Effect of free-air-ball palladium distribution on palladium-coated copper wire corrosion resistance

Since the development of palladium-coated copper wire (CuPd) 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 compared with bare Copper (Cu) wire. CuPd 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 CuPd wire is caused by corrosion of the ball bond intermetallic compound (IMC) bond interface layer induced by chlorine and moisture. However, when the Free-air-ball (FAB) is formed by electronic flame-off (EFO), the Pd may not be necessary to uniformly cover the FAB shell and consequently bonded ball surface. There are many factors affecting the Pd distribution on FAB, like CuPd manufacturing process, EFO currents, FAB size, shielding gas condition, etc. 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 CuPd corrosion resistance. Corrosion resistance methods were developed in this work to simulate the CuPd ball bond reliability. Different Pd distributions on FAB were studied by varying the EFO current and ball size. 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 uniformly covering the FAB shell will result in good corrosion resistance, whilst Pd diffusing into the FAB will lead to bad corrosion resistance. Further study was also performed on developing the next generation CuPd-Improved wire to improve the Pd distribution on CuPd wire with different EFO conditions and to improve the corrosion resistance.