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Effect of Free-Air-Ball Palladium Distribution on Palladium-Coated Copper wire Corrosion Resistance
Keywords: Palladium distribution, Palladium coated copper wire, corrosion resistance
Since the development of palladium-coated copper wire (PdCu) in recent years, it has been widely used in the electronic industry due to its good performance, like better bondability on lead surface, longer floor life performance, etc. compared with bare Copper (Cu) wire. PdCu wire was also expected to have better reliability than bare Cu, as it has a noble metal Pd as a coating covering outside. Generally, the reliability failure of PdCu wire is caused by corrosion of the first bond intermetallic compound (IMC) bond interface layer induced by chlorine and moisture. However, when the Free-air-ball (FAB) is formed by electronic flam-off (EFO), the Pd may not be necessary to nicely cover the FAB shell, and consequently bonded ball, like that on the wire surface. There are many factors affecting the Pd distribution on FAB, like PdCu manufacturing process, EFO currents, FAB size, shielding gas condition, ect. It is necessary for the semiconductor industry to understand the Pd distribution on FAB and how it affects the wire reliability performance. The work here investigated the Pd distribution on FAB and its effect on PdCu corrosion resistance Corrosion resistance methods were developed in this work to simulate the PdCu wire reliability. Different Pd distributions on FAB was studied by varying the EFO current. In general, it is observed that there are 3 types of Pd distribution on FAB: 1) Pd homogeneously covers on the FAB shell; 2) Pd partially covers on the FAB shell and partially diffuse into the ball; 3) major Pd diffuses into the ball and become Cu-Pd alloy. It is found that Pd homogeneous coverage of the FAB shell will result in good corrosion resistance, whilst Pd diffusion into the FAB will bring about bad corrosion resistance, which is even worse than pure Cu. Further study was also performed on developing the next generation PdCu wire to improve the Pd distribution on PdCu wire with different EFO conditions and to improve the corrosion resistance.
Jin Zhi (Lois) Liao, Assistance R&D Manager
Heraeus Bonding Wire Singapore
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