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@fairchildsemi.com; 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.
1-4244-0152-6/06/$20.00 ©2006 IEEE19182006 Electronic Components and Technology Conference
