In order to study the generation mechanism of electrochemical migration in a printed wiring board, paper/phenol‐resin composites with and without an adhesive layer, epoxy resin, and a laminate of epoxy resin and a paper/phenol‐resin composite were aged at 85 °C and 85% relative humidity (RH) with and without application of a DC voltage, and the space charge distributions in the samples were observed by the pulsed electroacoustic method. In the case of the composite with the adhesive layer, a large amount of negative charge is induced at the interface between the composite and the adhesive layer on the anode. It was found that such formation of space charge enhances the electric field intensity at the anode. The reason for this is because the conductivity is much lower in the adhesive layer than in the composite. Electrochemical migration does not occur when the same electric field is applied to the composite without the adhesive layer, whereas it does occur if the adhesive layer was replaced by epoxy resin with a similar conductivity to the adhesive layer. These results clearly indicate that electrochemical migration generates because the electric field at the anode surface is enhanced due to the low conductivity of the adhesive layer. © 2011 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Pulsed electroacoustic (PEA) method was applied as a nondestructive method to detect electrochemical migration in a sheet of epoxy resin and a sheet of laminate consisting of the same resin and a paper/phenol-resin composite. A significant decrease in the amount of negative charge was observed at the interface between the resin and the composite when the sample was the laminate. This is assumed to be due to the progress of electrochemical migration in the thickness direction. On the other hand, such a change in charge density was hardly observed when the sample was a sheet of epoxy resin. The electric field intensity in the epoxy resin layer in the laminate is assumed to be significantly enhanced, since the resistivity is much higher in the epoxy resin than in the composite. This seems to be the main reason why the electrochemical migration was induced only in the epoxy layer in the laminate. This means that observation of electrochemical migration in epoxy resin by the PEA method becomes possible by forming a laminate structure using the epoxy resin and paper/phenol-resin composite.

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