaction, increasing the occurrence of migration. When using Sn-9 mass%Zn solder plating, the Sn effectively prevents the excessive dissolution reaction of Zn. However, since reflow processing causes each element to separate out, reflow processing lessens the effectiveness of Sn, thus promoting the occurrence of migration. The flux processing of lead-free solders suppresses anodic dissolution and effectively prevents the occurrence of migration. However, with Sn-9 mass%Zn, the lowered adhesion between the flux film and the electrodes is a factor in speeding the growth of migration. The ionic migration of lead free solder in severe NOx circumstance will be commented. Next, electric reliability of the newly developed copper printed boards by NEDO project (L/S = 4/6, 8/12, 12/16 μ□) using polyimide as the substrate have been evaluated under 85 °C, 85%RH. The generation of the migration was tested by conventional time dependence of current under bias of 5.0 V. The progress of the migration could be investigated by EIS (Cole-Cole plots of the impedance). The degradation of the diameter of the impedance loop suggested the intimation of migrations. Finally, in order to establish the micro-fabricated printed circuit boards, “semi-additive” process will be one of the candidates. The process involves the copper etching processes by use of FeCl3 or CuCl2 aqueous solution. But even after the cleansing of such etched substrate, some residue of the iron or copper contaminants will affect the electric reliability of the boards. Such effects have been evaluated by the QCM method. The obtained results suggested that scarce residue of iron contaminants will diminish the reliability rather than residue of copper contaminants.