Influence of a New Abnormal (CuNi)6Sn5 / (NiCu)3Sn4 Layer Growth at Temperatures Above 175°C in Tin Silver Based Lead-Free Solder Joints
Authors: Timo Herberholz, Ph.D., Andrey Prihodovsky, Ph.D., Mathias Nowottnick, Ph.D. Company: Robert Bosch GmbH, Deggendorf Institute of Technology, and University of Rostock Date Published: 9/22/2019
Abstract: The demands on the temperature stability of solder joints are growing steadily. Increasing current density of power devices and changing boundary conditions will lead to operation temperatures above 175°C in the near future. Their consequences on the reliability of solder joints are not described and understood in detail thus far. Therefore, first studies on the aging behavior of tin based lead free solder joints above 175°C have been conducted on the system chip resistor type 1206 on a PCB (printed circuit board). A new phenomenon was found in the temperature range between 175°C and 200°C: a new abnormal growth of a single (CuNi)6Sn5 phase into a double intermetallic layer consisting of (CuNi)3Sn4 and (CuNi)6Sn5 phase has been observed. A Ni3Sn4 phase forms on top of the chip resistor on a boundary that is not wetted by the liquid solder during the reflow soldering process. This is the nucleus for the further transformation during isothermal aging. The transformation begins at the not wetted boundary on top of the chip resistor and moves toward the PCB (printed circuit board). After the transformation is completed and the boundary of the chip resistor is completely covered with the double intermetallic layer, the whole double layer grows perpendicular to the wetted surface. This phenomenon has negligible influence on the reliability of the investigated solder design element CR1206 soldered on PCB.