In-situ solder surface tension measurements using mechanical resonances

We propose a method for in-situ measurements of the surface tension of solder joints on microelectronics chips. The method uses actual chips placed in an inert atmosphere, in a heating chamber with a transparent lid. A confocal microscopy probe is used to oberve the top of solder balls through the lid while a piezoelectric actuator induces vibrations in the system. The setup allows the measurement of the resonance frequencies of the balls by comparing the system's response to sweeps in excitation frequency, for temperatures below and above the liquidus temperature of the solder joints. A number of phenomena were observed, such as undercooling during solidification and continuous formation of surface oxydes.