Flip Chip Bump Pad Structure with Copper Reinforcement
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Solution Overview
Problem
As flip chip package sizes decrease and low-k dielectric usage increases, mechanical stresses from coefficient of thermal expansion mismatch lead to failures such as cracking of the undoped silicon glass and low-k dielectric due to the weakened bump pad, particularly in conventional bump pad structures and direct bump on copper structures.
Innovation Solution
Incorporating a copper pad beneath the aluminum pad on a low-k dielectric layer, with vias connecting the copper and aluminum pads, increases the Young's modulus of the structure, reducing peeling and shear stresses by up to 22% compared to conventional designs, and additional copper pads or thicker layers can further enhance mechanical strength.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Volume of moving object
If the bump pad structure is made thinner to enable smaller flip chip packages, then the scalability and compactness are improved, but the mechanical strength and resistance to thermal stress are reduced
Solution Approach 1:
The patent applies composite materials by combining copper and aluminum pads in a multi-layer structure. The copper pad provides high mechanical strength and low-k dielectric compatibility, while the aluminum pad provides electrical connectivity. This composite structure enables smaller package sizes while maintaining the mechanical strength needed to resist thermal stress and prevent cracking.
2Speed
If low-k dielectric materials are used to reduce signal delay, then the electrical performance is improved, but the susceptibility to mechanical stress and cracking increases
Solution Approach 1:
The copper pad acts as an intermediary layer between the low-k dielectric and the aluminum pad. This intermediary structure provides mechanical support to the low-k dielectric, reducing its susceptibility to stress-induced cracking while allowing the low-k dielectric to maintain its electrical performance benefits for signal transmission.
Solution Approach 2:
The combination of copper pad with low-k dielectric creates a composite structure where the copper provides mechanical reinforcement to the inherently weaker low-k dielectric material, enabling it to withstand thermal cycling and mechanical stress while maintaining its low signal delay properties.
3Length of stationary object
If the aluminum pad is made thinner to reduce overall structure size, then the compactness is improved, but the resistance to peeling and shear stress is reduced
Solution Approach 1:
The patent uses a composite pad structure where a thin aluminum pad is combined with a copper pad. The copper pad provides the mechanical strength to resist peeling and shear stress, while the thin aluminum pad maintains electrical connectivity. This composite approach allows reduced overall pad thickness while maintaining stress resistance through the copper layer's mechanical properties.
Data Source
AI summary
An embodiment is a bump bond pad structure that comprises a substrate comprising a top layer, a reinforcement pad disposed on the top layer, an intermediate layer above the top layer, an intermediate connection pad disposed on the intermediate layer, an outer layer above the intermediate layer, and an under bump metal (UBM) connected to the intermediate connection pad through an opening in the outer layer. Further embodiments may comprise a via mechanically coupling the intermediate connection pad to the reinforcement pad. The via may comprise a feature selected from the group consisting of a solid via, a substantially ring-shaped via, or a five by five array of vias. Yet, a further embodiment may comprise a secondary reinforcement pad, and a second via mechanically coupling the reinforcement pad to the secondary reinforcement pad.