Microstructure Analysis of Transient Liquid Phase Bonded Joints with Sn based Rapidly Solidified Braze Ribbons
Authors: Iskandar Riza , Pongratz Ludwig , Wiesner Stefanie , Öte Mehmet , Schwedt Alexander , Bobzin Kirsten , Mayer Joachim 
Published: December 2016

Transient liquid phase (TLP) bonding is a diffusion based joining process that has been applied to different metallic systems. It is based on diffusion of filler metal constituents into the base material and/or vice‐versa leading to isothermal solidification. Combining isothermal solidification with a subsequent solid state homogenisation treatment offers the possibility of producing bonds that are almost chemically identical to the base material and have no significant microstructural discontinuity at the bond line [1]. In this work, braze ribbons as filler material were produced by melt spinning process resulting in flexible ribbons with homogenous element distribution, which in the end will enable equally homogenous melting characteristics of the braze material [2]. The aim of this work is to analyse the microstructure of the braze ribbons to understand their melting and solidification behaviour in a TLP bonding process.

In this work, we present the microstructure analysis of two braze ribbons made of Sn78Cu22 and Sn75Cu20Ge5. Germanium was added for improving the glass forming properties. Electron backscatter diffraction (EBSD) and SEM EDS analysis were performed on a JEOL field emission SEM JSM‐7000F equipped with EDS and EBSD detectors by EDAX. The TEM lamellae were prepared by the focused ion beam technique (FIB) using an FEI single beam FIB Strata 205. Scanning transmission electron microscope (STEM) investigations were conducted using a 200 kV field emission in column corrected omega filter TEM Zeiss Libra 200 FE equipped with HAADF and EDS detectors from Fischione and Bruker, respectively.

EBSD shows the differences in grain morphology after rapid solidification by the melt spinning process for the Sn75Cu20Ge5 braze ribbon (figure 1). It has been also revealed by EBSD analysis that metastable hexagonal CuSn phase could be found in the Sn matrix of the Sn75Cu20Ge5 braze ribbon. More detailed analyses for both braze ribbons were done by means of STEM combined with EDS analysis. These results revealed that CuSn phases in a Sn matrix can be found in both braze ribbons. Furthermore, cross sections of the Sn75Cu20Ge5 braze ribbon show different phases as well as different microstructures (figure 2). Crystalline CuSn phases were formed in a Sn matrix with amorphous Ge at the grain boundaries, whereas amorphous SnG areas were formed in the upper and lower part of the cross sections respectively. Moreover, DSC measurements were conducted to determine the melting behaviour of two ribbons and the measurement curves reveal two exothermic peaks up to 450 °C [3].

The brazing processes were performed in high vacuum by putting each braze ribbon in between aluminium cast material AlSi7Mg0.3. To achieve a uniform contact between the filler and base material during brazing, each joint was statistically loaded with p = 5.3 N/mm
2
during the brazing process. SEM EDS analysis was used to reveal the phases present at the brazed joints. As can be seen in figure 3, all phases are enclosed in a Sn matrix for both brazed joints. The Sn diffuses into the aluminium base material and the aluminium diffuses into the brazed joint material (see inset). In the Sn78Cu22 braze joint, intermetallic Cu
x
Al
y
phases can be detected. In the Sn75Cu20Ge5 braze joint, smaller CuAl precipitates spread over the joint with addition of GeAl. The results suggest that Ge induces a change of the intermetallic CuAl phase, while Ge binds a part of the Al.