Conductive Ball Tin Solder Copper Layer Reliability
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Solution Overview
Problem
Existing conductive balls in electronic devices form brittle Ni3Sn4 layers during reflow heating, leading to thermal instability, crack generation, and electromigration-induced voids, compromising connection reliability between upper and lower wiring substrates.
Innovation Solution
A conductive ball structure comprising a copper ball with a nickel layer and a tin-based solder, where the copper layer's thickness is adjusted to achieve a copper concentration of 0.7 wt % to 3 wt % in the solder, forming a thermally stable (Cu, Ni)6Sn5 intermetallic compound that prevents crystal growth and electromigration-induced failures.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a Ni layer and Sn/Bi solder are used to form a conductive ball, then the connection between upper and lower wiring substrates is achieved, but a brittle Ni3Sn4 layer forms during reflow heating causing thermal instability and crack generation
Solution Approach 1:
The patent segments the protective layer into multiple distinct layers: a Ni layer (50-200 nm) adjacent to the Cu ball core, and a Sn-based solder layer (1-5 μm) on the outer surface. This segmentation prevents the formation of a continuous brittle Ni3Sn4 layer by introducing a Cu diffusion barrier and controlling intermetallic compound formation at each interface separately, thereby maintaining thermal stability while achieving reliable connections.
Solution Approach 2:
The patent employs a composite structure consisting of Cu ball core, Ni intermediate layer, and Sn-based solder outer layer. This composite material system controls diffusion processes during reflow heating: the Ni layer forms a protective Ni3Sn4 layer at the Ni-Sn interface, while the Cu layer prevents excessive Cu diffusion into the solder. The composite structure thus achieves both connection reliability and thermal stability by managing intermetallic formation at controlled interfaces.
2Reliability
If a Ni layer and Sn/Bi solder are used to form a conductive ball, then the connection between upper and lower wiring substrates is achieved, but Ni moves by electromigration and a void is generated
Solution Approach 1:
The patent introduces a Cu layer as an intermediary between the Ni layer and the Sn-based solder. This Cu layer acts as a diffusion barrier that prevents Ni atoms from migrating through the solder layer under electromigration stress. The Cu layer intercepts and blocks the migration path of Ni atoms, thereby preventing void formation at the Ni-Sn interface while maintaining the electrical connection functionality.
3Stability of the object's composition
If the copper layer's thickness is adjusted to achieve a copper concentration of 0.7 wt % to 3 wt % in the solder, then a thermally stable (Cu, Ni)6Sn5 intermetallic compound is formed, but the manufacturing precision requirement increases
Solution Approach 1:
The patent specifies a Cu concentration range of 0.7-3.0 wt % in the Sn-based solder layer, which corresponds to optimal Cu layer thicknesses of 0.01-0.5 μm. This parameter range is designed to promote the formation of the thermally stable (Cu, Ni)6Sn5 intermetallic compound while avoiding excessive Cu diffusion that would occur at higher concentrations. The specified range provides a manufacturing window that balances thermal stability achievement with practical fabrication tolerances.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The (Cu, Ni)6Sn5 layer enhances connection reliability by preventing cracks and voids, ensuring stable electrical connections under thermal stress and maintaining high strength and density, thus improving the overall reliability of the electronic device.
Implementation Method 1
copper in the copper layer diffuses into the tin-based solder and a concentration of copper in the tin-based solder becomes 0.7 wt % to 3 wt % when reflow heating the tin-based solder
Implementation Method 2
a (Cu, Ni)6Sn5 layer is formed between the nickel layer and the tin-based solder... prevents crystal growth and electromigration-induced failures
Data Source
AI summary
A conductive ball includes a copper ball, a nickel layer covering an outer surface of the copper ball, a copper layer covering an outer surface of the nickel layer, and a tin-based solder covering an outer surface of the copper layer. A copper weight of the copper layer relative to a summed weight of the tin-based solder and the copper layer is 0.7 wt % to 3 wt %.