Conductive Pillar Cap Layer Mitigates Stress Cracking
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Solution Overview
Problem
Copper pillar technology in semiconductor packaging faces reliability concerns due to stress-induced cracks along the interface with solder, which can propagate to low dielectric constant dielectric layers, leading to leakage currents and underfill cracking.
Innovation Solution
A structure and method involving a cap layer, such as nickel, formed over the conductive pillar to reduce stress and prevent cracking, combined with a buffer layer and underfill to enhance reliability and electrical performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If copper pillar technology is used instead of solder bumps, then finer pitches and higher frequency performance are achieved, but stress-induced cracks occur at the interface with solder
Solution Approach 1:
The patent applies composite materials by creating a multi-layer structure consisting of a copper pillar core, nickel intermediate layer, and solder outer layer. This composite structure resolves the technical contradiction by allowing the copper pillar to provide high-frequency performance while the nickel intermediate layer prevents stress-induced cracking at the interface, thus maintaining both high frequency performance and interface reliability.
Solution Approach 2:
The nickel intermediate layer serves as an intermediary between the copper pillar and the solder layer. This mediator layer reduces the thermal expansion mismatch and mechanical stress between the two dissimilar materials, preventing crack formation while allowing the copper pillar to maintain its high-frequency electrical performance characteristics.
2Use of energy by moving object
If copper pillar technology is used, then capacitance load is reduced, but stress causes cracks that lead to leakage currents
Solution Approach 1:
The composite structure of copper pillar with nickel intermediate layer and solder outer layer maintains the low capacitance load advantage of copper while preventing leakage currents through the nickel layer that acts as a barrier to crack propagation, thus preserving both energy efficiency and electrical performance reliability.
Solution Approach 2:
The nickel intermediate layer provides beforehand cushioning by being deposited on the copper pillar before solder attachment. This pre-applied protective layer cushions against thermal and mechanical stress during subsequent processing and operation, preventing crack formation that would otherwise lead to leakage currents and maintain electrical performance.
3Volume of moving object
If copper pillar technology is used, then size and flexibility are improved, but underfill cracking occurs along the interface
Solution Approach 1:
The composite pillar structure with nickel intermediate layer provides a more compliant interface that better accommodates the stress from underfill material, preventing underfill cracking while maintaining the compact package size advantages of flip chip technology.
Solution Approach 2:
The nickel intermediate layer changes the mechanical parameters of the pillar interface, specifically increasing ductility and reducing thermal expansion coefficient mismatch. This parameter change allows the interface to better withstand the stress from underfill material without cracking, while preserving the compact package design.
Data Source
AI summary
An embodiment of the disclosure includes a conductive pillar on a semiconductor die. A substrate is provided. A bond pad is over the substrate. A conductive pillar is over the bond pad. The conductive pillar has a top surface, edge sidewalls and a height. A cap layer is over the top surface of the conductive pillar. The cap layer extends along the edge sidewalls of the conductive pillar for a length. A solder material is over a top surface of the cap layer.