EFFECTS OF THERMAL AGING ON INTERMETALLIC COMPOUNDS AND VOIDS FORMATION IN AuAl WIRE BONDING

There are several issues related to the mechanical and electrical wirebond failure during wirebonding process. Major factors are associated with AuAl intermetallic system. AuAl intermetallic compounds (IMC) can easily form at room temperature and can be accelerated with the elevated temperature. In this paper, pattern of intermetallic compounds growth and potential degradation due to voids in thermosonic ball bonding were studied on AuAl intermetallic compounds. The thermosonic ball bonding process used 99.99% gold wire and aluminized pad of Si chip. Results after HTS at 150oC for 500 hours demonstrated that voids were generated around the diffusion layer because of the Kirkendall effect and severe voids was clearly exhibited after thermal aging at 150oC for 1000 hours. Prolong aging time can lead to bonding failure associate with Kirkendall voids. INTRODUCTION AuAl intermetallic compounds is describes as intermediate alloy that present at the bonded interface of Au bonding wire and aluminized pads of a Si chip by diffusion welding. The intermetallic compounds growth via crystal vacancies made available by defects, contamination, impurities, grain boundaries and mechanical stress. With time and temperature the AuAl intermetallic compounds advance the Kirkendall voids and degrade the long-term reliability of AuAl bonding system. This is generally accepted as the cause of bond failure influenced by the different diffusion rates of Au and Al atoms through intermetallic phase layers [1]. AuAl Intermetallic Compounds Formation Observations suggest that the initial growth rate of the AuAl intermetallic compounds usually follows a parabolic relationship: x = K t (1) where x is the intermetallic layer thickness, t is the time, K is the rate constant and: Solid State and Technology, Vol. 16, No 2 (2008) 240-246 ISSN 0128-7389 Corresponding Author: [email protected] K = C e -E/KT (2) where C is a constant, E is the activation energy for layer growth (in electron volts), k is the Boltzman constant, and T is the absolute temperature [2]. The phase diagram of the Au-Al alloy system exhibits five intermetallic compounds as shown in Figure 1. Detail study on the intermetallic compounds have been done by Philofsky [3] with Au5Al2 (tan), Au4Al (tan), Au2Al (metallic gray), AuAl (white) and AuAl2 (deep purple) intermetallic phase being observed. It is apparent that Au5Al2 grows much faster than the other phases and often associated with the bond failures. Figure 1: Al-Au phase diagram taken from Hansen“Constitution of Binary Alloys” [4]. EXPERIMENTAL METHOD Samples for this test were prepared with a 120kHz Kulicke & Soffa 8028 thermosonic wire bonder using 25 μm gold (99.99% Au) wire. Bonding parameter setup was carried out on bond pads with an opening of 90 μm and constant bonding temperature of 200oC. Optimized parameter setting have been used for this purpose, where optimization of the parameter was performed using the standard method of response surfaces. Bonded samples were then molded with a commercial molding compound.