CoWoS Metallization Stack Asymmetry for Warpage-Aligned Yield
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Solution Overview
Problem
Chip-on-wafer-on-substrate (CoWoS) packaging faces yield reduction due to warpage issues during thermal processing, which can lead to defects such as bump wetting failure and bridging, despite the use of low CTE planar cores, as the thermal expansion of underfill materials induces warpage in opposite directions between the CoW wafer and the package substrate.
Innovation Solution
Adjusting the total metal thickness of the top and bottom metallization stacks, ensuring Ttop > Tbottom, to align the warpage direction of the package substrate with the CoW wafer, while maintaining a metric (Ttop-Tbottom)/Tbottom ≤ 0.12 to prevent underfill cracking, thereby ensuring proper alignment and stability during curing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the total metal thickness of the top metallization stack is increased to be greater than the bottom metallization stack to align warpage direction, then the reliability of the CoWoS assembly is improved, but the device complexity increases due to asymmetric metallization design
Solution Approach 1:
The patent applies asymmetry by designing the top metallization stack with greater total metal thickness than the bottom metallization stack. This asymmetric configuration creates a compensating warpage moment that aligns the warpage direction of the package substrate with the CoW wafer, preventing misalignment defects such as bump wetting failure and bridging during thermal processing.
Solution Approach 2:
The patent changes the physical parameter of metal thickness in the metallization stacks. Specifically, it increases the total metal thickness of the top stack relative to the bottom stack, creating a controlled warpage effect. The metric (Ttop-Tbottom)/Tbottom ≤ 0.12 provides a quantitative parameter control to achieve the desired warpage alignment while preventing underfill cracking.
2Manufacturing precision
If the metric (Ttop-Tbottom)/Tbottom is controlled to prevent underfill cracking, then the manufacturing precision is improved, but the flexibility in metallization design is reduced
Solution Approach 1:
The patent establishes a specific parameter range for the metric (Ttop-Tbottom)/Tbottom ≤ 0.12 to control warpage. This quantitative parameter control ensures that the asymmetric metallization design achieves the desired warpage alignment without causing underfill cracking, providing a clear design guideline that balances precision with practical implementation.
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 yield-reducing failures by ensuring the CoW wafer and package substrate warp in the same direction, maintaining structural integrity and preventing underfill cracking, thus enhancing the reliability of the CoWoS assembly.
Implementation Method 1
the thermal expansion of underfill materials induces warpage in opposite directions between the CoW wafer and the package substrate
Data Source
AI summary
A chip-on-wafer-on-substrate (CoWoS) semiconductor assembly is formed which includes a chip-on-wafer (CoW) sub-assembly of integrated circuit (IC) dies mounted on an interposer, which is in turn mounted on a package substrate with a top metallization stack and a bottom metallization stack using bonding bumps connecting the backside of the interposer and the front side of the package substrate. The bonding bumps provide electrical connections between the ends of through-vias exposed at the backside of the interposer and the top metallization stack of the package substrate. The likelihood of certain failure mechanisms that can adversely affect CoWoS yield are reduced or eliminated by ensuring a total metal thickness of the top metallization stack is greater than a total metal thickness of the bottom metallization stack, but not so much greater as to induce cracking of the underfill material during curing thereof.