Mechanical Properties of Intermetallic Compounds in the Au–Sn System

The mechanical properties of intermetallic compounds in the Au–Sn system were investigated by nanoindentation. Measurements of hardness and elastic modulus were obtained for all of the confirmed room-temperature intermetallics in this system as well as the β phase (8 at.% Sn) and AuSn4. Overall, it was found that the Au–Sn compounds have lower hardness and stiffness than common Cu–Sn compounds found in solder joints. This finding is in contrast to common knowledge of “Au embrittlement” due to the formation of either AuSn4 or (Au,Ni)Sn4 intermetallic compounds. This difference in understanding of mechanical properties of these phases and the resulting joint strength is discussed in terms of reliability and possible failure mechanisms related to interface strength or microstructural effects. Indentation creep measurements performed on Au5Sn, Au–Sn eutectic (29 at.% Sn) and AuSn indicate that these alloys are significantly more creep resistant than common soft solders, in keeping with typical observations of actual joint performance.

Elimination of Au-embrittlement in solder joints on Au/Ni metallization

Systematic experimental work was carried out to understand the mechanism of Au diffusion to the solder interface, and a novel method was proposed to eliminate Au embrittlement by circumventing the continuous layer of (Au,Ni)Sn4 at the solder interface. Contrary to the usual expectation, it was determined that utilization of a very thin Ni metallization in the Au/Ni/Cu under bump metallization (UBM) was an effective means of maintaining mechanical integrity of the solder joint. It was found that the out-diffusion of Cu during the aging period changes the chemistry and morphology of the intermetallic compounds at the interface.

Effects of Cu and Ni additions to eutectic Pb–Sn solders on Au embrittlement of solder interconnections

Effects Cu and Ni additions on Au embrittlement of eutectic Sn–Pb solder interconnections during a solid-state aging treatment were evaluated using a ball shear testing method. The addition resulted in a formation of Au-containing ternary intermetallic compounds, either Au–Sn–Cu or Au–Sn–Ni phase, inside the solder matrix during aging treatment. The fracture energy of the solder interconnection containing 2.9 wt% Cu remained almost the same up to 200 h of aging treatment at 150 °C, demonstrating the possibility of suppressing the Au embrittlement by forming ternary intermetallic phases inside the solder matrix.

Characterization of Gold Embrittlement in Solder Joints

Mechanical strength, integrity, and reliability of solder connections used in the microelectronics industry are important factors in overall quality and reliability of the finished product. In most cases tin (Sn) rich solders are attached to a base metal plated with nickel (Ni) and then with gold (Au). Formation of AuSn4 intermetallics in the solder may result in loss of more than 80% of the initial impact toughness, resulting in loss of reliability of the connection. Gold (Au) embrittlement is a major concern in tin/lead (Sn/Pb) soldering or any other joining process with Au and Sn as major constituents. Noncompliance to Au plating-thickness specifications by vendors or insufficient Sn wicking of Au surfaces can result in embrittled joints and unreliable parts.