Among the many processes involved in assembling and manufacturing PCBs, PCB wire bonding is used to develop long-lasting and stable electrical connections.
Once the design has been completed, wiring transitions to PCB fabrication. The board is etched with conductive traces, and vias are drilled to connect multiple layers. Durable, reliable, and solidly performing outcomes depend on effective wiring; thus, careful design and execution are essential in wiring.
What is PCB Wire Bonding?
PCB Wire Bonding Vs. PCB Routing
|
Feature |
PCB Routing |
PCB Wire Bonding |
|---|---|---|
|
Concept |
Involves PCB Copper Layers routing |
Connecting components by attaching wires to PCB |
|
Application |
For operational applications |
For prototype, and testing setups. |
|
Flexibility |
Restricted to PCB layers and traces |
Wired connections not depending upon PCB layers and traces |
|
Scalability |
Low cost and mass production |
Small scale and high cost |
|
Appearance |
Neat and clean appearance |
Complex circuitry |
|
Equipment Requirements |
Depending on PCB softwares |
Depends upon technical and hardware skills |
|
Limitations |
Restricted due to software and size of the PCB |
Requires more time and expertise |
|
Use Cases |
Used in consumer electronics itemsa |
Used in research and development process |
Types of PCB Wire Bonding
Ball Bonding
Ball bonding is used in high-volume production because of its well-known high-speed and high-precision capabilities. It involves using a high-temperature flame-quenching process to form a ball at the end of a very thin wire (usually made of gold or copper). The ball is then bonded to the pad using heat, pressure, and ultrasonic energy. The same technique is used to bond the other end of the wire.
Stitch Bonding
Stitch bonding is an inexpensive wire bonding technique that is mainly utilized with aluminum wire and is thus an economic solution for a range of electronic packaging applications. Unlike ball bonding, stitch bonding is conducted without any application of heat, with single-point bonding being achieved at room temperature. However, stitch bonding is inappropriate for high-arc wiring due to low loop height capability. Additionally, stitch bonding usually requires secondary pressure welding to provide reliable connections.
Wedge Bonding
The wire is pressed against the pad using a wedge-shaped tool, and ultrasonic vibrations create friction, which forms a metallurgical bond. Unlike ball bonding, the technique of wedge bonding often uses aluminum wire, as it is cheaper and better suited to particular applications. This technique works very well when working at higher temperatures or in situations that require strong mechanical connections.
PCB Wire Bonding Material
Bonding Wire Materials
Gold Wire
Gold wire is a good connection material. It is a good conductor for electricity, does not rust, and is highly reliable. This is ideal for critical applications such as in aircraft, medical equipment, and sophisticated communication systems. One key benefit of gold wire is its ability to resist extreme conditions for a really long duration. Gold is expensive, and hence is used mostly where reliability and performance are extremely critical.
Aluminum Wire
Aluminum wire is a low-cost bonding material that is light and a good conductor. Many choose to utilize it for low-budget projects and gadgets for this reason among others. Its low cost and ability to be manufactured in volume are main advantages; thus, they benefit items in great quantities. Nonetheless, aluminum wire has a few disadvantages. It is stiff and prone to rusting, and thus, restricted to low-arc wiring or rough environments where reliability and enduring power are a priority.
Copper Wire
Copper wire is a malleable material that joins objects. It is more affordable and better than gold since it can transmit electricity and withstand heat. This is what has made it popular in cars, factories, and normal electronics. It is a good alternative to using gold since it is readily available and is good for electricity and heat. Nonetheless, copper wire rusts and requires specialized tools to use since it is hard, and connections are harder.
Bond Pad Materials
The bonding wire and PCB pad have to be in perfect alignment to form a strong bond. Aluminum wires work best when connected to plated aluminum pads or aluminum surfaces. Plated gold pads provide a best possible connection with gold wires.
- Gold plating is exceptionally effective against rust, adhering with great power, and it's perfect for critical applications.
