Wafer Level Chip Scale Package Stress Reduction via Polyimide Layers
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Solution Overview
Problem
As larger dies are bonded to printed circuit boards, the stresses on solder balls increase, and existing methods to reduce these stresses are inadequate, especially since underfill is avoided to maintain re-workability, leading to potential package failures if defective dies are not replaceable.
Innovation Solution
The use of polyimide layers with reduced Young's modulus, such as DFS with a Young's modulus of 0.48 GPa, and varying thickness ratios between these layers to decrease the overall stiffness of the wafer-level chip scale package, thereby reducing stress on solder balls, while maintaining re-workability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stress or pressure
If underfill is applied to reduce stress on solder balls, then stress reduction is achieved, but re-workability is lost and defective dies cannot be replaced
Solution Approach 1:
The patent introduces polyimide layers as intermediary stress-absorbing structures between the die and solder balls. These polyimide layers have reduced Young's modulus compared to conventional materials, allowing them to absorb and distribute thermal and mechanical stresses away from the solder balls, achieving stress reduction without requiring underfill that would compromise re-workability
Solution Approach 2:
The patent changes the material parameter (Young's modulus) of the polyimide layers to reduced values. By selecting polyimide materials with lower Young's modulus and optimizing their thickness, the structure achieves better stress absorption characteristics while maintaining the re-workability of the bonding interface
2Area of moving object
If larger dies are bonded to PCBs, then functional capability is improved, but stress on solder balls increases
Solution Approach 1:
The patent changes the material parameter (Young's modulus) of the polyimide layers to reduced values. By selecting polyimide materials with lower Young's modulus and optimizing their thickness, the structure achieves better stress absorption characteristics while maintaining the re-workability of the bonding interface
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 normalized accumulated stress in solder balls by up to 10% and improves the reliability of the die by minimizing overall stiffness, as demonstrated through simulation results and material combinations.
Implementation Method 1
The use of polyimide layers with reduced Young's modulus, such as DFS with a Young's modulus of 0.48 GPa, and varying thickness ratios between these layers to decrease the overall stiffness of the wafer-level chip scale package, thereby reducing stress on solder balls
Data Source
AI summary
A structure includes a metal pad over a semiconductor substrate, a passivation layer having a portion over the metal pad, and a first polyimide layer over the passivation layer, wherein the first polyimide layer has a first thickness and a first Young's modulus. A post-passivation interconnect (PPI) includes a first portion over the first polyimide layer, and a second portion extending into the passivation layer and the first polyimide layer. The PPI is electrically coupled to the metal pad. A second polyimide layer is over the PPI. The second polyimide layer has a second thickness and a second Young's modulus. At least one of a thickness ratio and a Young's modulus ratio is greater than 1.0, wherein the thickness ratio is the ratio of the first thickness to the second thickness, and the Young's modulus ratio is the ratio of the second Young's modulus to the first Young's modulus.