Reliability Study of Silver, Copper and Gold Wire Bonding on IC Device

Abstract

Copper wire bonding in IC packages is not always suitable for devices with active circuit under bonding pad because higher bonding power required for copper wire bonding may cause top aluminum metal splash and mechanically impact the circuit underneath. Silver wire is an alternative solution to this problem based on its physical properties and lower cost compared to gold wire. Ag88%Au8.5%Pd2.5%X1% and Ag95%Au1.5%Pd2.5%X1% alloyed silver wires are used in the study to compare with copper and gold wires of 99.99% in purity. As bonding power plays a dominating role in wire bonding, we focused on the effects of silver, copper and gold bonding wires with different bonding power on the top aluminum metal splash of power device by Optical Microscope(OM) and Scanning Electron Microscope(SEM). The ball shear strength of the bonding wires with different bonding power in samples without mold compound encapsulation was investigated before and after 24, 48, 96 and 192 hours of pressure cooker test (PCT). The intermetallic compound (IMC) formed between silver and aluminum was confirmed by focus ion beam (FIB) and transmission electron microscope (TEM). Although the top surface of the silicon device shows no significant difference after aluminum layer removal for all three wire types, the severity level of vertical deformation and side splash of aluminum layer due to copper wire bonding is much more than silver or gold wire using same amount of bonding power. Ball shear strength of non-encapsulated silver wire decreases dramatically after PCT aging compared with copper wire or gold wire and some samples show zero shear strength after PCT 96 hours and PCT 192 hours for silver wires doped with Pd/Au. Furthermore, larger bonding power induces higher ball shear strength. The major IMC compositions between silver and aluminum are Ag3Al and Ag2Al. A thermo dynamic model was built to explain why silver wire is prone to corrosion compared with copper wire by humidity although copper is easier to be ionized than silver. No electrical test was performed as the samples cannot be tested without package encapsulation and singulation. Furthermore, silver wire samples in SO8 package with mold compound encapsulation were subjected to highly accelerated stress test (HAST), PCT, temperature cycle test (TCT) after MSL1 preconditioning test as well as high temperature operation life test (HTOL) according to JEDEC procedures. The encapsulated samples using either Ag 88wt% or Ag95wt% alloys all passed MSL1 and PCT/HAST/TCT/HTOL. Drain to source on-resistance (Rdson) of the device including package parasitics was measured and it has no significant difference between silver wire and gold wire. The results from this study shows promising data using silver alloy wires but care should be taken to further understand the degradation of silver-aluminum interface under severe humidity condition. Using other metallization on silicon top surface such as NiAu or CuAu can significantly alleviate the interface problem related to AgAl.