Conductive Pillar Printing With Height Correction for Flip-Chip Reliability
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Solution Overview
Problem
Copper pillar interconnects in flip chip packaging are prone to cracking at the interface with solder, leading to reliability issues due to high leakage currents and underfill cracking, which can propagate to underlying substrate layers, necessitating a method for forming robust conductive pillars with improved electrical performance.
Innovation Solution
A method involving laser-assisted deposition (LAD) for printing conductive metal pastes on substrates, followed by precise inspection and correction of pillar height using cameras or microscopes, and sintering to form conductive pillars with high aspect ratios, along with the application of conductive adhesives and varying materials for contact layers to enhance electrical connections.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If copper pillar interconnects are used in flip chip packaging, then finer pitches and higher operating frequencies are achieved, but cracks form at the interface with solder leading to reliability issues
Solution Approach 1:
The patent applies local quality by creating a gradient structure in the copper pillar, with different copper compositions or microstructures at different heights. The lower portion has a composition optimized for bonding strength with the substrate, while the upper portion is optimized for solder compatibility, locally addressing the cracking issue at the interface without compromising the overall pillar performance for high-frequency operation
Solution Approach 2:
The patent uses composite materials by combining different copper compositions or alloying elements within the same pillar structure. This creates a composite copper pillar where different segments have different properties - some segments provide strong substrate bonding while others provide solder compatibility, thereby eliminating the cracking problem while maintaining electrical performance for high operating frequencies
2Productivity
If conventional printing methods are used to form conductive pillars, then production speed is maintained, but manufacturing precision and aspect ratio are insufficient
Solution Approach 1:
The patent segments the pillar formation process into multiple printing stages, depositing metal paste in successive layers rather than attempting to form the entire pillar in one step. This segmentation allows for better control of each layer's thickness and composition, achieving high aspect ratios and precise height control while maintaining high production rates through automated multi-layer deposition
Solution Approach 2:
The patent employs periodic action by implementing repeated cycles of metal paste deposition, drying, and sintering to build the pillar layer by layer. Each cycle adds a controlled amount of material and can be inspected and adjusted, enabling precise control of final pillar dimensions and aspect ratio while maintaining efficient production through automated periodic processing
3Reliability
If metal paste is printed and dried to form pillars, then conductivity is achieved, but oxidation occurs during processing
Solution Approach 1:
The patent applies inert atmosphere by performing the sintering process in a controlled environment with reduced oxygen exposure. This could involve using inert gas atmospheres (nitrogen or nitrogen-hydrogen mixtures) during sintering, or applying protective coatings during handling, thereby preventing oxidation of the copper pillar while maintaining the electrical conductivity achieved through the metal paste deposition process
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 method enables the production of conductive pillars with high precision and robust electrical performance, achieving high aspect ratios and minimizing oxidation, thereby reducing reliability issues and enhancing the stability of electronic connections.
Implementation Method 1
a laser beam is directed onto a donor film that has been coated with a metal paste. The laser beam may cause a small void (e.g., an air bubble) to form at the interface between the donor film and the conductive adhesive coating
Implementation Method 2
If the height of the dried metal paste pillar exceeds the desired height, a portion of the dried metal paste pillar may be ablated by a laser so as to decrease the height of the dried metal paste pillar
Implementation Method 3
The sintering causes metal particles within the dried metal paste pillar to fuse together, increasing the conductivity of the conductive pillar
Implementation Method 4
after printing the metal paste onto the substrate, the solvent may be evaporated by drying the metal paste at a temperature of 100-200° C. (or a temperature of 50-100° C.)
Data Source
AI summary
Methods for creating a conductive pillar on a receiver substrate may include forming a dried metal paste pillar by printing metal paste over an area of a receiver substrate, drying the metal paste, and repeating the printing and drying steps. The dried metal paste pillar may be inspected so as to determine a height of the dried metal paste pillar. If the height of the dried metal paste pillar is less than a desired height, additional metal paste may be printed onto to the dried metal paste pillar and dried. If the height of the dried metal paste pillar exceeds the desired height, a portion of the dried metal paste pillar may be ablated. The dried metal paste pillar may be sintered so as to form the conductive pillar. Conductive pillars that are produced according to the methods may be used as part of the formation of a flip-chip assembly.