Dual Underfill Materials for Flip Chip Corner Stress
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Solution Overview
Problem
Flip chip IC assemblies experience high stress near the corners of the IC die and underfill voiding issues, particularly for larger IC die sizes with fine bump pitches, due to the need for underfill materials with high fracture strength and good filling and flow properties, which are typically mutually exclusive.
Innovation Solution
A dual underfill material approach is used, where a core underfill material with low fracture strength and good flow properties is applied in the center region, and a second underfill material with high fracture strength is applied at the corners, ensuring strong adhesion and minimizing voiding.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If a single underfill material with high fracture strength is used, then bump and underfill cracking at die corners is reduced, but underfill voiding increases for fine solder ball pitches
Solution Approach 1:
The patent applies different underfill material properties to different regions of the die. The first underfill material with higher fracture strength is applied at the die corners where stress concentration occurs, while the second underfill material with lower viscosity and higher flowability is applied in the central region where fine bump pitch requires good filling properties. This local differentiation resolves the contradiction by matching material properties to regional requirements.
Solution Approach 2:
The patent uses a composite underfill system comprising two distinct underfill materials with different property profiles. The first underfill material provides structural strength at corners, while the second underfill material ensures complete filling between fine bumps. This composite approach allows the system to simultaneously achieve both high fracture strength and good flow properties that would be mutually exclusive in a single material.
2Reliability
If a single underfill material with good flow properties is used, then underfill voiding is minimized for fine bump pitches, but fracture strength is insufficient at die corners
Solution Approach 1:
The patent applies different underfill material properties to different regions of the die. The first underfill material with higher fracture strength is applied at the die corners where stress concentration occurs, while the second underfill material with lower viscosity and higher flowability is applied in the central region where fine bump pitch requires good filling properties. This local differentiation resolves the contradiction by matching material properties to regional requirements.
Solution Approach 2:
The patent uses a composite underfill system comprising two distinct underfill materials with different property profiles. The first underfill material provides structural strength at corners, while the second underfill material ensures complete filling between fine bumps. This composite approach allows the system to simultaneously achieve both high fracture strength and good flow properties that would be mutually exclusive in a single material.
3Ease of manufacture
If underfill material is applied in a single layer, then manufacturing process is simple, but cannot simultaneously satisfy both fracture strength and flow properties requirements
Solution Approach 1:
The patent segments the underfill application process into two distinct steps: first applying the corner underfill material to the die corners, then applying the central underfill material to the remaining areas. This segmentation allows each material to be optimized for its specific function while maintaining a relatively simple manufacturing process that builds upon conventional underfill techniques.
Solution Approach 2:
The patent applies the corner underfill material in advance before applying the central underfill material. This preliminary action ensures that the high fracture strength material is in position to protect against corner cracking before the final underfill application, allowing both material types to perform their optimized functions without complex simultaneous application.
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 solution effectively reduces bump and underfill cracking at the corners and voiding, enhancing the reliability of flip chip IC packages by providing adequate mechanical strength and flow properties, suitable for finer ball pitches and larger die sizes.
Implementation Method 1
pushing the IC die with a sufficient force for the core underfill material to be displaced laterally by the bumps so that the bumps contact the land pads
Implementation Method 2
The second underfill material is cured resulting in it having a higher fracture strength as compared to the core underfill
Data Source
AI summary
A method of assembling a flip chip IC package includes applying core underfill material to a surface of a package substrate in a pattern including an area corresponding to a core region of an IC die thereon that is to be attached, that excludes of an area corresponding to corners of the IC die. The IC die is bonded to the package substrate by pushing the IC die with a sufficient force for the core underfill material is displaced laterally by the bumps so that the bumps contact the land pads. After the pushing the corners of the IC die are not on the core underfill. Edge underfilling includes dispensing a second underfill material that is curable liquid to fill an area under the corners of the IC die. The second underfill material is cured resulting in it having a higher fracture strength as compared to the core underfill.