Measuring Sodium Migration in Mold Compounds Using a Sodium Amalgam Electrode as an Infinite Source

Abstract

Mold compounds used for chip encapsulation are one of the main contamination sources for mobile ions in semiconductor devices. These ions are a prominent reliability risk, especially, when it comes to high power devices such as high voltage transistors. Due to strong electrical fields, ions from the mold compound can migrate into sensitive areas of these devices resulting in chip failure. Due to the high natural abundance and the small ionic radius, sodium ions are considered to be the main risk. Therefore, it is of utmost importance to keep the sodium concentration of the materials used for encapsulation as low as possible. However, the concentration value for sodium is not sufficient for risk evaluation since ion migration is also strongly dependent on the diffusion coefficients. Within this work we will describe a new approach to measure field induced ion migration in mold compounds using a sodium amalgam electrode setup. The new technique is compared to conventional techniques such as electrochemical impedance spectroscopy and the advantages and disadvantages are discussed. The described technique can be an important tool when it comes to the selection of the encapsulation material for new high power semiconductor devices.