Electronic devices contain many internal connections between copper substrates and soldering materials. Historically, lead-based alloys have been used as the principal soldering material for this purpose; however, due to associated health risks, lead is no longer used in the manufacture of electronics. Lead has been replaced with tin-based alloys containing several metallic components, including a compound known as Cu6Sn5. This compound is of significant interest to the manufacturing industry as it is present in the majority of soldered interconnects, including those of future 3D electronic devices.
During the soldering process, a fine layer of Cu6Sn5 forms spontaneously between the copper substrate and the solder itself. If cracks appear in this layer of Cu6Sn5 (when, for example, a mobile phone is dropped), the connection within the electronic device will fail. The chance of failure is increased substantially by any flaw or instability originating in the manufacturing process; therefore, the Cu6Sn5 must be mechanically and electronically sound to optimise device performance. Cracks causing joint failure can form during soldering or during use of a product because the crystal structure of Cu6Sn5 changes as it heats and cools, resulting in expansion and contraction.
Associate Professor Kazuhiro Nogita and his team from The Nihon Superior Centre for the Manufacture of Electronic Materials (NS CMEM) at The University of Queensland and ANFF-Q have recently found that adding different metallic elements to the soldering alloy can stabilise the Cu6Sn5 and alleviate this issue. In particular, they have discovered that additions of zinc, gold and indium can help to minimise stresses caused by thermal expansion.
With technical support from ANFF-Q Professional Officer Dr Javaid Khan, the team used ANFF-Q’s high-precision differential scanning calorimetry (DSC) equipment to detect the very small thermal changes that took place during soldering when different metals were included in the solder alloy. They mapped this information with diagrams that can now be used to improve processing technology and develop more resilient alloys. The outcomes are not only of great scientific value, but will also influence the electronic packaging process and the designs of future joining materials, and other products containing Cu6Sn5. The team’s work with their industry sponsor, Nihon Superior, has produced four co-patents so far, and will play an important role in Nihon’s current and future products.