Wirebonding and Ribbonbonding
The most common interconnections in a hybrid are wirebonds. In high-power and/or microwave applications, a single wirebond connection does not always meet thermal or electrical requirements. In high-power FETs, the current, and thus the heat, are dissipated through the gate connection; a single wirebond used for this connection, even if it has a large diameter (0.005 in.), might not be able to handle the thermal transfer. When the heat travels from the bond pad surface of the gate to the much smaller surface area of the wirebond, the power is bottlenecked and cannot transfer fast enough, resulting in a bumed-out gate (this power loss should not be confused with the power loss in the silicon due to FET switching). Consequently, multiple wirebonds are often used, although the wire itself may be rated for the carried current. Sometimes it is better to use three or four smaller-diameter gold wirebonds instead of a single larger-diameter aluminum wire-bond; the increased surface area of multiple bonds, coupled with the higher thermal conductivity of the gold wire, can greatly decrease the potential for a thermal failure under high-power conditions.
There are several different kinds of wirebonds, including the wedge bond, the ball bond, and the stitch bond (Harper, 1970). Wedge bonds are made with a wedge or chisel-shaped tool that applies pressure to the lead wire on a preheated bonding pad. Difficulties with wedge bonding include imprecise temperature control, poor wire quality, inadequately mounted silicon chips, and a poorly finished bonding tool.
Ball bonding is a process in which a small ball is formed on the end of the wire by severing the wire with a flame; the ball is then deformed under pressure against the pad area on the silicon chip. The number of steps in this bonding operation is small and the strength of the bond obtained is strong. Aluminum wire cannot be used because of its inability to form a ball when melted with a flame. However, gold wire, which is an excellent electric conductor and is more ductile than aluminum, can be used. A disadvantage of ball bonding is that a relatively large bond pad is required. Figure 22.6a shows the steps in forming a ball bond.
Stitch bonding combines some of the advantages of both wedge and ball bonding. The wire is fed through the bonding capillary, but the bonding area is smaller than for ball bonds, and no hydrogen flame is required. Both gold and aluminum wires can be bonded at a high rate. Figure 22.6b depicts the formation of a wedge bond.
Gold wire is typically bonded using thermocompression; aluminum wire is typically bonded using an ultrasonic process. Thermocompression wirebonding depends on heat and pressure. In general, the bonding equipment includes a microscope, a heated stage, and a heated wedge or capillary that applies pressure to the wire at the interface with the bonding surface. In addition, a wire-feed mechanism is required, as is some mechanism for manipulation and control. Three primary parameters affecting thermocompression bonding are force, temperature, and time. These parameters are interdependent and are affected by other conditions and factors. Minor changes in these variables can cause significant differences in bond characteristics. Low bonding temperature is desirable to avoid degradation of wire bonds due to gold-aluminum interactions. Low pressures avoid fracturing or otherwise damaging the silicon beneath the bond. The bonding tool used in the process may be of tungsten carbide, titanium carbide, sapphire, or ceramic (Schafft, 1972).
In a ball bond the weakest link occurs in the annealed wire leading to the bond. In a stitch or wedge bond, it occurs in the region of the wire in which the cross section has been reduced by the bonding tool.
Ultrasonic wirebonding also involves heat and pressure, but heat is supplied by ultrasonic energy rather than by a heated stage or capillaries. Pressure is also used, but is incidental to the effect of the ultrasonic energy. Three primary factors affecting ultrasonic wirebonds are force, time, and ultrasonic power. The ultrasonic power available for making the bond is dependent on the power setting of the oscillator power supply and the frequency adjustment of the tool. The force used is generally of the order of tens of grams and is large enough to hold the wire in place without slipping and to channel the ultrasonic energy into the bonding site without causing deformation of the wire. High power and short bonding time are usually preferred to avoid metal fatigue and to prevent the initiation of internal cracks. Lower power nevertheless gives a good surface finish and a large pull strength.
The third bonding method is a combination of ultrasonic and thermocompression wirebonding. In ultrasonic ball bonders, the ultrasonic heat is identical to that in ordinary ultrasonic bonders, but a straight-wire capillary is used to feed the wire, as on the thermocompression bonder. Also included is the flame-off device necessary to form the ball on the gold wire (Pecht, I99l).
In ribbon bonding, a gold ribbon is split-tip welded to the die and substrate. Ribbons range from 0.005 to 0.050 in. wide and from 0.001 to 0.005 in. thick. The cross-sectional area of a ribbon can offer more current-carrying capacity than a wirebond, and the larger surface area allows power transfer from the die to the ribbon. An example comparison of cross-sectional and bonding surface area is given below.

patterned using photolithography or etching. If the design requires multilayering, another layer of Kapton is placed on top of the patterned copper and the etching is repeated for the second layer. The copper between the layers of Kapton is sealed as protection against corrosion; the exposed copper, including the leads, is nickel- and gold-plated to generate a corrosion barrier and a bondable surface, respectively,
Figure 22.15 shows a close-up of tape thai has been patterned and plated. Bumped tab leads arc shown in Figure 22.16. The die and the substrate or chip carrier are connected to bumps of copper and/or plating at the ends of the leads by a process similar to resistance welding. After the die has been bonded, it can be probed and tested while still in tape form. Figure 22.17 shows a tape-automated-bonded die. Note that the leads fan out to larger probing pads, which enable the die to be functionally tested. These probe pads are cut off when the die and its bonds are punched out of the tape. After removal from the tape, the leads must be formed, or bent, to be mounred and bonded to the chip carrier or substrates.
TAB has several advantages: The bonding area is much larger than thai of a ball or wedge bond; the lead itself provides a larger cross-sectional area for carrying the current and power; and the copper also gives one of the highest possible thermal conductivities and current capacities. The disadvantages of TAB are the lime and cost of designing and fabricating the tape and the capital expense of the bonding equipment. Each die must have its own tape, patterned for its bonding configuration, and each TAB design requires its own equipment program and setup. Consequently, TAB has typically been limited to high-volume production applications.

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