The second bond, a wedge bond, is performed by pressing the capillary onto the package pad, applying either thermal energy only or thermal energy and US excitation to assist bond formation. Along a programmed trajectory, the wire is now looped to the location of the second bond on the substrate (loop formation). This initial bond is referred to as the ball bond. The first bond of a bonding cycle is achieved by pressing the ball onto the bonding pad on the chip surface, while applying either heat only (TC bonding) or a combination of heat and US energy (TS bonding).
The ball–wedge bonding sequence ( Figure 3(a)) starts with the formation of a ball at the end of the bonding wire, e.g., with a high-voltage electric discharge (electronic flame off). In case of ball–wedge bonding, the bonding wire is fed through a capillary in case of wedge–wedge bonding it is fed through a bonding wedge. One distinguishes two wire bonding sequences, ball–wedge bonding and wedge–wedge bonding, and three basic bonding processes, ultrasonic (US), thermosonic (TS), and thermocompression (TC) bonding ( Baldwin 2001 ). Typical process temperatures are around 400 ☌, which limits its application mainly to ceramic packages.Īfter the die attach, thin metal wires are bonded, one after the other, from the chip’s bond pads to the substrate pads.
Glass die bonding uses a glass frit material, which is either screen/stencil printed or dispensed onto the substrate (see also glass frit bonding in Section 1.15.3.1). Solder die attach provides an excellent thermal conductivity, high-temperature stability in case of hard solders, and is typically used in conjunction with metal and ceramic packages. Typical process temperatures range from 200 ☌ to 400 ☌. The solder can be applied by dispensing from a hot nozzle, by dispensing and printing solder pastes or by applying solder performs.
In solder die bonding, different solder systems, such as AuSi (eutectic), AuGe, PbSn, AuSn, and PbAgIn, are used to attach the chip to the substrate. Solder and glass die bonding generally form a more rigid interface between chip and substrate and typically require higher process temperatures. Many MEMS are sensitive to packaging-induced stresses and soft die attach adhesives are widely used to minimize the stress acting on the sensor chip (see Section 1.15.4). The advantages of polymer die bonding are its flexibility with respect to material properties, the low process temperature, and the relatively low cost. Subsequently, the polymer is cured and the assembly is ready for wire bonding. The die attach process is typically highly automated: a die bonder feeds the substrates, dispenses the die attach adhesive, picks the chip from a carrier, and attaches it to the substrate. The characteristics of the adhesive, namely its rheological, mechanical, electrical, and thermal properties, can be controlled by the polymer matrix itself, but also by adding filler materials, such as gold, silver, alumina (Al 2O 3), or boron nitride particles. In case of polymer die bonding, an adhesive, commonly an epoxy, a polyimide or a silicone, is used to attach the chip to the substrate. One distinguishes polymer die bonding, solder die bonding, and glass die bonding ( Tummala and Rymaszewski 1989, 1997), with polymer die bonding being the most common method.
Wire bonding always requires a preceding die attachment process to mount the die onto the substrate, such as a leadframe or multilayer substrate. Brand, in Comprehensive Microsystems, 2008 1.15.2.2.1 Wire bonding
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