First introduced by Bell Labs in 1957, wire bonding is an integrated circuit (IC) manufacturing method used in more than 40 billion microelectronic devices each year. Wire bonding is used extensively for interconnecting semiconductor chips to package leads and many other applications that allow RF devices to meet stringent size, weight, power, and cost requirements (SWaP-c).
What is Wire Bonding?
A solid phase welding technique, wire bonding is a method of transferring electrical connections between semiconductor chips and the external leads of RF devices using very small bonding wires. Bonding wires range in size from 15 microns to as large as 500 microns depending on the application. Bonding wires are developed from Gold (Au), Copper (Cu), Silver (Ag), Aluminum (Al) and alloys such as Palladium-Silver (PdAg). Wire bonding is a very popular and effective welding technique due to its reputation of survivability, performance, and adaptability.
Wire Bonding Methodologies
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Thermocompression Wire Bonding
Pressure and heat are applied to the wiring and substrate to achieve an effective bond. Ultrasonics are not applied in this wire bonding process. This approach does have its limitations in that only Au wire and materials can be used. Due to this limitation thermocompression is not a common approach in modern high-reliability wire bonding applications.
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Thermosonic Wire Bonding
Heat, pressure, and ultrasonic energy is applied to the wiring and the substrate in order to achieve an effective bond.
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Ultrasonic Wire Bonding
Pressure and ultrasonics are applied to the wiring and substrate without the use of heat to achieve an effective bond. An advantage of the ultrasonic bonding approach is that this method can be applied with any type of wire material.
How are Wire Bonds Formed?
The two primary wire bond formations are wedge bonds and ball bonds. The vast majority of wire bonds are developed via ball bonding. Advantages of this wire bond method includes enhanced functionality characteristics and a much smaller footprint. Additionally, ball bonding allows for a faster rate of production, enhanced flexibility, and densities.
A form of wedge bonding, ribbon bonding is another method that is gaining widespread attention due to increasing current and frequency requirements. At frequencies above 20 GHz, ribbon wire performs much better than round wire electrically. Additionally, ball-stitch-on-bonding. This approach is ideal for devices that require die to die bonding in multi-chip modules (MCMs), system in packages (SIPs), and stacked die applications.
Wire Bonding Considerations
While widely utilized, wire bonding does present certain challenges and risks. Typical failure examples include bond pad adherence and peeling of the pads. Also, wires can sometimes be positioned poorly, resulting in failures at the bonding points.
For qualifications, nondestructive and destructive bond pull tests are performed to analyze survivability and bond strength. There are other means of qualification such as; mechanical shock and vibration, bond ball shear testing, stabilization bake, constant acceleration, and internal visuals. Wire bond qualifications can vary widely depending on the mission requirements. For high-reliability and harsh environment applications the typical qualification standard is MIL-STD-883 test methods 2011.7 bond strength (destructive bond pull test) and 2023.5 nondestructive bond pull. To learn more about qualification standards for high-reliability and harsh environment applications, click here.
Conclusion
With the right approach wire bonding can be successful on a range of substrates, from printed circuit boards (PCBs) to flexible circuits and ceramics. While some technologies such as flip chip allow designers to reduce package footprints, wire bonding has a strong history of survivability, performance, and adaptability.
In addition to practice, a very important element to wire bonding is the equipment being used. Criteria Labs utilizes the latest wire bonding equipment and technologies available to produce outstanding yields with speed and accuracy.
*Source: NASA Electronic Parts and Packaging Program (nepp.nasa.gov)
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