Segmented Copper Pillar Bump Layout for Reflow Stress Absorption
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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 or cracking of adjacent vulnerable layer structures during reflow processes or thermal cycling, 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 electrical and thermal conductivity, then signal transduction and heat dissipation are improved, but larger stress is induced in the packaging due to thermal expansion differences
Solution Approach 1:
The copper pillar is divided into two distinct parts: a lower copper pillar portion and an upper metal bump portion. This segmentation allows each part to serve different functions - the copper pillar provides electrical connectivity and heat dissipation, while the metal bump absorbs thermal expansion stress, thereby resolving the contradiction between improved conductivity and reduced packaging stress.
Solution Approach 2:
The invention uses a composite structure combining copper material for the lower pillar portion and a different metal material for the upper bump portion. This composite material approach leverages the high electrical and thermal conductivity of copper while utilizing the stress-absorbing properties of the metal bump material, effectively addressing both the conductivity improvement and stress reduction requirements.
2Productivity
If copper pillar bump technology is used to increase fine pitch to 40 μm, then capacity for flip chip package is increased, but adjacent vulnerable layer structures suffer delamination or crack issue during reflow process
Solution Approach 1:
By segmenting the copper pillar into a lower copper portion and an upper metal bump portion, the invention enables the metal bump to specifically absorb stress during reflow processes, protecting vulnerable layer structures from delamination and cracking while maintaining the high-density 40 μm fine pitch configuration.
Solution Approach 2:
The metal bump acts as an intermediary element between the copper pillar and the surrounding vulnerable layer structures. It absorbs and buffers the thermal expansion stress, preventing direct transmission of stress to adjacent layers, thereby protecting them from delamination and cracks during reflow processing.
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 crack of adjacent vulnerable layers, thereby enhancing the reliability of the flip chip package.
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, 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 two diagonally opposite quarter circles, 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.