Wire bonding of Cu and Pd coated Cu wire: Bondability, reliability, and IMC formation

Wire bonding with bare Cu and Pd coated Cu (PdCu) wire have been adopted quickly as a mainstream packaging technology for high pin count and fine pitch devices. The differences between Au and Cu wire bonding are well understood as a result of extensive research. However, the differences between Cu and PdCu wire have not been investigated in as much detail. This paper is a result of collaborative work to study the wire bonding process using Cu wire and PdCu wire. 0.7 mil Cu and PdCu wires are used in the study with bonded ball diameter of about 33μm. This is the leading edge of fine pitch Cu wire bonding. This study showed that Cu and PdCu wire can be bonded with similar 1st bond parameters, and the bonding results for these two wire types are similar. Process window test results with respect to the ultrasonic level and force level showed that shear, ball size and pad splash are very similar between Cu wire and PdCu wire. 1st bond pull strength is about 11% higher with PdCu wire indicating higher PdCu wire tensile strength. The after bake pull test results are very different between the two wires. PdCu wire has much higher pad peeling failure rate than Cu wire and the peeling showed up after 24 hour bake at 175°C. The after bake pull strength is also much lower. The poor after bake results indicate that PdCu wire may need different bonding parameter settings than Cu wire. The effect of forming gas versus nitrogen should be examined as well. Second bond study showed the advantage of PdCu wire. The most significant difference is the tail pull strength. PdCu wire has 50% higher tail strength than Cu wire, indicating a more robust 2
^nd
bond process and less chance for short tails. A deeper understanding of these and other differences will assist in the proper selection between these wire types. High resolution transmission electron microscopy (HRTEM) with Energy Dispersive X-ray Spectrometer (EDX) studies show that the approximately 80 nm Pd coating dissolves into the Cu bulk during ball formation process, therefore, almost no Pd is present at the bond interface in the as-bonded state. However, Pd congregates and diffuses back to the bond interface, especially, a Pd-rich layer forms in the peripheral interface. The congregation of Pd near the bond interface appears to be detrimental to the bond strength, causing high peeling failure.