Introduction to Two-Source Bonding Wires
Two-source bonding wires refer to a type of wire used to connect semiconductor chips to the pins or leads of integrated circuit (IC) packages. Unlike traditional single-source bonding wires, two-source bonding wires utilize two different wire materials joined together. This allows the wires to benefit from the advantages of both materials.
The main components of a two-source bonding wire are:
- Fine wire section: Typically made of gold, copper or aluminum. Provides flexibility and good bondability.
- Coarse wire section: Typically made of copper or aluminum. Provides mechanical strength.
The fine wire is generally connected to the chip bond pad, while the coarse wire is connected to the package lead. The joint between the two wires is usually located above the highest loop arc.
Two-source bonding wires offer benefits like:
- Cost savings - Less gold or copper is needed compared to single-source gold or copper wires.
- Performance - Optimizes wire properties for both the chip and package bonds.
- Reliability - Alleviates thermal stresses between bonds.
However, two-source bonding is also more complex compared to traditional single-source bonding. Let's explore the mysterious world of two-source wires in more detail.
The Evolution of Two-Source Bonding Wires
The use of two-source bonding wires evolved over decades of semiconductor and packaging development:
- 1950s - Gold wire bonding is introduced for connecting silicon chips.
- 1960s - Aluminum bonding wires emerge as a cheaper alternative to gold.
- 1970s - Copper bonding wires introduced to provide better electrical and thermal conductivity.
- 1980s - Gold bonding wires become thinner (down to 18 μm) enabling connections for high I/O chips.
- Early 2000s - Two-source bonding wires introduced combining different materials.
Driving Forces Behind Two-Source Wires
Several factors drove the need for two-source bonding wires:
- Miniaturization - Wires need greater strength as spans get longer on smaller ICs.
- Complexity - High density ICs require optimizing wire properties at each end.
- Cost - Reducing precious metal usage by utilizing copper or aluminum for most of the span.
Two-source bonding provided an innovative solution to balance these conflicting requirements.
How Two-Source Wire Bonding Works
The process of connecting a chip to an IC package with two-source wires involves the following key steps:
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Ball bonding - The fine wire section is ball-bonded to the chip bond pad using thermocompression or ultrasonic techniques.
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High looping - The wire is looped in an arc up to the required height above the chip.
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Coarse wire attachment - The coarse wire is welded onto the fine wire using a laser or electrical discharge.
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Stitch bonding - The coarse wire is stitch bonded onto the package lead.
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Trimming - The wire is trimmed leaving a small stitch tail.
The joint between the fine and coarse wires is crucial. Parameters like alignment, metallurgical mixing and grain structure affect joint strength and reliability.
Fine Wire Materials Used in Two-Source Bonding
The fine wire carries current from the chip pad and forms the ball bond. Common options include:
- Gold - Excellent conductivity and bondability but expensive. Diameters down to 15 μm possible.
- Copper - Very good conductivity and cost but challenging bonding properties. Limited to diameters above 20 μm.
- Aluminum - Cheaper alternative but with reduced performance compared to Au and Cu. Oxide concerns during bonding.
The choice of fine wire material depends on factors like:
- Costtargets
- Required electrical performance
- Bond pad metallization
- Availability of advanced bonding tools
Trend Towards Copper Fine Wire
Copper is an attractive option for the following reasons:
- Significantly lower cost compared to gold.
- High electrical and thermal conductivity - about 30% higher than gold.
However, copper oxidation and hardnesses require advanced bonding systems to achieve high yields and reliability. The industry is progressively adopting copper fine wire bonding as the technology matures.
Coarse Wire Selection Criteria and Options
The coarse wire portion must provide the required strength to span from chip to package as well as suit the packaging metallization.
Key selection criteria for the coarse wire include:
- Strength - Depends on wire diameter, material properties and span length.
- Stitch bondability - Compatibility with stitch bonding onto the typically Cu or Al package pads.
- Cost - Strong drivers to reduce material costs.
Widely used coarse wire options:
- Copper - Low cost, high strength and conductivity. Requires palladium plating for stitchbonding.
- Aluminum - Lower cost alternative to copper for shorter spans. Higher electrical resistance.
- Nickel-plated Copper - palladium plating alternative with lower cost. Unproven long term reliability.
The Secret Art of Joining Fine and Coarse Wires
A robust metallurgical joint between the fine and coarse wire sections is critical for wire performance and reliability.
However, joining dissimilar metals like gold and copper or aluminum and copper is tricky:
- Intermetallic intermixing can lead to a brittle joint.
- Shrinkages, voids or alignment issues can reduce strength.
- Heating and cooling can induce stresses in the joint.
Common methods for joining the wires include:
- Laser welding - A laser pulse is used to melt the wire surfaces for joining. Fast, consistent and flexible.
- Ball bonding - Heat and ultrasonics create a ball to bond the two wires. Simple but wire size constrained.
- Electrical discharge - A voltage pulse vaporizes and fuses the wire surfaces. Challenging process control.
Extensive process development and qualification is required to optimize the joint integrity. Parameters like laser energy, pulse duration, discharged voltage profile, wire prep and cleanliness are critical.
I use laser welding for its precision, speed and joint quality. The technique provides the control needed for a reliable two-source bonding process.
Impact of Two-Source Wires on Wire Bonding Tools
Using two-source bonding wires required innovations in wire bonding equipment and tools:
- Dual-clamp capability - Independent wire clamps are needed for the fine and coarse wires.
- Precise wire alignment - Coaxial alignment between the two wires during joining is crucial for joint strength.
- Wire positioning and height detection - Dual capability to detect and position both wire types.
Advanced dual wire bonders allow programming parameters independently for the fine and coarse wire processes. This provides the control and repeatability needed for high-yield two-source bonding.
Features like programmable bond force, ultrasonic power and temperature are optimized for each wire section. Real-time measurements like wire height detection allow responsive process adjustments.
The latest smart bonders ease the challenges of two-source bonding. Their capabilities help unlock the benefits of using hybrid fine-coarse wires.
The Future of Two-Source Wire Bonding
Two-source wire bonding provides an evolving solution to connect chips with increasing I/O density to their packages. Some key trends shaping its future are:
- Adoption of Cu fine wire - Progress in Cu bonding will drive broader use of Cu-X two-source wires.
- Coarse wire diversity - Alternatives to Cu and Al for speciality applications.
- Interconnect pitch reduction - Demands for finer wire diameters and optimized loops.
- Process monitoring - In-line monitoring to detect joint defects and ensure reliability.
- Modeling and simulation - Computer modeling to predict performance and optimize designs.
In summary, two-source wire bonding is an advancing technology gaining adoption to reap its benefits while managing the complexity. With diligent engineering and smart equipment, reliable two-source interconnects are within reach.
The mysterious world of hybrid fine-coarse wires is being progressively illuminated to power next-gen semiconductor packages. Through persistent innovation, their full potential can be realized by the industry.