4.3 Bonding Principles .1 Wire Bonding Types
4.3.2 Thermocompression Bonding
A thermocompression bond (or weld) is the result of bringing two metal surfaces (bonding wire and the substrate or pad metallization, for example)
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0.1 1 10 100 1000
Chip or Package Area, mm^2
Number of I/O
Area array
One row Two rows Three rows Four rows
Fig. 4.5 Number of I/O’s as a function of chip or package area for both perimeter (1 row to 4 rows) and area array interconnection points (bonding pads)
Fig. 4.6 Layouts of Multiple Rows of Bonding Pads on an Integrated Circuit. (a) Two rows at effective 50mm pitch; (b) Three rows at an effective 30mm pitch; (c) Four rows at an effective 10mm pitch
together in intimate contact during a controlled time, temperature, and pressure (or force) cycle. During this ‘‘bonding cycle’’, the wire and, to some extent, the underlying metallization undergo plastic deformation and interdiffusion on the atomic scale. This atomic interdiffusion can result in a uniform welded inter- face, if both gold wire and gold pad or substrate metallization are used. Gold- aluminum intermetallics [67] are formed when gold wire and aluminum pads (or vice versa) are used. Regardless, the plastic deformation that occurs at the bonding interface ensures: intimate surface contact between the wire and the
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1 10 100 1000
Chip or Package Area, mm^2
Input/Output Density
Area array
Four ro ws Three ro
ws
One ro w Two ro
ws
Fig. 4.7 Package or chip I/O density (I/O per unit area) as a function of the area for both perimeter (1–4 rows) and area array interconnection points (bonding pads)
(a) (b)
Fig. 4.8 Examples of Wirebonded Circuitry. (a) Static RAM Module using MCM-D technol- ogy. Unit contains 300 gold thermosonic wirebonds; (b) Experimental X-ray Detector for use in space. 36-detector chips with bond pads on both sides of the chip. Each chip has over 200 wirebonds per side. Total wirebonds on assembly exceed 18,000. Chips mounted on open frame to allow wirebonder access to both sides
pad, provides an increase in the interfacial bonding area, and breaks down any interfacial film layer (oxide, contamination, etc.). Surface roughness, voids, oxides, and absorbed chemical species or moisture layers can all impede the intimate metal-to-metal contact and limit the extent and strength of the inter- facial weld; thus, causing a poor bond. In some cases, this interfacial contam- ination (usually on the pad) is so extensive that it prevents bonding altogether.
The inclusion of contamination at the weld interface can lead to serious relia- bility problems [10].
The interfacial bonding temperatures are typically in the range of 300–4008C [45] for bonds made by thermocompression bonding. The bonding cycle, exclu- sive of bond positioning, takes a fraction of a second. In thermocompression bonding, the required heat for interface formation is applied by either a heated capillary (the bonding tool through which the wire feeds) or by mounting the substrate and/or package on a heated stage (column). With stage or column heat, the die and package combination must come into thermal equilibrium with the stage, which can take seconds to minutes depending upon mass.
Because of the high stage or column temperatures (>3008C) involved in ther- mocompression bonding, IC or device die attachment is usually limited to the gold-silicon eutectic or certain metal alloy attaches. Also, long times on heated stages can cause reliability problems with previously placed wirebonds, such as uncontrolled intermetallic growth. Most modern thermocompression bonders use a combination of both capillary and column heat. The capillary is made of ceramic, ruby, tungsten carbide, or other refractory material. Special capillary shapes are needed for fine-pitch and deep access applications. Controlled capillary resistance is needed to prevent damage to electrostatically sensitive circuitry.
A typical ball bonding cycle is illustrated in Fig. 4.9. There are five major steps in the ball bonding process: (1) ball formation (Views a and b, Fig. 4.9);
(2) ball attachment to IC or substrate pad (first bond) (View c, Fig. 4.9); (3) traverse to second bond location (View d, Fig. 4.9); (4) wire attachment to package or board pad (second bond) (View e, Fig. 4.9); and (5) wire separation (View f, Fig. 4.9). The initial ball formation step is accomplished by cutting the wire end as it extends through the capillary with an electronic discharge. This cutting is called flame-off due to the fact that in the early days of wire bonding an open flame hydrogen (or forming gas) torch was used to cut the wire. Once cut, the ends of the wire ball up due to surface tension and capillary action.
Figure 4.10 illustrates free air balls produced with gold wire by a negative electronic flame-off system. Heat, time, and pressure or force are the major determining factors in the formation of thermocompression bonds. Typically, the forces used in thermocompression bonding are higher than in other ball bonding methods (i.e., thermosonic ball bonding), resulting in a much more flattened ball. Thus, the first bond is ‘‘nail head’’ shape rather than just a slightly flattened ball as obtained with standard pitch thermosonic ball bonding (e.g., see Fig. 4.2a). Fine pitch thermosonic ball bonding produces very flat, minimal diameter and height balls as described below in Sections 4.8 and 4.9.
Fig. 4.9 Schematic representation of the ball bonding cycle: (a) Flame-off; (b) ball formation;
(c) first bond; (d) transition to second bond; (e) second bond; and (f) separation of wire after second bond
Gold wire is used in most thermocompression wire bonding processes because it is easily deformed under pressure at elevated temperature and is very resistant to oxide growth that can inhibit proper ball formation. Alumi- num wire, because of its rapid oxide growth, has difficulty in forming properly shaped balls on standard bonding machines. Successful aluminum wire ball bonds have been formed using an inert atmosphere around the bonding head to minimize oxide formation [30, 65]. Copper and other materials (e.g., palladium and platinum) have also been ball bonded [52] in both thermocompression and thermosonic applications. Also, wedge style thermocompression bonding with many different materials has been performed [55, 8].