- Silver plating has high electrical conductivity and adheres to surfaces with great tenacity, but tarnishes and dulls more rapidly than does gold.
Incompatible materials can lead to poor bonding, intermetallic compound formation, and long-term reliability issues.
The Wire Bonding Process
Key Process Steps
Surface Decontamination and Pre-Treatment (Removal of Oxide Layers)
Eliminating pollutants and oxide layers that could compromise the connection requires a clean bonding surface. Pretreatment ensures better adhesion and bonding strength.
Parameters of Ultrasonic Energy and Pressure
Strong bonds must be produced by carefully optimizing ultrasonic energy and pressure without breaking the wire or the substrate. Proper parameter settings ensure consistent and reliable bonding.
Microscopic Interconnect Mechanism
The strength of the bond depends on the formation of a metallurgical bond at the microscopic level. A strong, long-lasting bond results from the interaction between the atoms in the pad and the atoms in the wire.
Optimizing Design Strategies
- Bond Pad Layout Design and Spacing Rules:
Design rules need to take into account wire loop height and the mechanics of stress. Bond pad arrangement and spacing are critical to minimize interference and ensure reliable connections.
- Administration of Wire Loop Elevation and Mechanical Strain Mitigation:
Loop height management helps to reduce mechanical stress, thereby improving reliability. The use of stress relief measures, like looping, successfully reduces the incidence of wire failure.
- Thermal Management Design (Matching the Expansion of Materials at High-Temperatures):
Differences in thermal expansion between various materials can lead to stress and possible failure. Design ensures that materials perform well under high temperature conditions.
Comparison Between Bonding and Traditional Welding
|
Aspect |
Bonding Technology |
Traditional Welding (Manual/Reflow Soldering) |
|---|---|---|
|
Precision |
High precision, suitable for fine-pitch connections and miniaturized designs. |
Lower precision, limited by solder flow and manual control. |
|
Thermal Stress |
Minimal thermal stress due to localized energy application (ultrasonic or thermosonic). |
Higher thermal stress, as the entire component is exposed to heat. |
|
Suitability for Miniaturization |
Ideal for miniaturized and high-density designs due to fine wire bonding capabilities. |
Less suitable for miniaturization due to larger solder joints and lower precision. |
|
Process Complexity |
More complex process requiring specialized equipment and precise parameter control. |
Simpler process, but requires careful temperature and solder dosage management. |
|
Key Parameters |
Ultrasonic energy, pressure, and wire loop height control. |
Soldering iron temperature (recommended 300-350°C) and solder dosage control. |
|
Applications |
High-frequency, high-reliability, and miniaturized applications (e.g., 5G, medical). |
General-purpose electronics, where cost and simplicity are prioritized. |
|
Limitations |
Higher cost and complexity, requires advanced equipment and expertise. |
Limited precision, higher thermal stress, and less suitable for advanced packaging. |
Conclusion
Wire bonding is one of the fundamentals of modern electronics, making reliable contacts in a wide range of applications. In this guide, we start with soldering and wiring information so that you can understand the wire bonding process. Later on, we discuss wire bonding types and PCB bond material selection. In this way, FCPCBA is trying to make you know what you want to know about PCB wire bonding, so, don't hesitate to reach out to our team for guidance.
Frequently Asked Questions
What is the typical diameter of bonding wires?
Bonding wires typically range from 15 µm to 50 µm in diameter, depending on the application and current requirements.
What is wire sweep in wire bonding?
Wire sweep refers to the unintended movement or deformation of wires during the encapsulation process, which can lead to short circuits or open connections. It is a common issue in high-density wire bonding.
What are common defects in wire bonding?
- Non-stick on pad (NSOP): The wire fails to bond to the pad.
- Lifted bonds: The bond detaches from the pad.
- Wire breakage: The wire breaks during or after bonding.
- Sweeping: Wires deform or shift during encapsulation.
