Annealing effect and crystallization characteristics of copper wire bonding on pre-plated leadframe

Copper wire bonding on pre-plated leadframe (PPF) is an ever-increasing mode of interconnection in electronic packaging to substitute the gold wire bonding in the volume production in the forthcoming years. Reliability from copper wire bonding should however be well known before putting the process into manufacturing. In this letter, the 4N copper wire with 50 μm diameter bonded on PPF substrate was investigated. It was found for some samples that due to the work hardening effect of the copper, low bonding strength in the heel of the 2^nd bond between the copper wire and the PPF surface was noticeable. An anneal process was introduced to recover the microstructural weakness, and wire pull tests revealed that it remarkably enhanced the bond strength of the 2^nd joint. Interfacial crystallization characteristics have shown the difference between the 1^st and the 2^nd bonds which were attributed to hot forming and cold forming respectively.