As the industry is pushed toward reducing costs and saving space, while also increasing complexity and reliability, wire bonding techniques will play an ever increasing role in the area of electronics manufacturing. For that reason, it is beneficial to review basic wire bonding techniques.

A wire bond is the welded electrical interconnection on a PCB, usually from the semiconductor die to the non-common lead frame or substrate pad. Normally, gold wire is used for interconnecting techniques.

There are three basic wire bonding techniques:

• thermosonic bonding: utilizes temperature, ultrasonic and low impact force, and ball/ wedge methods. Figure 5-1 illustrates a thermosonic ball bond.
• ultrasonic bonding: utilizes ultrasonic and low impact force, and the wedge method only. Figure 5-2 illustrates an ultrasonic wedge bond.
• thermocompression bonding: utilizes temperature and high impact force, and the wedge method only.

The following are the basic requirements needed for successful wire bonding:

1) Cleaning is the fundamental process needed prior to wire
bonding. The metallization must be free of organic and inorganic contamination. For example, finger print oil on the bonding area reduces the reliability of the interconnection.

There are two popular cleaning methods, which are plasma cleaning and ultraviolet (UV) ozone cleaning.

• Plasma cleaning is effective for removing epoxy bleed-out, which is caused by outgassing.
• UV ozone cleaning emits significant amounts of radiation (wavelengths 1848A and 2537A ) to remove organic contaminants.

2) Setting the proper temperature for thermosonic, ultrasonic, and thermocompression techniques are critical for consistent wire bonding.

• Thermosonic bonding must be set at 100oC-150oC.
• Ultrasonic bonding can be set at 25oC or ambient temperature.
• Thermocompression bonding must be set at 300oC-500oC.

3) Setting the proper force for the thermosonic, ultrasonic,
and thermocompression techniques provides the pressure needed for reliable wire bonds.

• Thermosonic bonding requires 0.5- 2.5g force per wirebond.
• Ultrasonic bonding requires 0.5- 2.5g force per wire bond.
• Thermocompression bonding requires 15-25g force per wire bond.

4) Setting the proper power is essential for the thermosonic
and ultrasonic bonding techniques. To ensure quality bonds, increase the power setting without exerting or over-stressing the wire. You will know over-stressing is taking place when the pull testing device indicates a low break.

5) Make sure the unit is properly clamped in the work hold
er, as it is critical that no movement takes place. You can verify this by nudging the object with tweezers. If movement takes place, the unit must be secured during high speed bonding.

6) Make sure the tool (the capillary) is in functional condi-
tion. Factors such as bond size, bond pad pitch, wire diameter, harness type, and metallization have an effect on bonding performance. The proper tool selection is essential for consistent wire bonding.

Bonding Evaluation:
After bonding, the wire bond may be evaluated using visual methods and mechanical testing, depending on the requirements and situation. The visual method uses an optical microscope, scanning electron microscope (SEM), and other analytical instruments to find the undesirable bonds. Mechanical testing is employed for the evaluation of bond strength. Wire bond evaluation methods are listed in MIL-STD-883D. They include the following:

1) internal visual (method 2010, test condition A and B): checks for internal material, construction, and workmanship of micro-circuit.

2) delay measurement (method 3003): measures the propa-gation delay of micro-circuit.

3) destructive bond pull test (method 2011): evaluates the bond strength by hooking and pulling the wire until failure takes place.

4) non-destructive bond pull test (method 2023): detects unacceptable wire bonds while avoiding damage to acceptable wire bonds.

5) ball bond shear test: determines the ball bond strength by detecting a "cratering" anomaly that the pull test typically does not.

6) constant acceleration (method 2001, test condition E):finds improper interconnected wires through subjection to high acceleration using 10Kg of force.

7) random vibration (method 2026): rigidly fastens wire bonded devices and subjects them to random frequencies and intensities of vibration to detect discrepancies.

8) mechanical shock (method 2002): applies sudden force to wire bonds to produce failures.

9) stabilization bake (method 1008): aims at determining the effect of storage at high temperature without electrical stress.

10) moisture resistance test: performed to evaluate, in an accelerated manner, the corrosion resistance to high humidity and heat.

Wire bonding techniques may pose a new challenge to your manufacturing assembly. However, by understanding the different wire bonding techniques with MIL-STD 883D screening methods, you can optimize and refine the wire bonding process that is dictated by your design and the manufacturer’s requirements.

* Figure 5-1- image courtesy of Aprova

** Figure 5-2- image courtesy of K/S Micro-Swiss