Study on interface of Pd-plated Cu wire stitch bonding

Abstract

Cu wire is the alternative material to the Au wire in many aspects, such as better electrical and thermal conductivity, higher mechanical strength and its lower cost for the high volume manufacture. Due to the surface oxidation of the Cu wire, the Pd-plated Cu wire (Pd-Cu wire) has been adopted quickly in many fine pitch and high density package devices. Cu wire bonding has been well understood in recent years, while Pd-Cu wire is still under intense investigation. Here we present a study on the interface of the stitch bonding by TEM analysis. Our experiments were performed on 1 mil (∼25um) Pd-Cu wire bonding with Ag plated leadframes under the 5% Hydrogen forming gases. The Pd coating thickness of the Cu wire was about 100nm, while the thickness of the lead-frame's Ag coating was about 6um. The leadframes were subjected to pre-heating treatments at 200°C in normal, 5 minutes extended and 20 minutes extended condition, total 3 samples, in order to investigate whether it will form oxide material on surface of the lead-frames and degenerate the bondability. The research tried to understand the underlying microscopic mechanism of the stitch bonding interface by means of micro-topography and elements distribution analysis. The samples for TEM examination were prepared by specific manual grinding followed by FIB micro-machining. No oxide layer or Oxygen gathering region was found at the interfaces of three samples by TEM topography. The results indicated that the existing Pd coating at the Cu wire surface was able to prevent Cu from being oxidized, and the surface of the lead-frames also showed no oxygen gathering. A thin “dark” Pd layer at the interface of three samples between copper and silver layers was observed by TEM with EDX, and the peak value of the Pd at the interface was about 40 atomic %. The thickness of the Pd layer was about 30nm which was much thinner than the original coating thickness. It indicated Pd coating has been seriously deformed during the bonding process. Each element (Cu, Pd, and Ag) at the interface had low interdiffusion rates under the pre-heating temperature (200°C) from the analysis of the abrupt elements distribution curves. It was observed that the content of the Cu in Ag coating was varying for the three samples. When the pre-heated time of the lead-frame increased at 200°C, the content of the Cu in Ag coatings increased from 10% to 20% and then to 30%. The details of Pd layer microstructure and topography need further investigation with improved analytical means.