Increase in capacitance and tan δ between conductors onprinted circuit board at low frequency due to ionic migration

An endurance test for a printed circuit board to ionic migration was carried out for 2000 h in a chamber controlled at 85°C and 85%RH. The capacitance C and tan δ between the conductors on the board were measured to find their correlation with ionic migration in the board. The measurement frequency of C and tan δ ranged from 1 to 1000 Hz. The configuration of the test circuit was designed according to IPC-SM-840. The material of the insulating board was fiberglass-reinforced epoxy resin. The distance between the Cu conductors was 0.165 mm, and 70 V dc was applied continuously. In the low frequency region (<30 Hz), C and tan δ gradually increase after ~800 h voltage application. During the gradual increase, steeply transient increases in C and tan δ are detected at about the same time when a deposit was seen to have formed on the board between the conductors, although no significant decrease in insulating resistance between the conductors is observed. It was confirmed that the deposit could be detected early from the steep increases in C and tan δ in the low frequency region. In the high frequency region (>30 Hz), on the other hand, gradual slight increases are observed from ~1100 h after voltage application, but no steep increases. The dependence of C and tan δ on frequency at the measurement is due to the low mobility of Cu ions in the water film on the insulating board