August 17, 2015, anysilicon
With the creeping rise in the price of gold, many microelectronics manufacturing companies have been looking for alternatives to reduce the cost of ball bonding. Recently, the trend has been to look into the use copper wire. However, there are many drawbacks to using copper wire, including the short shelf life of pure copper wire, EFO must occur in a forming gas (N2 + 3H2) adding cost and infrastructure, and Palladium-coated copper wire also adds cost to the wire.
In addition, because copper is harder than gold, bond pad structures must be scrutinized so the bonding does not damage the die. Pure silver wire has been tried as another alternative to gold but is often eliminated as an option due to the phenomenon of silver migration.
IMAPS held their annual “Topical Workshop & Tabletop Exhibition on Wire Bonding” in San Jose, CA last month where a presentation was delivered on Silver (Ag) alloyed wire as an alternative to copper or gold wire. Within the last few years, some wire manufacturers have created silver alloy bonding wire with their own specific recipes for the alloy. Alloying silver has been useful to eliminate the silver migration phenomenon. The price of silver alloy wire is somewhere between Pd-coated copper and gold. Silver alloy wire has similar wire bonding properties, such as a similar hardness to gold. A couple of purported useful applications are in LED manufacturing and solar panels.
On the other hand, there are challenges in using silver alloy wire as well. With the alloys comes added electrical resistance, which is a negative for many applications. Also, silver alloy ball formation cannot be done in room atmosphere (it needs N2 only). There is also the drawback concerning the suitability of silver alloy wire to any given application; for example, it is probably not suitable for use in radio-frequency applications.
In recent years, the price of gold has approached the price of platinum, making Pt a contending option for wire bonding versus Au wire. The excellent biocompatibility of platinum, combined with its good stimulation and sensing properties, makes this precious metal a top choice for use in life-saving cardiovascular implants such as defibrillators and pacemaker electrodes.
This is a guest post by Palomar Technologies, formerly Hughes Aircraft, which is the global leader of die attach solutions, wire bonding equipment, optoelectronic packaging systems and precision assembly services.