Ionic migration has been the subject of intensive study, both theoretical and experimental, over the past 40 years. It is known as a reliability concern for printed wiring boards (PWBs) in high density microelectronic packaging and power electronic packaging. Ionic migration is an electrochemical phenomena that occurs primarily under normal ambient conditions: i.e. when the local temperatures and current densities are low enough to allow moisture on the surface. Standardised test 85°C/85%RH is typically used for accelerating and predicting ionic migration failure, however, the possibility of moisture condensation — a prerequisite for ionic migration — at a relatively high temperature and low relative humidity is unlikely. In order to assess more realistic and less thermally severe environments, this work examines prolonged steady state exposure of PWBs. Steady-state conditions of 90%RH at 30°C under a bias of 5V DC were tested over a 210 day period with continuous in-situ monitoring of dendritic growth. Investigative techniques were conducted to evaluate the migration development on the PWBs after testing using optical microscopy, scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDS). This paper will demonstrate that steady-state thermal humidity bias (THB) tests appear to provide ionic migration behaviour similar in service conditions, however, do not demonstrate the dramatic failure associated with ionic migration.

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