Abstract

Devices that are sealed with a process using flux (all solder seals, some brazed seals some weld seals, etc.) will have residual flux entrapped. Devices with an internal construction such that areas exist where solvents can not completely clean surfaces exposed to flux vapor will also have residual flux entrapped. This entrapped flux will cause these devices to be susceptible to electromigration induced failure prior to the normal end-of-life of the device. If solder flux is trapped within a device, elevated temperature operation will shorten the life of the device. At elevated temperature, the activators in the flux will become a vapor and act as a catalyst for electromigration. Metal will be distributed across the surface between the “anode” and the “cathode” of the applied potential. The metal will be pulled from the “anode” to the “cathode”. The electromigration can be stopped (or slowed) by reducing the maximum temperature the device will be exposed to when a potential is applied. If the amount of entrapped flux is not sufficient to bridge the gap between the “anode” and the “cathode” of the potential when in a non-vapor state (liquid or solid), the electromigration will be stopped when the temperature is below that needed for the flux activators to be in a vapor state. This paper contains details of this failure mode in hermetically sealed EMI filters and includes life test data (insulation resistance at elevated temperature), life reduction calculations, and photographs of the electromigration.

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