Graded Modulus Underfill for Semiconductor Assembly Stress Mitigation
Solution Overview
Problem
The coefficient of thermal expansion (CTE) mismatch between semiconductor packages and dies leads to excessive stress on interconnects, causing open-circuit failures and reliability issues due to fatigue, with stiff underfills protecting interconnects but potentially causing defects in semiconductor dies, while flexible underfills fail to adequately safeguard interconnects.
Innovation Solution
The use of underfill materials with graded elastic moduli, featuring a stiff region near the package substrate and a flexible region near the semiconductor die, achieved by segregating filler particles of different sizes within a matrix material, to mitigate stress and prevent defects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If stiff underfill material is used, then interconnects are protected from stress, but semiconductor dies may develop defects due to redistributed stress
Solution Approach 1:
The underfill material is designed with spatially varying mechanical properties: a first region with higher elastic modulus near the substrate to protect interconnects, and a second region with lower elastic modulus near the die to prevent die defects. This local differentiation allows each region to perform its specific function optimally without compromising the other.
Solution Approach 2:
The underfill material comprises a composite structure with multiple filler particles of different sizes distributed in a matrix material. The first filler particles (larger size) concentrate in the first region to provide stiffness, while the second filler particles (smaller size) concentrate in the second region to provide flexibility, creating a composite material with graded mechanical properties.
2Reliability
If flexible underfill material is used, then semiconductor die defects are prevented, but interconnects are not adequately protected from excessive stress
Solution Approach 1:
The underfill material is designed with spatially varying mechanical properties: a first region with higher elastic modulus near the substrate to protect interconnects, and a second region with lower elastic modulus near the die to prevent die defects. This local differentiation allows each region to perform its specific function optimally without compromising the other.
Solution Approach 2:
The underfill material comprises a composite structure with multiple filler particles of different sizes distributed in a matrix material. The first filler particles (larger size) concentrate in the first region to provide stiffness, while the second filler particles (smaller size) concentrate in the second region to provide flexibility, creating a composite material with graded mechanical properties.
3Adaptability or versatility
If CTE mismatch is present between substrate and die, then thermal expansion differences occur, but excessive stress is generated on interconnects causing fatigue failures
Solution Approach 1:
The elastic modulus of the underfill material is varied as a function of position to create a gradient that changes from higher modulus near the substrate to lower modulus near the die. This parameter change allows the underfill to progressively accommodate the CTE mismatch stress, with the stiffer region near the substrate providing structural support and the more compliant region near the die reducing stress transmission to the interconnects.
Solution Approach 2:
The underfill material comprises a composite structure with multiple filler particles of different sizes distributed in a matrix material. The first filler particles (larger size) concentrate in the first region to provide stiffness, while the second filler particles (smaller size) concentrate in the second region to provide flexibility, creating a composite material with graded mechanical 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 effectively reduces the risk of defects in semiconductor dies while protecting interconnects from excessive stress, enhancing the reliability and mechanical coupling of semiconductor device assemblies.
Implementation Method 1
an underfill material with a graded elastic modulus between the package substrate and the semiconductor die
Implementation Method 2
achieved by segregating filler particles of different sizes within a matrix material
Data Source
AI summary
Underfill materials with graded moduli for semiconductor device assemblies, and associated methods and systems are disclosed. In one embodiment, the underfill material between a semiconductor die and a package substrate includes a matrix material, first filler particles with a first size distribution, and second filler particles with a second size distribution different than the first size distribution. Centrifugal force may be applied to the underfill material to arrange the first and second filler particles such that the underfill material may form a first region having a first elastic modulus and a second region having a second elastic modulus different than the first elastic modulus. Once the underfill material is cured, portions of conductive pillars coupling the semiconductor die with the package substrate may be surrounded by the first region, and conductive pads of the package substrate may be surrounded by the second region.