High Temperature Solder (Au/Sn) Failures from Nickel Plating Impurities

High temperature gold/tin eutectic (80 Au/ 20 Sn) solder is used in manufacturing for multiple reasons. These motives may include the ability to post solder a part/device without reflow, high temperature field applications, and allow soldering to thick Au layers without the possibility of precipitating AuSn4 brittle intermetallics. In the following military case, Au/Sn eutectic was employed because of high temperature service and the guarantee of no occurrence of gold embrittlement when soldering to the thick Au outer plating. The Au was plated over an electroplated nickel (Ni) layer on a Kovar (iron/nickel/cobalt) housing. The soldering resulted in an extremely poor bond strength of a duroid circuit to the Kovar housing. The results showed contamination in the supplier’s electroplated Ni bath caused the plating to have poor bond strength. The failure occurred within the Ni plating layer.

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.