Palladium coated copper wire — Characteristics and wire behavior in BSOB process and it's reliability performance

The successful introduction of copper wire to semiconductor industry had a significant impact on the manufacturing cost thus copper wire which is 60%-70% cheaper than gold as an interconnecting material between die and lead have become the preferred wire for new devices and conversion status for semiconductor companies indicate that migration from gold to copper wire is a major accomplishment. In addition to this, reliability of devices which could not pass MSL-1 at 260 deg C requirement owing to development of kerkendall voiding (which results from very fast IMC growth rate especially after TC500 cycles) have been overcome. But as compared to quality responses and manufacturability, especially on BSOB wirebond process (Bond-stitch on ball) copper wire had been found out also to be more susceptible to issues like lifted ball, cratering and stitch problems (short tail, lifted stitch) as that of gold wire process. So the cost savings is not justifiable because of quality issues. Most of the issues have been attributed mainly due to the fast oxidation characteristics of the copper wire itself and its free air ball, although newer copper kit designs with nitrogen supply focused on free air ball and bond pad area have been developed, the issues had not been mitigated as wirebonding is still done at elevated temperatures. Studies have been conducted across the industry and most of the solution presented is to coat the copper wire material with Palladium, thus the introduction of CuPd wire (Palladium coated copper wire). A CuPd wire still maintains the electrical characteristics of the copper (lower resistivity than gold, copper core at 99.99%) while palladium as a noble metal prevents oxidation of the copper wire. The wire cost is 2-3 times that of a bare copper but still much cheaper as compared to gold wire. This paper aims to provide the results of studies conducted on a CuPd wire on a DFN package of 0.50mm package thickness which was done in conjunction with bare copper wire utilizing a BSOB process. Study was focused on the free air ball, distribution of palladium after electro flame-off application, bondability for 1st and 2nd bond, oxidation response and reliability tests. The results of studies will show the difference between CuPd wire bare copper wire and why reliability failure was encountered on the bare copper BSOB and not on the CuPd BSOB.