Thin Die Stack Assembly Using Gaseous Flux for Flatness
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Solution Overview
Problem
As semiconductor circuits miniaturize, the thinness and tight spacing of die stacks pose challenges in maintaining die flatness and preventing residual flux issues, leading to defects, reduced yields, and premature failures due to adhesion and corrosion problems.
Innovation Solution
The use of handler fixtures that maintain die flatness and employ gaseous fluxes to minimize residual flux, combined with precise adhesive application and bonding techniques to ensure planarity and reliable connections.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If traditional water or solvent spray-cleaning techniques are used to remove adhesive residue, then cleaning effectiveness is improved for larger dimensions, but cleaning effectiveness deteriorates at much smaller interconnection pitch dimensions
Solution Approach 1:
The patent transitions from liquid spray-cleaning to gaseous flux that evaporates without residue. This parameter change in the cleaning medium's physical state (from liquid to gas) enables effective cleaning at small pitch dimensions where liquid spray cannot reach or effectively remove residue.
Solution Approach 2:
The patent replaces the mechanical spray-cleaning system with a gaseous flux system that chemically cleans surfaces during the bonding process itself, eliminating the need for separate mechanical cleaning steps and achieving better results at small dimensions.
2Object-generated harmful factors
If gaseous flux materials are used to reduce residual flux, then residual flux is minimized, but adhesive properties are reduced
Solution Approach 1:
The patent introduces a specifically formulated gaseous flux that acts as an intermediary substance, providing both the fluxing action needed for soldering and the adhesive properties needed to hold dies during assembly, without leaving harmful residues. This mediator combines multiple functions in one material.
Solution Approach 2:
The gaseous flux is formulated as a composite material that combines fluxing agents with adhesive components, enabling it to perform multiple functions simultaneously: cleaning surfaces, promoting solder wetting, and providing mechanical support during assembly without leaving harmful residues.
3Length of moving object
If thinner dies are used to achieve smaller dimensions, then circuit miniaturization is improved, but die handling difficulty increases
Solution Approach 1:
The gaseous flux acts as a temporary intermediary support medium during the assembly process, providing mechanical support to thin dies without requiring complex handling equipment. The flux environment allows thin dies to be positioned and bonded with minimal mechanical manipulation.
Solution Approach 2:
The gaseous flux is applied preliminarily before die assembly, creating a protective and supportive environment that prevents die bending and breaking during subsequent handling and bonding operations, enabling successful assembly of ultra-thin dies.
4Quantity of substance
If die stack spacing is reduced to increase density, then interconnection density is improved, but alignment precision becomes more difficult to maintain
Solution Approach 1:
The gaseous flux serves as a mediator that fills and stabilizes the reduced spacing between die stacks during assembly, providing a uniform environment that maintains alignment precision even at closely spaced dimensions, enabling high-density interconnections without sacrificing alignment accuracy.
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 enhances assembly yields and reliability by maintaining die flatness, reducing residual flux, and improving electrical conductivity, thereby supporting high-density interconnections and longer product lifespan.
Implementation Method 1
flux used to remove surface oxides from solder balls and pillars
Implementation Method 2
underfill adhesive fillers such as SiO2 or other fillers and viscosity of the underfill adhesive and ability to flow between interconnections
Implementation Method 3
thermal compression bonding
Implementation Method 4
thermal compression bonding or in some cases, traditional flip chip reflow assembly processes
Data Source
AI summary
Die stacks and methods of making die stacks with very thin dies are disclosed. The die surfaces remain flat within a 5 micron tolerance despite the thinness of the die and the process steps of making the die stack. A residual flux height is kept below 50% of the spacing distance between adjacent surfaces or structures, e.g. in the inter-die spacing.