New failure mode induced by electric current in flip chip solder joint

The effect of electric current on the failure mechanism of flip chip solder joints was studied. The solder used was Pb-Sn eutectic, and the joints had a diameter of 100 μm. The soldering pad on the chip-side had a Cu metallurgy, and that on the board-side had an Au/Ni/Cu metallurgy. The flip chip packages were placed in an oven set at 100°C, with 2×10⁴ A/cm² electric current passing through some of the joints in the packages. The rest of the solder joints, which were in the same package but without current passing through, were used as control. During current stressing, the chip surface temperature reached 157°C due to joule heating. A new failure mode induced by the electric current was found. The joints failed by very extensive Cu dissolution on the chip-side. Not only part of the Cu soldering pad was dissolved, but also part of the internal Cu conducting trace within the chip. The dissolved region was back-filled with solder. Large amount of Cu₆Sn₅ intermetallic was present inside the solder joint. The source of Cu in Cu₆Sn₅ was from the dissolved Cu pad and trace. The site of failure was at the conducting trace that had been back-filled with solder, where a much greater current density was present due to a smaller cross-section.