Hybrid Lead-Free Solder Preform for Die Attach
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Current high-temperature lead-free solder preforms fail to provide both high melting temperature performance and improved wetting for applications like die attach, while also being cost-effective, due to issues with Bi-Ag alloys and the formation of low melting phases or Ag3Sn intermetallic compounds.
Innovation Solution
A lead-free solder preform with a core layer of high melting temperature Bi alloy and an adhesion layer of Sn or Sn-based alloy, where the adhesion layer is designed to enhance wetting and prevent low melting phases, ensuring reliable bonding and thermal fatigue resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If Bi-Ag alloy is used as high temperature solder, then melting temperature requirement is satisfied, but wetting performance on surface finish materials deteriorates
Solution Approach 1:
The solder material is divided into two distinct layers: a core layer containing Bi-Ag alloy for high temperature performance, and an adhesion layer containing Sn-based alloy for improved wetting. This segmentation allows each layer to independently fulfill its specific function without compromising the other.
Solution Approach 2:
Different regions of the solder preform are assigned different material compositions tailored to local requirements: the core layer uses Bi-Ag for high melting point and thermal stability, while the adhesion layer uses Sn-based alloy for superior reaction chemistry with surface finish materials like Cu, Ag, Ni, and Au.
2Reliability
If Sn is alloyed into Bi-Ag to improve wetting, then reaction chemistry with surface finish materials improves, but low melting Bi-Sn phases form which compromise high temperature performance
Solution Approach 1:
Instead of homogeneously alloying Sn into Bi-Ag, the material is segmented into separate layers where Sn-based alloy is confined to the adhesion layer and Bi-Ag remains in the core layer, preventing formation of low melting eutectic phases while maintaining both improved wetting and high temperature performance.
Solution Approach 2:
The adhesion layer acts as an intermediary between the Bi-Ag core and the surface finish materials, providing the necessary Sn for good reaction chemistry and wetting, while the core layer maintains high temperature integrity without being contaminated by low melting phase-forming Sn.
3Temperature
If high melting temperature solder is used for die attach, then thermal stability is improved, but compatibility with standard reflow soldering profiles becomes difficult
Solution Approach 1:
The solder preform is segmented into two layers with different melting characteristics: the adhesion layer melts first at lower temperature to enable wetting and bonding, while the core layer melts at higher temperature to provide thermal stability. This allows the material to accommodate standard reflow profiles while maintaining high temperature performance.
Solution Approach 2:
The solder material exhibits temperature-dependent phase changes: at reflow temperatures, the adhesion layer transitions to molten state for bonding, while the core layer remains solid providing structural support; at higher temperatures, the core layer also melts to achieve complete fusion and thermal stability.
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 solution achieves high melting temperature solder joints with improved wetting and thermal fatigue performance, maintaining structural integrity and cost-effectiveness in high-temperature solder applications.
Implementation Method 1
the adhesion layer includes a second solder alloy or metal having a solidus temperature below 245° C.
Implementation Method 2
These free Sn diffuse to the interface and react with surface finish materials to provide wetting
Implementation Method 3
the formation of Ag3Sn intermetallic compound (IMC) phases in the alloy
Implementation Method 4
the high thermal/electric conductivity
Data Source
AI summary
A lead-free solder preform includes a core layer and adhesion layer coated over surfaces of the core layer, where the preform delivers the combined merits from constituent solder alloys of the core and adhesion layers to provide both high temperature performance and improved wetting in high-temperature solder applications such as die attach. The core layer may be formed of a Bi Alloy having a solidus temperature above 260° C., and the adhesion layer may be formed of Sn, a Sn alloy, a Bi alloy, In, or an In alloy having a solidus temperature below 245° C. The solder preform may be formed using techniques such as: (1) electroplating a core ribbon with an adhesion material, (2) cladding an adhesion material foil onto a core ribbon, and/or (3) dipping a core ribbon in a molten adhesion alloy bath to allow thin layers of adhesion material to adhere to a core ribbon.