Temperature rise and fusing current in wire bonds for high power RF applications

Wire bonds in any semiconductor package represent an inductor between the die level bond pad and the lead frame post. As current flows through the wire bond, there is some loss of power due to Joule´s heating effect. This power is dissipated by means of conduction through the length of the wire, and convection and radiation from the surface of the wire. For DC applications, the fusing current of the wire is readily available. This study examines the phenomena as it applies to RF applications. In RF applications, the current is transmitted only through the outer layer (skin) of the conductor, and the skin depth is dependent on the frequency. A closed form solution is derived for a simplified case where both pad and post are at the same temperature. Analytical results were validated with the use of IR Scan measurements. A numerical solution methodology was also developed using couple field finite element method to account for both electrical and thermal loading. Fusing current predictions were obtained for 0 Hz (DC), as well as from 880 MHz to 2 GHz; which are frequencies of interest for cellular applications. The differences between analytical and experimental results were found to be less than 15% and between numerical and experimental results were found to be less than 10%.