Low cost silver alloy wire bonding with excellent reliability performance

Abstract

In semiconductor packaging technology, replacement of bonding wire material has received much attention in recent years due to the expensive gold price. Although many novel wire materials, such as Cu, Pd coated Cu, Ag, Pd coated Ag and Ag alloy, had been proposed to be substitute for the conventional gold wire, a comprehensive, industrial standard reliability testing and verification of bonding interface properties must be conducted before widely use in mass production. In this paper, a novel Ag-Au-Pd alloy bonding wire with excellent reliability performance is demonstrated. This bonding wire has been successfully used in mass production and it is fully comparable to the existing gold wire bonding processing. The bonding interface properties between the Ag alloy wires and the Al pads under thermal and humidity condition were systematically investigated. To verify the capability of Ag alloy bonding wire, the bonded test chips were tested under harsh environmental conditions, including 600 hours highly accelerated temperature and humidity stress test (HAST), 600 hours pressure cooker test (PCT), 2000 cycles temperature cycle test (TCT), 200 °C and 2000 hours high temperature storage life test (HTSL), 2000 hours high temperature operation life test (HTOL), and 6000 hours mother board test (MBT). Ag-Au-Pd bonding wires pass all these packaging reliability and product life testing. The interface microstructures and composition of the samples were analyzed by scanning electron microscopy (SEM), focus ion beam (FIB), scanning transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS). Emphasis in this exercise was on the possible interface corrosion, oxidation, segregation and intermetallic compounds (IMCs) formation behaviors after thermal and humidity stress conditions. TEM and EDS mapping results indicated two interface layers were formed between Ag alloy wire and Al pad. EDS analysis revealed that the layer close to Ag alloy wire side could possibly be Ag2Al alloy phase while the layer close to aluminum pad side could possibly be metastable Ag3Al electronic compound. In addition, the concentrations of Au and Pd elements were uniformly distributed in Ag alloy wire and Ag2Al alloy region. Moreover, there exist some oxygen-rich clusters in between the Ag2Al and Ag3Al layers, while the oxygen level in another region was barely detectable. This demonstrates that the addition of Pd and Au elements can support better oxidation resistance. The aluminum element could diffuse across the interface to form Ag2Al, Au-Al, and Pd-Al related intermetallic compounds during wire bonding. The coexistence of multi-phase intermetallic compounds can suppress the interface corrosion and provide excellent adhesion with Ag-Au-Pd alloy wire and Al pad. In the meantime, the Ag, Pd and Au could diffuse across the native oxygen-rich aluminum surface to form Ag3Al intermetallic phase during bonding action. The formation of Ag3Al metastable phase is speculated to be correlated with the possible existence of temperature difference between two ends. It can provide good adhesion and contact for electrical conduction. The existence of Pd in Ag3Al IMC layer could possibly limit the growth rate of IMCs under further stressing. It is concluded that Ag-Au-Pd alloy wire had been proven to be a low cost, highly reliable bonding wire material and can be widely used in packaging industry.