The increased price of gold is making gold ball bonding, a mainstay process in package assembly, cost prohibitive. Copper provides a viable alternative, but also poses challenges due to physical properties. With proper equipment adaptation, copper is on its way to replacing gold as the material-of-choice for fine- and ultra-fine pitch packaging.
BY CHRISTOPHER BREACH, Ph.D, Oerlikon Esec
Thermosonic ball bonding is a major interconnect process in microelectronics packaging. However, the primary wire material used in fine-pitch (FP) and ultra-fine pitch (UFP) ball bonding is gold, and due to drastic price increases, gold ballbonding has become a costly process that has a considerable economic effect on package assembly. An alternative wire material to gold is copper, which is much cheaper ($20/kg vs. $28,000/Kg) and has several technical benefits including better electrical conductivity. It has been widely used in discrete and power devices with wire diameters typically larger than 30μm for many years.1,2 With only minor differences in the drawing process used to manufacture the wires, there are potentially huge cost savings.
 Figure 1. Calculated graphs of (a) electrical conductivity (b) fusing current for Cu and Au wires. |
However copper wire behaves quite differently than gold due to its different physical properties, some of which are beneficial and others detrimental to bonding performance.
Electrical Properties of Cu and Au
Figure 1a shows that the electrical conductivity of Cu is significantly better than Au. Another property of interest is the fusing current, defined as the current required to melt the wire at its center.3 The graph in Figure 1b shows how fusing current varies with wire diameter and wire length in air and is calculated using the procedure in reference 3.3 Copper wire has a significantly larger fusing current than gold over a range of wire lengths, although the difference becomes much less significant in long wires. However, when wires are surrounded by molding compound, fusing current generally increases significantly due to the high thermal conductivity of the compound (relative to air). In general, Cu wire is capable of carrying higher currents than the same diameter Au wire. Of course, the specific current carrying properties of Cu wire will depend on the nature of the wire i.e. the purity. Increasing amounts of impurities and alloy/micro-alloy content reduce electrical conductivity.4