Characterisation of Electrical-Thermal-Mechanical Deformation of Bonding Wires Under Silicone Gel Using LF-OCT

Bonding wires are one of the most failure-prone component of power electronics modules (PEMs), and silicone gel is usually employed to encase bonding wires. To study the deformation of silicone gel-encased bonding wires, in this paper we report the use of line-field optical coherence tomography (LF-OCT) technique to precisely measure the electrical-thermal-mechanical (ETM) deformation of bonding wires. A LF-OCT system was developed to capture the whole cross-sectional image (B-scan) of the boning wire sample in a single shot fashion due to its advantageous parallel detection scheme. This, together with the Fourier phase self-referencing technique, allows the deformation of the bonding wires to be quantitatively measured down to the range of 0.1 nm. The maximum sampling rate (framerate) of the deformation measurement achieved was 400 Hz when setting camera imaging size to 1920200 pixels, providing a temporal resolution of 2.5 ms for monitoring the ETM deformation dynamics of the bonding wire. We found that the ETM deformation of the gel-encased bonding wire was about three times smaller than that of the bare bonding wire. These results represent the first experimental demonstration that the LF-OCT could be a useful analytical tool for studying the time-dependent ETM deformation of bonding wires encased by silicone gel.