Simulation and Experimental Analysis for a Ball Stitch on Bump Wire Bonding Process
above a Laminate Substrate
Yong Liu, Howard Allen, Timwah Luk and Scott Irving
Fairchild Semiconductor Corp.
82 Running Hill Road, Mail Stop 35-2E, South Portland, ME 04106
Email: yliu@; Tel: (207) 761-3155; Fax: (207) 761-6339
Abstract
This study will focus on a ball stitch on bump (BSOB) wire bonding process above a laminate substrate by modeling and experiment. The goals of our study are: (1) to determine the stress and deformation mechanism of BSOB wire bonding process on laminate substrate; (2) to understand the impact of wire bonding parameters. The simulation will include the ultrasonic transient dynamic bonding process, and the stress wave transferred to the interface between bond structure and laminate substrate. Different laminate material parameters are studied for the optimized solution. Different ultrasonic parameters of bonding force and frequency are studied and discussed for the effects of bonding process on laminate substrate structures with partial supports. Experimental test work includes a DOE study with different parameters of ultrasonic power and bonding force. Finally, the comparison of modeling and experimental results is provided. Introduction
Currently there is a substantial volume of modeling work on standard ball and wedge wire bonding process, most of which consider pure mechanical bonding loads with static or quasi dynamics methodology to simulate the free air ball (FAB), compressive bonding process on silicon and wedge bonding on lead frame substrate [1-3]. In 2004, we presented a complete transient dynamic modeling work for the wire bonding process of both FAB and the bond pad metallization system at ECTC54 [4]. In recent years, there have been studies of wire bonding for bond pad structure with low K film above silicon substrate [5-6]. However, little work has been reported about a new wire bonding process called BSOB and its use with laminate-based substrate packages. In standard wirebonding the first bond is a ball bond to a bondpad on the die followed by a wedge bond to a bondfinger on the substrate. The BSOB process is different in that the first ball bond is attached to the die and the wire to the ball is then broken, leaving a gold bump. The next bond is a second ball which is bonded to the bondfinger on the substrate. From this ball the wire is played out back to the first ball upon which a wedge bond is made, completing the bonding cycle. BSOB wire bonding is used where exceptionally short wires are needed such as in very thin packages where loop height control is extremely critical and the distance between wire to die edge is very small.
It is known that in reality, wire bonding is a complicated, multiple physical transient dynamic process and is completed within a very short time. The dynamic impact of wirebonding to both devices and the substrate is critical and significant. This study will evaluate the BSOB wire bonding process on a laminate substrate by use of both modeling and experimentation. The goals of our study are to determine the stress and deformation mechanism of the bonding process on a laminate substrate and to understand the impact of different wire bonding parameters to the stress balance and deformation of a bond pad with partial support at the bottom of laminate. The simulation will consider both the ultrasonic transient dynamic bonding process and the stress wave transferred to the interface between bond structure and laminate substrate. The model considers the bonder capillary as a rigid body due to its high hardness, thus the rigid and elastic plastic contact pair between capillary and FAB is introduced. The contact surfaces between the FAB and bond pad are a non-linear contact pair with consideration of the dynamic friction. The Pierce strain rate dependent model is utilized to model the wire bonding stain behavior. Different laminate material parameters are studied to understand their impact on the bond pad structure. Different ultrasonic parameters such as bonding force and frequency are studied and discussed for the effects of the bonding process on laminate substrate structures with partial supports. Experimental test work includes a DOE study with different parameters of ultrasonic power and bonding force. Ball shear strength is used for the DOE test response. Finally, the trend comparison and discussion of modeling and experimental results are presented.
Problem Definition, Material Properties and assumptions
The basic bond pad structure of the laminate substrate, shown in Fig.1, is created using Cu, Ni and Au layers plated onto the laminate material. The wire bonding area is located near the via, which is also very close to the edge of the die. Furthermore, due to the substrate design the bottom is only partially supported. This increases the difficulty of BSOB wire bonding. Before starting the actual assembly BSOB wire bonding process, carefully simulation and analysis are necessary for cost savings. In order to conduct an effective simulation, the following assumptions are made: (1) The temperature of FAB is the same as substrate (in reality, there is some difference due to the transient temperature cooling from FAB forming and moving to contact bond pad). (2) FAB is rate dependent elastic plastic material during bonding process. The bond pad and other metal layers are treated as elastic plastic material. All the other materials are considered to be linear elastic. (3) The contact intermetallic effect, diffusion in bond formation due to ultrasonic energy and heat induced by friction, will not be considered in this paper. (4) The capillary is a rigid body due to much higher Young’s modulus and hardness. The inertia force from capillary transferred to FAB is not considered in this paper. The related material properties are listed in Table 1.
