Study of process responses, intermetallic formation and reliability of palladium-coated copper wire bonds
Lim, Adeline Bee Yen
Date of Issue2016-05-24
School of Materials Science and Engineering
Kulicke & Soffa Pte Ltd
This thesis aims to address the knowledge gaps in terms of bondability, intermetallic formation and corrosion performance between palladium-coated Cu (Pd-Cu) and bare Cu wire bonds. The research specifically targets industrial fine pitch interconnect applications where a bonding wire (bare Cu and Pd-Cu) of approximately 15 μm diameter is used. In this research project, the effect of key process parameters such as Electronic Flame Off (EFO) current and cover gas type on the palladium distribution in Pd-Cu Free Air Ball (FAB) and bonded balls was studied. It was observed that Electronic Flame Off (EFO) current has a significant effect on the palladium distribution in Pd-Cu FABs. In addition, the hardness of Pd-Cu FABs is strongly affected by the EFO current and palladium distribution. Cu-Al intermetallic compounds (IMCs), typically CuAl2, CuAl and Cu9Al4 can be present between copper bonded ball and aluminum metallization in the as-bonded state and after thermal annealing. Bulk alloys of Cu and Cu-Al IMCs with varying concentrations of palladium (0, 1, 3 and 10 wt.% replacement of Cu) were prepared. The mechanical properties of these Cu and Cu-Al IMCs were studied using nano-indentation. Effects of the addition of palladium on these mechanical properties were also investigated. Among the three Cu-Al IMCs studied, CuAl (H~8.5 GPa) was found to be much harder than CuAl2 and Cu9Al4 (H~6.0 GPa). Young’s moduli of CuAl and Cu9Al4 IMCs were around 155 GPa and are much stiffer than the one for CuAl2 (~110 GPa). The addition of a few percent of Pd only slightly increases the hardness and Young’s modulus of the IMCs. The humidity reliability of bulk Cu, Al and the Cu-Al IMCs with various concentrations of Pd addition was investigated using a conventional three-electrode cell and an acidic chloride solution as the electrolyte. Among the three Cu-Al IMCs, Cu9Al4 has the highest corrosion rate followed by CuAl2 and CuAl. Pure Al is also more easily corroded than pure Cu. Palladium was shown to slightly improve the corrosion resistance of the metals and the Cu-Al IMCs. Interfacial evolution and bond reliability in Pd-Cu wire bonds during isothermal annealing at 175°C was investigated and compared to that of bare Cu wire bonds. The IMC thickness for Pd-Cu bonds with Pd at the bond interface was found to be thinner as compared to that for Pd-Cu wire bonds with no Pd at the bond interface. The presence of palladium at the bond interface has slowed down the IMC growth. Corresponding bond pull test finds that Pd-Cu wire bonds with Pd at the bond interface have best preserved the bond strength after long hours of annealing due to the beneficial presence of Pd. This research project provides fundamental information on the bondability, intermetallic growth and corrosion susceptibility of Pd-Cu wire bonds which will be useful to ensure good quality Pd-Cu wire bonding and reliability. Pd addition was shown to improve the corrosion resistance and thermal reliability of copper wire bonds, and its distribution can be controlled through the bonding process. Based on the generated understandings, guidelines for packaging engineers for reliable bonding using Pd-coated Cu bonding wires were generated.