Effects of Lead Design on Ultrasonic Bond Quality of Wire Bonds Narasimalu Srikanth and Charles J. Vath, III Research and Development Department, ASM Technology Singapore No. 2 Yishun Avenue 7, Singapore Phone: (65) 67503172, Fax: (65) 67510447, email: srikanth@asmpt.com Abstract Wire bonding is an important method of interconnection in microelectronics. The micro welding of the contact pad to the gold wire is achieved by a thermosonic wire bonding process. Ultrasonic energy is known to soften metaUic materials and hence when used in the wirebond process to decrease the flow stress similar to thermal energy. But experimental results shows for some lead frame designs neck fatigue, squashed bonds and poor wire pull strength are observed for some particular leads. Detailed resonance studies shows that those leads have closeby resonance compared to the bonding frequency which causes enhanced vibration resulting in smeared and squashed ball as well as localized fatigue at both the neck and heel of the wirebond. Hence precautions during leadframe design are necessary to avoid closeby resonance motion that are in the in-plane direction along the capillary vibration direction. This can be determined apriori using finite element method. 1. Introduction Microelectronic packaging is moving towards increased number of electrical interconnections (I/Os) and hence uses finer pitch wire bonds with very thin gold wires to pack more bonds at the periphery of the device. The thermosonic wire bonding process involves the simulataneous application of mechanical pressure, temperature and ultrasonic energy [1]. Gold is the preferred wire material even though copper is seen as a alternative [2, 3]. The first bond involves the micro-forging of a small metallic ball followed by the application of ultrasonic energy to bond the wire and the device. While the second bond involves applying an impact force and ultrasonic energy to the formed wire and the lead of the leadframes. Leadframes are essentially made of metals and alloys such as copper and alloy42, using a stamping or etching route (see Fig. 1). The first bonds are generally placed close to the periphery of a die along the edge. In order to minimize the chances of wires contacting each other during wire sway due to mold flow induced drag forces, the leads in the leadframes are spread apart such that it ensures uniform pitch distance between the bonded wires [4]. Neck fatigue of wire bonds are seen occasionally in some leadframe designs of surface mount package types such as QFP, TQFP, etc. Literature review shows no detailed study has been made to investigate their cause. The presence of such defects on the wire bonds affects the wirebonds' performance during further reliability tests. The effect of neck fatigue can be quantified in terms of pull strength. Hence in the present study a detailed investigation was undertaken to understand the reasons of such damage. Since the ultrasonic process involves application of a vibration at very high frequency (138KHz), dynamic forces such as inertial forces and damping forces exist other than the spring forces. Hence vibration characterisitcs of the bonding process was taken for detailed study for any clues as to what can induce such a wire damage. 2. Materials and Processes In the present study, a metallic leadframe made of Cu (Olin brand) with a geometry is shown in Fig. 1. In a wirebonder the leads are generally clamped by clamp plates that are placed on both sides under a clamping pressure as shown in Fig. 3. A gold wire of 1 mil diameter was used to electrically connect the silicon device and leads using a ASM's Eagle.m wirebonder. Fig. 1 Typical 100 Lead QFP leadframe used in packaging. 3. Finite element model FEM involves dividing a complicated geometry into a finite number of elements, connected to each other by conditions of compatibility and equilibrium. Thus the infinite number of degrees of freedom associated with a continuous system is reduced to a finite number of degrees of freedom, which can then be examined individually. Finite element method (FEM) has been successfully used in the resonance prediction of metallic structures [5]. In the present study, the leadframe details were modeled using an Abaqus finite element software. All the details of the various leads in the leadframe including the coating such as Ag, Ni and Pd layers were modeled. The resonant 2005 Electronics Packaging Technology Conference 0-7803-9578-6/05/$20.00 .2005 IEEE 756 Authorized licensed use limited to: National University of Singapore. Downloaded on July 17,2010 at 03:12:33 UTC from IEEE Xplore. Restrictions apply.