Mumbai University > Electronics Engineering > Sem 8 > MEMS Technology

Marks: 5M

• Wire bonding is the oldest method for connecting a chip or die to the inner leadframe of the package.
• Gold wires with 25 to 50 μm diameter or aluminum wires with 25 μm diameter are commonly used.
• The bonding of the wires occurs by application of heat and ultrasonic energy to form a thermosonic bond;
• Application of ultrasonic energy alone to form an ultrasonic bond (e.g., a type of cold weld);or application of heat alone and compression to form a thermocompressionbond.

a) Ball bonding:

• A ball bond tool implements a thermo-compression bond by pressing heated gold balls onto metallized pads.
• A torch heats a gold wire tip above the melting point (~400°C), which allows surface tension to form a ball on the end of thewire.
• The tool with a mold on the end presses the ball onto the metallized bond pad.
• material (i.e., gold ball–metallized bond pad).
• The thermosonic ball bond process begins with an electric discharge or spark to melt the gold and produce a ball at the exposed wire end (see figure).
• The tip—or capillary—of the wire-bonding tool descends onto the aluminum bond pad, pressing the gold ball into bonding with the bond pad. Ultrasonic energy is simultaneously applied.
• The capillary then rises and the wire is fed out of it to form a loop as the tip is positioned over the package lead—the next bonding target.
• The capillary is lowered again, deforming the wire against the package lead into the shape of a wedge—the stitch bond.
• As the capillary rises, special clamps close onto the wire, causing it to break immediately above the stitch bond.
• The size of the ball dictates a minimum in-line spacing of approximately 100 μm between adjacent bond pads onthe die. This spacing decreases to 75 μm for stitch bonding.

b) Wedge bonding:

• A wedge bond tool implements an ultrasonic bond by feeding a gold or aluminumwire, which is pressed onto the metallized bond pad and ultrasonic energy (~50 kHz) applied. Figure 9.14 is a schematic the operation of a ball and wedge bond tool.
• Sealing the package well is the final step in packaging a microsystem. Sealing the package cavity can be accomplished by use of a eutectic solder.
• The use of eutectic solder such as AuSn requires high temperature to braze the package lid.
• Alternatively, epoxy can be used to attach the package lid; this requires a much lower temperature, but it produces a nonhermetic seal and has the issue of outgassing discussed earlier.
• Glass frit seals are often used to attach a ceramic lid to a ceramic package, but this requires temperatures higher than brazing.