Core-Shell Cu-Ag Sintering for Whisker-Free Interconnects
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Solution Overview
Problem
The microelectronics industry faces challenges with Pb-free solder alloys due to their poorer drop/impact behavior, higher processing temperatures, anisotropic solidification, thermal expansion, and propensity to form tin whiskers, necessitating the development of alternative interconnect solutions that are dimensionally stable, corrosion-resistant, and cost-effective.
Innovation Solution
The method involves sintering Cu—Ag core-shell nanoparticles at a temperature where the silver shell diffuses to form sintered necks between particles, creating a solderless interconnect that is mechanically and chemically compatible with Cu board pads and components, without forming tin whiskers, using a process that can be controlled by the initial Ag layer thickness and annealing conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If Pb-free solder alloys (Sn-Ag-Cu) are used for electronic interconnects, then the interconnect material does not contain lead, but the material forms tin whiskers and has poorer drop/impact behavior
Solution Approach 1:
The invention uses core-shell particles with a copper core and silver shell, creating a composite material that combines the advantages of both metals. The copper core provides excellent electrical conductivity and mechanical strength, while the silver shell enables low-temperature sintering through fast diffusion, resulting in a reliable interconnect material that is both whisker-free and mechanically robust
Solution Approach 2:
The invention changes the sintering temperature parameter from traditional soldering temperatures (240°C for Sn-Ag-Cu) to lower temperatures (220°C or below) by utilizing the fast diffusion of silver atoms. This parameter change allows sintering to occur at temperatures where tin diffusion is negligible, preventing whisker formation while still achieving strong interparticle bonds
2Reliability
If Pb-free solder alloys are used, then the interconnect is lead-free, but the processing temperature increases to 240°C
Solution Approach 1:
The invention changes the sintering temperature parameter from traditional soldering temperatures (240°C for Sn-Ag-Cu) to lower temperatures (220°C or below) by utilizing the fast diffusion of silver atoms. This parameter change allows sintering to occur at temperatures where tin diffusion is negligible, preventing whisker formation while still achieving strong interparticle bonds
Solution Approach 2:
The silver shell acts as an intermediary that facilitates low-temperature sintering through its fast diffusion characteristics. The silver layer enables atomic transport between particles at lower temperatures, mediating the sintering process without requiring the high temperatures needed for traditional Pb-free solders
3Reliability
If Sn-Ag-Cu solder is used, then the interconnect is Pb-free, but the solidification and thermal expansion become highly anisotropic
Solution Approach 1:
The invention replaces the mechanical solidification process of traditional solders with a diffusion-based sintering process. Instead of relying on liquid-solid phase transitions that cause anisotropic solidification, the core-shell particles bond through atomic diffusion during sintering, creating isotropic mechanical and thermal properties in the final interconnect structure
4Strength
If traditional sintering of semiconductor particles is performed, then the particles sinter completely, but the temperature required is very high
Solution Approach 1:
The invention applies local quality by having different shell thicknesses or compositions on different regions of the particle surface, or by using a shell material (silver) with specific diffusion properties that enable localized sintering at lower temperatures. This allows the sintering process to occur at reduced temperatures while still achieving the desired structural integrity
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 results in a denser, dimensionally stable, and corrosion-resistant interconnect with enhanced mechanical and electrical characteristics, potentially replacing traditional solder joints at lower temperatures than required for traditional semiconductor sintering, offering a viable Pb-free solder alternative.
Implementation Method 1
Ag diffusion from the particle surfaces to particle-particle contacts during heating leads to enhanced interparticle sintering compared with uncoated Cu nanoparticles
Implementation Method 2
sintering of Cu—Ag core-shell particles is an exemplary embodiment disclosed herein
Data Source
AI summary
A sintered structure and method for forming it are disclosed. The method includes obtaining core-shell particles having a core material and a shell material, forming the particles into a powder compact, and annealing the powder compact at an annealing temperature. The shell material is a metal that diffuses faster than the core material at the annealing temperature and diffuses to the contacts between the core-shell particles during annealing to form sintered interfaces between the core-shell particles. The sintered structure can have discontinuous regions of shell material between the sintered interfaces. The core material can be a metal, semiconductor or ceramic. The core material can be copper and the shell material can be silver. The sintered interfaces can be almost purely shell material. The annealing temperature can be significantly lower than the temperature needed to form interfaces between particles of the core material without the shell material.