Test device design and test procedure for non-destructive characterization of Bi phase accumulation during electromigration

Low Temperature Solder (LTS) composed of Sn and Bi is becoming common in microelectronics applications. They are preferred for applications where the components are temperature sensitive. Using LTS also brings down the energy requirements during the reflow process. One of the major issues with LTS is that as the current density increases, Bi phase accumulates at the anode due to electromigration. This increases the resistance of the joint which eventually causes the device to fail. Commonly used test devices require cross sectioning to analyze the Bi thickness after failure. This causes a discontinuity in the experiments, and is therefore difficult to obtain continuous data on the Bi growth. Once a device has been tested, the test cannot be continued further on the same device. A non-destructive test methodology can avoid such issues and produce more reliable data on Bi thickness accumulated with time. The test device contains multiple Cu-Solder-Cu junctions directly observable under a microscope. The solder is deposited using solder paste allowing us to create test devices with varied compositions easily with the same procedure. The test devices are subjected to a current using a 4-wire Kelvin probe.