Ni Plating on BLM Edge for Pb-Free C4 Undercut Control
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Solution Overview
Problem
Pb-free Controlled Collapse Chip Connection (C4) with Ball Limiting Metallurgy (BLM) structures experience chip-level cracking during back-end-of-line (BEOL) processes due to chemical and thermal undercutting, leading to mechanical weakness at the TiW/intermetallic interface, which is exacerbated by different coefficients of thermal expansion between the semiconductor chip and packaging laminate.
Innovation Solution
A BLM structure is formed with a metal seed layer and a barrier metal layer that covers the first circumferential edge but not the second, preventing undercut and reducing tensile stresses during chip-join cool-down, while allowing compressive stress edges to remain uncovered to avoid delamination.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If wet etching is performed on the Cu seed layer to form BLM structures, then the BLM structure is formed with proper adhesion and diffusion barrier properties, but chemical undercut of 1-2 μm occurs at the edges of the C4 solder bump structures
Solution Approach 1:
A Ni barrier layer is electroplated on the Cu seed layer before wet etching to prevent chemical undercut. This preliminary protective layer is deposited in advance to prevent the harmful chemical reaction between the etchant and Cu seed layer, thereby maintaining edge precision while preserving the necessary adhesion and diffusion barrier properties.
Solution Approach 2:
The Ni barrier layer serves as an intermediary between the Cu seed layer and the wet etchant. This intermediate layer prevents direct contact between the etchant and Cu, eliminating the chemical undercut problem while allowing the BLM structure to maintain its adhesion and diffusion barrier functions.
2Reliability
If solder reflow is performed to complete the C4 bump formation, then the solder bump is properly formed and bonded, but thermal undercut of 2-5 μm occurs due to thermal reaction between Sn and exposed Cu
Solution Approach 1:
The Ni barrier layer is deposited on the Cu seed layer before solder reflow to prevent thermal undercut. This preliminary protective layer prevents the thermal reaction between Sn and Cu during the reflow process, maintaining edge precision while allowing proper solder bump formation and bonding.
Solution Approach 2:
The Ni barrier layer acts as an intermediary that prevents direct thermal reaction between Sn in the solder bump and Cu in the seed layer during reflow. This intermediate layer eliminates the harmful thermal undercut while preserving the necessary bonding properties.
3Reliability
If the Cu seed layer is fully exposed to allow good adhesion, then adhesion to passivation layer and bonding pad is improved, but mechanical weakness occurs at the TiW/intermetallic interface due to undercut
Solution Approach 1:
The Ni barrier layer is deposited on the Cu seed layer before subsequent processing steps to prevent undercut. This preliminary protective layer maintains the full exposure of the Cu seed layer for good adhesion while preventing mechanical weakness by eliminating the undercut that would create weak interfaces.
Solution Approach 2:
The Ni barrier layer serves as a protective intermediary that prevents the formation of weak intermetallic interfaces. By preventing undercut, it maintains the mechanical strength of the structure while allowing the Cu seed layer to remain fully exposed for optimal adhesion properties.
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
This approach reduces chip-level cracking by preventing undercut and maintaining the integrity of the Cu seed layer, enhancing the mechanical strength of the BLM structure and minimizing stress-induced damage during thermal expansion.
Implementation Method 1
chemical undercut of 1-2 μm can occur at the edges of the C4 solder bump structures
Implementation Method 2
thermally driven reaction that occurs between the Sn of the solder and any exposed Cu of the Cu seed layer
Implementation Method 3
thermal undercut can add an additional undercut of 2-5 μm from the edges of the chemically undercut C4 solder bump structure
Implementation Method 4
different coefficients of thermal expansion between the semiconductor chip and packaging laminate
Implementation Method 5
chip-level cracking during back-end-of-line (BEOL) processes
Data Source
AI summary
A structure and a method of manufacturing a Pb-free Controlled Collapse Chip Connection (C4) with a Ball Limiting Metallurgy (BLM) structure for semiconductor chip packaging that reduce chip-level cracking during the Back End of Line (BEOL) processes of chip-join cool-down. An edge of the BLM structure that is subject to tensile stress during chip-join cool down is protected from undercut of a metal seed layer, caused by wet etch of the chip to remove metal layers from the chip's surface and solder reflow, by an electroplated barrier layer, which covers a corresponding edge of the metal seed layer.