Fast and Reliable Ag–Sn Transient Liquid Phase Bonding by Combining Rapid Heating with Low-Power Ultrasound

Abstract

Ag–Sn transient liquid phase (TLP) bonding is capable of producing joints with excellent mechanical and thermal properties, and with operation temperatures at or even above the processing temperature. However, reduction of the comparatively long processing times required to achieve high-quality joints is highly desirable. To this end, the use of fast heating and low-power ultrasonic (US) pulses for accelerated Ag–Sn TLP bonding is investigated with particular focus on defect generation and avoidance. Employing rapid heating with 20K s-1, the process time can indeed be strongly reduced, yielding a high average shear strength of (58 ± 22) MPa. The large variance of the shear strength is caused by gas entrapment and pronounced lateral squeeze-out of liquid Sn, facilitated by the suppression of Ag 3Sn formation upon rapid heating. Introduction of a weak US pulse leads to enhanced Ag 3Sn formation and efficient removal of entrapped gas, and thus to avoidance of large-scale bond defects, which results in an enhanced shear strength of (104 ± 8) MPa.