Copper Pillar Bump Structure for Thermal Stress Relief
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Solution Overview
Problem
The copper pillar bump technology induces larger stress in packaging due to thermal expansion differences, leading to delamination and crack issues in adjacent vulnerable layer structures during reflow processes or thermal cycling tests, affecting the reliability of flip chip packages.
Innovation Solution
Replace parts of the copper pillar with a metal bump to absorb excess stress, using a copper pillar structure with a lower part and an upper part, where the metal bump directly contacts the second upper surface of the lower part, increasing the volume ratio of the metal bump to prevent delamination or cracking.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If copper pillar bump technology is used to improve signal transduction and thermal conductivity, then electrical and thermal performance is improved, but larger stress is induced due to thermal expansion differences causing delamination and crack issues
Solution Approach 1:
The copper pillar is divided into multiple segments along its length, with each segment having different cross-sectional areas. This segmentation allows the pillar to better accommodate thermal expansion differences by creating zones of varying stress distribution, thereby reducing delamination and cracking while maintaining electrical and thermal conductivity.
Solution Approach 2:
Different sections of the copper pillar are designed with different cross-sectional areas, creating local variations in mechanical and thermal properties. The varying cross-sectional areas provide localized stress relief zones that accommodate thermal expansion differences, preventing delamination and cracking while preserving overall conductivity.
2Productivity
If fine pitch between copper pillar bumps is scaled down to increase package capacity, then integration density is improved, but stress concentration increases leading to higher risk of delamination and cracking
Solution Approach 1:
The segmented copper pillar structure distributes stress across multiple zones, preventing stress concentration even when pitch is reduced. Each segment acts as an independent stress management unit, allowing fine pitch scaling while maintaining structural integrity and preventing delamination.
Solution Approach 2:
The cross-sectional area parameter of the copper pillar is varied along its length, creating a gradient structure that adapts to stress distribution requirements. This parameter change allows the pillar to maintain strength and resist delamination at reduced pitch dimensions.
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
The design effectively absorbs excess stress during reflow soldering, preventing delamination or cracking of adjacent vulnerable layers, thereby enhancing the reliability of the copper pillar bump structure.
Implementation Method 1
absorb excess stress through the metal bump and prevent the delamination or crack of adjacent vulnerable layer structures
Implementation Method 2
performing a first electroplating process using the first photoresist as a mask to form a lower copper pillar
Data Source
AI summary
A copper pillar bump (CPB) structure, including a passivation layer covering a substrate and exposing a pad and a copper pillar on the passivation layer and the pad and connecting directly with the pad, wherein a horizontal cross-section of the copper pillar is circle. The copper pillar is provided with an upper part and a lower part, and an upper surface of the lower part includes a first upper surface and a second upper surface. The second upper surface is on one side of the first upper surface, a horizontal cross-section of the second upper surface of the lower part is quarter circle, and the upper part of the copper pillar is on the first upper surface of the lower part. A metal bump is on the copper pillar, wherein parts of the metal bump directly contact the second upper surface of the lower part.