By Dr. Ivy Wei Qin, Kulicke & Soffa, Willow Grove, Pa.
In the semiconductor industry, chip-to-substrate interconnections provide the electrical paths for power and signal distribution. There are three common interconnect methods: flip chip, TAB (tape automated bonding) and wire bonding. Although flip-chip applications are growing, wire bonding continues to be employed for the majority of interconnects. It is currently responsible for more than 90 percent of today's chip interconnects and continues to grow at a phenomenal rate. The Wire Bonding Process Wire bonding involves connecting pads on the die to a leadframe (or substrate) using very fine diameter wire. There are two different wire bonding technologies: ball bonding and wedge bonding. The basic steps for both types of wire bonding include forming the first bond (normally on the chip), forming the wire loop and forming the second bond (normally on the substrate). One key difference is that in ball bonding, a free air ball is formed at the beginning of each bond cycle and the first bond is achieved by bonding the ball to the pad (Figure 1). In wedge bonding, the wire is bonded directly to the device using force and ultrasonic energy (Figure 2).
Capillaries The bonding tools employed in ball bonding are called capillaries, which are axial-symmetric ceramic tools with vertical-feed holes. The bonding tools used for wedge bonding are called wedges, which are generally made of tungsten carbide or titanium carbide with an angled feedhole at the bottom of the wedge. Figure 3 shows wire bonding tools used in fine-pitch applications. The tool tips are shaped to give the clearance needed in fine-pitch bonding. The bonding wire used in ball bonding is gold (Au) wire of 99.99% or greater purity. Alloy wires (99% or less purity) are sometimes used to meet special application requirements, such as high wire strength. Additionally, copper wires can be ball bonded with some modifications to the wire bonder. The modification mainly consists of forming a gas environment to prevent Cu oxidation during the free air ball formation. Both Au and Cu bonding are performed at elevated temperatures (normally 150-200°C); this process is called thermosonic bonding because it employs heat and ultrasonic energy. Wedge bonding can bond Au wire at elevated temperatures and Al wire at room temperature. Most small diameter Al wire used in wedge bonding is Al with 1 percent silicone for increased wire strength. Al wire bonding is attractive for some applications because room temperature bonding is possible and because there are no Au-Al intermetallic compounds, which occasionally cause reliability concerns(1). There are no restrictions on the direction for looping the wire from the first to the second bond in ball bonding. This makes ball bonder looping extremely flexible. Traditional wedge bonding equipment can only place the bonds parallel to the wire direction. A rotary bond head enables wedge bonders to work at different angles by rotating the bond head to the programmed angle before finishing the looping motion to the second bond. With Au wire, stable wedge bonding processes typically can be achieved for wire angles of less than 35 degrees.(2)
Since ball bonding can loop in different angles from the first bond, no rotational q axis is required. The table and bond head move in X, Y and Z directions. The rotational motion in rotary head wedge bonders requires a theta motor and additional moving parts. This moving mass is much larger than the bondhead on a typical ball bonder, resulting in slower speed. Current ball bonding speed can be more than twice that of the fastest wedge bonder. Due to its higher speed, lower cost and more flexible looping capability, ball bonding is the most commonly used interconnect method today. Wedge bonding serves as a complementary technology for applications that require special process considerations. Fine-Pitch Wire Bonding There are several critical factors in fine-pitch wire bonding, including achieving small, reliable bonds (first bond, second bond and tail bonds in ball bonding), maintaining straight loops, and positioning the bonds accurately. Wire bonding's fine-pitch capability has been demonstrated in the laboratory at 35µm in-line pitch on both the leading edge ball and wedge bonders. For 35µm pitch ball bonding, 15µm wire was used with a bonded ball diameter of 27µm. For 35µm wedge bonding, 20µm diameter wire was employed with a bond width of 23µm (Figure 4).
Despite the smaller squash width produced by wedge bonders, the finest pitch devices in production today (45µm) are produced with ball bonders, due to speed and cost advantages, as well as looping flexibility. Ball bonders can also achieve greater accuracy. For accurate bond placement, a wire bonder needs to be able to teach and maintain accurate offsets between the vision system's optical center and the tool center.
In ball bonding, only one X and one Y offset are needed. In wedge bonding, accurate offset has to be achieved in all bonding directions, which can be a challenge in fine-pitch applications. Stacked Die Bonding Applications One of the fastest growing trends in the semiconductor industry is the use of stacked die packages. The need for smaller, lighter, smarter devices drives this three-dimensional packaging technology. Stacked die applications present wire bonding challenges that include low loop and multi-level wire bonding loop clearance requirements, bonding to overhang unsupported die edges and loop resistance to wire sweep during molding.
Stacked die interconnect technologies include forward ball bonding, reverse ball bonding and wedge bonding (Figures 5-6). A reverse ball bonder can achieve lower loops by placing a ball bump on the die, then bonding the wire from the substrate to the die. Although this method results in a lower loop, the extra ball bump process increases the wire cycle time. With looping motion optimization and advances in wire bonding technology, wire bonding equipment can readily provide loop trajectories to meet the requirements of stacked packaging.(3, 4) Conclusions The advantages offered by wire bonding include a large, existing infrastructure, programming flexibility, die shrink capability and low cost. Continuing advances in wire bonding technology, combined with increased process integration, will allow wire bonding to continue meeting the majority of semiconductor interconnect requirements for many years. References 1. G. Harman, Wire Bonding in Microelectronics, McGraw-Hill, 2nd Edition, pp. 115-148. 2. I. W. Qin, P. Bereznycky and D. Doerr, "Wedge Bonding for Ultra Fine Pitch Applications," Proc. Advanced Packaging Technologies Seminar, SEMICON Singapore 2001. 3. M. Klossner and S. Babinetz, "Assembly Solutions for 3-D Stacked Devices," Proceedings, SEMICON Singapore 2002. 4. Qin, et. al, "Providing Stacked Die Solutions with Wedge Bonding Technology," Proceedings, SEMICON Singapore 2002.
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