Copper Pillar Bump Structure With Metal Bump Stress Buffering
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Solution Overview
Problem
Copper pillar bump technology induces larger stress in packaging due to thermal expansion differences, leading to delamination and cracking of adjacent vulnerable layer structures during reflow processes and thermal cycling, affecting the reliability of flip chip packages.
Innovation Solution
Replace parts of the copper pillar with a metal bump that directly contacts the copper pillar, absorbing excess stress and preventing delamination or cracking by increasing the volume ratio of the metal bump.
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 cracking
Solution Approach 1:
The patent employs a composite structure consisting of a copper pillar integrated with a metal bump (such as solder material). This composite design combines the high thermal and electrical conductivity of copper with the stress-absorbing properties of the metal bump, which has different thermal expansion characteristics. The metal bump acts as a stress buffer that compensates for thermal expansion mismatches between the copper pillar and surrounding substrates, thereby preventing delamination and cracking while maintaining excellent signal transduction and thermal conductivity.
Solution Approach 2:
The patent modifies the physical and material parameters of the bump structure by transitioning from a pure copper pillar to a hybrid copper-metal bump structure. This parameter change involves selecting metal bump materials with specific thermal expansion coefficients that match or complement the surrounding package materials, thereby adjusting the overall stress distribution and preventing harmful stress-induced failures during thermal cycling.
2Productivity
If fine pitch between copper pillar bumps is scaled down to increase capacity, then integration density is improved, but stress concentration increases leading to higher risk of delamination
Solution Approach 1:
In fine-pitch applications, the copper-metal bump composite structure becomes even more critical for stress management. The metal bump portion acts as a compliant layer that absorbs and distributes stress more effectively than rigid copper alone, enabling higher integration densities without proportionally increasing stress concentration risks.
Solution Approach 2:
The patent applies different material properties to different parts of the bump structure - the copper pillar provides rigid structural support and electrical conductivity, while the metal bump provides localized stress absorption and compliance. This local differentiation of material qualities allows the structure to simultaneously achieve fine pitch compatibility and stress resistance.
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 stress, preventing delamination and cracking of adjacent layers, thereby enhancing the reliability and integrity of flip chip packages.
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 connecting the bonding pad
Data Source
AI summary
A copper pillar bump (CPB) structure is provided in the present invention, including a substrate, a pad on the substrate, a passivation layer covering the substrate and exposing the pad, and a copper pillar on the passivation layer and the pad and connecting directly with the pad. The copper pillar is provided with an upper part and a lower part, and a top surface of the lower part includes a first top surface and a second top surface. The second top surface is on one side of the first top surface, and the upper part of the copper pillar is on the first top surface of the lower part. A metal bump is on the copper pillar, wherein parts of the metal bump directly contact the second top surface of the lower part.