Wire Bonding Gold wire bonding is one of the main interconnect processes used in semiconductor packaging. It is widely used because it is a proven and stable technology. It is a high throughput process and is very appropriate in a high-volume manufacturing environment. Interconnect Process Interconnect process is the process or method of connecting the IO terminals of a die or what is commonly called bond pads to the terminals of the package. Wire bonding is the common interconnect method for lead frame packages and also for BGA-type packages. Another interconnect method is flip-chip, which is mainly for BGA-type packages. Typical Wire Bonder Machine The wire bonder is a complex, state-of-the-art machine, that has a very precise vision and three-axis motion system. The bonder is able to see and recognize tiny die features or patterns. These patterns are used by the machine to identify precise device location and also accurately target bonding area or position. Unique die features and patterns also serve as device identity and are recognizable by the machine. The capability of this machine is generally called a pattern recognition system or PRS. A bonding machine has several sub-assemblies, which include a material handling system, optic system, bonding system, and a computer that controls the whole system. The XY table carries the bond head and the optic system and precisely moves in the XY direction. The bond head controls the Z movement of the bonding process, and it includes the transducer that performs the actual bonding process by applying force and ultrasonic power. The optic system is the precise eye of the machine that provides information to the PRS. The XY table and the bond head control the XYZ movement to form the wire loop during bonding. Wire feed system controls wire movement and feeding to the bond head. Wire tension in the wire feed system is controlled through pneumatic controls and gauges. Bonded Wire First Bond - Bonded Ball The first bond is the bonding of the free air ball to the bond bad. The bond pad is a small area of a thin layer of aluminum. The first bond requires a small amount of force to press the free air ball onto the bond pad followed by ultrasonic power to form the bond between the two materials. The bond is a very thin gold-aluminum intermetallic layer and the strength of the bond depends on the amount of intermetallic formed. The bond intermetallic is formed within the contact area between the bonded or mashed ball and the bond pad. The larger the mashed ball, the larger the contact area, and the higher the percentage of intermetallic formed. More intermetallic means higher bond strength which is ideal in the bonding process. The bonded ball or mashed ball diameter is the diameter when viewed from the top view. This diameter typically should be in the range of two to four times the wire diameter or nominally three times the wire diameter. However, the mashed ball diameter must also be within the bond pad opening (BPO), otherwise there is a risk of bond shorting to the adjacent wire. Because of the inherent shape of the bonded ball, the contact area is always smaller than the mashed ball diameter. Also, the bonded ball height should be around 0.6 to 0.8 times the wire diameter. Wire Loop The wire loop is important to clear the wire from touching any surface of the die, particularly the die edge while it forms the connection towards the lead. The wire loop must not be tight or too low to avoid unnecessary stress on the wire and prevent it from breaking. Likewise, the wire loop must not be too high otherwise it will be at risk of being exposed to the top of the package. 4 and 5 are not acceptable. 6 fixes the problems of 4 and 5. Second Bond - Stitch The second bond is the bonding of the wire onto the lead which completes the bonding sequence. Similar to the first bond, bond force is applied to press the wire onto the lead, and ultrasonic power is used to form the bond. Because the second bond is on the lead, which is a much harder material compared to the aluminum bond pad, it allows a much higher bond force and ultrasonic power to make the proper bond. The bond strength is relative to the contact area of the wire to lead, which is limited by the wire diameter, capillary tip size and shape, and bond force. One side of the capillary is pressed onto the wire by the application of bond force. This forms the fishtail of the second bond. The lead surface will also have the capillary tip impression showing that sufficient bond force was applied during bonding. Bond Pad Integrated circuits are composed of hundreds or thousands of transistors and have many more input and output terminals. Each terminal corresponds to a bond pad. The more complex the IC, the more bond pads it has. ICs have similar bond pad compositions to diodes and transistors, but apart from bond pad metallization, two dimensions should be known for ICs, the bond pad opening (BPO) and the bond pad pitch (BPP). These two dimensions are critical for determining capillary design. Bonding Tool or Capillary Choosing the right capillary is very important to set