Flip-Chip Bump Anchoring via Lateral Undercut
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Solution Overview
Problem
Conventional flip-chip bump structures exhibit low adhesion and shear strength due to their design, leading to mechanical reliability issues during processing steps.
Innovation Solution
The introduction of a bump structure with a passivation layer and a metal bump featuring a lateral undercut and lip, formed through surface treatment, which enhances adhesion and shear strength by anchoring the bump to the metal pad.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If a conventional bump structure is used, then the manufacturing process is simple, but the adhesion and shear strength are low
Solution Approach 1:
The bump structure is segmented into multiple functional layers including a nickel under-bump metallization layer, an electroless palladium layer, and an immersion gold layer. This segmentation allows each layer to provide specific functions such as adhesion, solderability, and corrosion resistance, thereby improving overall strength while managing complexity through functional specialization
Solution Approach 2:
The patent introduces a lateral undercut feature that extends horizontally beneath the passivation layer, creating a mechanical anchor in the lateral dimension. This dimensional addition provides enhanced adhesion and shear strength by distributing stresses across a larger area and creating a interlocking geometry between the bump and substrate
2Reliability
If a bump structure with lateral undercut and lip is introduced, then adhesion and shear strength are enhanced, but the manufacturing process becomes more complex
Solution Approach 1:
The lateral undercut and lip features are formed through preliminary surface treatment and electroless plating steps performed before final bump deposition. By preparing the substrate surface in advance with the undercut geometry and adhesion promoter layers, the subsequent bump formation process is simplified, and the mechanical reliability is enhanced through pre-established anchoring features
Solution Approach 2:
An electroless nickel under-bump metallization layer is introduced as an intermediary between the copper interconnect and the solder bump. This intermediate layer provides both adhesion enhancement and a controlled diffusion barrier, improving mechanical reliability while allowing the manufacturing process to proceed through standardized electroless plating techniques
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 modified bump structure significantly improves adhesion and shear strength, providing exceptional mechanical reliability and resistance to loss of adhesion and shear forces.
Implementation Method 1
adhesion enhancement
Implementation Method 2
acts as a diffusion barrier
Implementation Method 3
provides solderability
Implementation Method 4
provides corrosion resistance
Implementation Method 5
provides oxidation resistance
Implementation Method 6
enhanced solderability
Implementation Method 7
anchoring the bump to the metal pad
Data Source
AI summary
An embodiment method of forming and a bump structure are disclosed. The bump structure includes a passivation layer formed over a metal pad, the passivation layer having a recess exposing a portion of the metal pad, and a metal bump formed over the metal pad, the metal bump having a lip extending beneath the passivation layer, the lip anchoring the metal bump to the passivation layer.