For the electronic equipment, various printed circuit boards are used. Recently, the electronic equipment is getting miniaturized, and insulation reliability of the printed circuit boards becomes the important problem. In this environment, paper phenol base printed circuit boards are produced a lot and mainly used for the consumer article, so that we examined the ionic-migration of paper phenol base printed circuit boards. Generally, the resistance value of the paper phenol base printed circuit board is lowered with continual rapid-dropping and recovering of the resistance value in the high-humidity/temperature test for ionic-migration when it passed about 1500 hours. However, when it passed after 300 hours, the rapid changes of the resistance value by the ionic-migration were observed in our test, and it was observed that the bridging is done between some electrodes of the printed circuit boards.
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