up a stable and reliable wire bonding process. Capillary selection starts with defining the correct design or dimension. Critical dimensions of capillary tip that affect the bonding process are the chamfer diameter (CD) and the chamfer angle (CA). MBD should always be within BPO to avoid shorting. A large BPO and BPP will always allow for a larger CD and CA. A larger BPO with a smaller BPP may still use the same CA, but the CD may be the same or smaller, just to guard the bond from shorting. For small BPO and BPP, a smaller CA and CD are recommended. The volume of the free air ball (Vfab) is equal to the volume of the mashed ball (Vmb). The diameter of the free air ball Øfab can be calculated as follows: A good stitch bond has the shape of a fishtail with thickness shaped with the capillary face angle. The stitch length (SL) can be estimated using the capillary dimensions; capillary tip size (T) and CD. Other capillary dimensions that affect stitch shape are face angle (FA) and outer radius (OR). Another thing that should be considered when selecting or defining capillary design is target loop height. Fine-pitch devices have small BPP and will have tight spaces between the wires. It is important that the capillary will have enough space so that it does not touch or disturb adjacent wires. Capillary and Transducer Just like any other tool, there is a limit to how long we can use a single capillary (capillary life). It is common to hear that the capillary is worn out when there is a bonding issue. But, in reality, capillaries are made out of ceramic, which is a very strong material and does not wear out by pressing a soft gold wire. What happens is that there is a gold buildup on the capillary tip and this affects the bond quality. In general, a capillary lasts for a million bonds. 1 wire is equal to two bonds. The capillary is mounted on the transducer, and when replacing the capillary, some things must be noted and done to ensure that the capillary and transducer work as intended. Transducer screw or clamp screw also has its own tool life. A worn-out screw may affect transducer efficiency, so it must be replaced as it approaches its tool life limit. Clamp screw quality is critical and it must also be tightened at a certain torque. Other Key Items Wire bonding is a complex process and involves many parts and parameters that are synchronized and interact at every bond cycle. Everything must be set right for the process to work. It does not need to be perfect, but everything has to be within specifications. P-parts design and quality are critical to a consistent and stable bonding process. P-parts hold and keep the lead frame or substrate from moving during the bonding process. Even the minor movement or bouncing of the leads or die attach pads will lead to a bonding issue, usually a non-stick bond, and for most devices, this is usually an automatic reject or yield loss. Yield loss translates to higher manufacturing costs. Correct bond program is also critical. PRS must be taught properly to make sure that the machine will find the correct eye spots at all times. This is followed by accurately teaching bond location, i.e., the camera crosshair is properly centered on the bond pad. Likewise, the crosshair must also be centered on the capillary tip impression or what’s called capillary offset. These things ensure proper bond placements. Preparation of devices for bonding is also very important. Variability in machines can also have a significant impact on achieving a stable bonding process. Variability can be within the same machine, between machines of a different make, or between machines of the same make. Bond Defects Because the bonds or wires are really small, the device must be inspected under a microscope at about 10x to 40x magnification. To see highly detailed images of the device we need to use magnification levels of 50x to 100x or even higher depending on device size. Bond Strength Tests Testing bond strength is very important. Visually the bond may look very good, but this can only be confirmed after bond strength testing. There are two industry-standard methods required to mechanically test bond strength. Both tests are destructive, which is why both tests are run during machine setup before production run. The machine or process is not released to production until the test is complete with the pass result. Depending on the number of wires, the tests are done on all wires or just a sample of wires. Normal Bonding Low-Loop Bonding Reverse Bonding - To Address Low-Loop Bonding Limitations BSOB / SSB Ball Stitch on Ball Bond stitch on Ball Ball Stitch on Bump stand-off Stitch Bonding BSOB / SSB Process BSOB is not only for reverse bonding, it is also useful for SIP or hybrid packaging, where different substrate materials and bond pads are used, which makes second bond or stitch bonding very difficult. There are many other applications for BSOB / SSB and reverse bonding techniques. Knowing the basic concepts and principles of these techniques will help address many issues that may be encountered in product development or on the production floor.
