Combined Effects of Polluted Particles and Environmental Factors on Electrochemical Migration Failure

Abstract

With the miniaturization of printed circuit board (PCB), the distance between the circuit traces or assembled component terminals is becoming smaller and smaller. Under the certain temperature, relative humidity, and bias voltage (THB), it is easier for the insulation performance between circuits to degrade due to electrochemical migration (ECM). The airborne dust deposited on the PCB during a long-term service may change the failure mechanism and time to failure (TTF) of ECM between circuits. In this paper, the mechanism and the characteristics of the combined effects of quartz particles in airborne dust, the temperature, the relative humidity and the bias voltage on the ECM failure of immersion silver finished PCB were studied by THB experiments. Then, the influencing significance of four factors on TTF of ECM were studied by the orthogonal test and range analysis. The results show that the silver cations migrate firstly but the exposed copper substrate on the positive electrode become the main migrated substance finally. The TTF of ECM changes non-monotonously with the increase of particle coverage density due to the bidirectional function of the insoluble particles, which can increase water condensation on PCB by the capillary action, but can extend the migration path of metal cations by physical barriers. The turning point is about the coverage density of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$350~\mu \text{g}$ </tex-math></inline-formula> /cm 2. The insoluble particles with high coverage density can significantly shorten the TTF of ECM on the PCB than that with low coverage density under the same THB conditions.