Wire bonding is often used to connect a power semiconductor chip face with other chips or connecting elements. Wires used with wire bonding are generally composed of aluminum, gold, or copper. Since a bond wire is limited in its current capability, several bond wires are often used in parallel for higher amperages. Stitching, i.e. multiple bonding of a wire on a surface, is employed to distribute the current more evenly on a chip. Wire bonding is a specific type of welding.
Wire bonds can be divided into two classes based on wire diameter:
- Thin wire bonding using wires with a diameter of approximately 17 - 100 µm
- Thick wire bonding using wires with a diameter of approximately 100 - 500 µm
Figure 1. Geometry of an ultrasonic welded connection: A) loop height B) loop length C) Bond foot hole
A distinction can be made between different wire bonding processes. In microelectronics thermal compression bonding is generally used (nailhead bonding, gold ball bonding).
Thermal Compression Bonding
Thermal compression bonding, or ball bonding, relies on pressure, temperature, and ultrasonic forces to achieve bonding.
Figure 2. Thermal Compression Technique
The Substrate is heated and welding is accomplished by atomic bonding forces and diffusion. There is no molten liquid phase and the wire material is gold.
The basic steps in thermal compression bonding are as follows:
- Gold ball is melted in H2 flame
- Wire is positioned, first bond (Nail head) is made, bonding tool presses the ball onto the substrate
- Bonding tool is pulled up, loop is made (top of figure 3), wire is positioned and second bond (Stitch) is made
- Bonding tool is pulled up, and excess wire is detached
Figure 3. Thermal Compression Bonding Nailhead and Stitch
Typical characteristics of thermal compression bonding include:
- Substrate temperature of approximately 350° C
- Pressing force of approximately 0.6 N
- Contact duration of approximately 50 ms
Disadvantages of thermal compression bonding include:
- Change of microstructure due to the melting of the gold ball
- Lowered wire strength equates to a higher minimum loop height (top of figure 3)
- The ball has 3 to 4 times larger diameter than the wire hence large connecting surfaces are requires
- With gold wires on aluminium pads, intermetal phases can occur when high temperatures are applied
Ultrasonic Bonding (Wedge Bonding)
Ultrasonic bonding is the most common bonding method used in power electronics. Ultrasonic bonding is often referred to as wedge bonding due to the shape of the bonding tool used in this process.
Figure 4. Ultrasonic or Wedge Bonding
The working principle of ultrasonic bonding is that ultrasonic forces generate friction, and pressure pulls the metals' oxide layers to the surface. The surfaces draw closer to each other and are welded together. No external heat is required.
The basic steps involved in ultrasonic bonding are:
I) Exact positioning of the wire
II) The first bond is made (ultrasonic force + pressure) to the die using the loop procedure
III) The wire is positioned and the second bond is made (ultrasonic force + pressure)
IV) Excess wire is detached
Typical characteristics of wedge bonding include:
- Bonding force of 0.3 - 0.6 N
- Oscillation amplitude of 1 - 2 µm
- Contact time of 5 - 60 ms
- Ultrasonic frequency of approximately 60 kHz
- Up to 30 W power
Current Load Capacity of Aluminium Wires
One of the key characteristics of bonding wires is electrical resistance, which causes them to heat up as current flows through them. The extent to which the wire is heated depends on the diameter and length of the wire. Heat dissipation occurs almost exclusively at the wire ends. The hottest part of the wire can be found in the middle of the wire bond inbetween both bond feet.
Figure 5. Heating of an aluminium wire as a function of current and the length of the wire
The surge current (load capacity with a sinusoidal current with a period of 10ms) depends only on the diameter of the wire - not the length. No remarkable heat dissipation occurs at the bond feet during short periods of current flow.
Figure 6. Bond foot cylinder surface
The chip metallisation directly under the bond feet is a critical factor in determining the thermal stability of bond sites. Current density is greatest at this point since metallisation of 2 - 8 µm is considerably smaller than the wire bond (100 - 500 µm). Elecromigration can occur, a migration of atoms from the area of metalisation to the wire, if the critical current density is surpassed, until no more metal is left in the vicinity of the chip. In this way, the bond is removed from the chip (lift off).
The ever increasing current density witin chips (over 250 A/cm²) present ever increasing demands on bonding. The number of bond wires and their diameter are increased such that many bonds are made with one and the same wire. This method is referred to as stitching.
Figure 7. Stitching
Aluminium bands can be used instead of wires since they have larger bond cross-sections.
Verifying the Bond Sites
The quality of bonding must be controlled by performing a pull test or a meaningful shear test as well as optical verification under a microscope.
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