Sn-Ag Solder Paste for PV Cell Interconnection
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Solution Overview
Problem
Conventional electrically conductive adhesives (ECAs) used in solar module assembly face issues such as inadequate thermal resistance, high cost, and reliability concerns due to flaky hardening agents and gel formation, leading to power losses and substrate damage, especially in extreme temperatures and during thermal cycling.
Innovation Solution
The use of a solder paste comprising Sn-containing solder alloy with a liquidus temperature below 225°C, dispersed in a solder flux, for interconnecting photovoltaic cells, which provides reliable, corrosion-resistant, and cost-effective joints with improved mechanical and electrical properties.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional electrically conductive adhesives (ECAs) are used to interconnect PV cells, then the interconnection process is simplified, but the reliability and thermal resistance are inadequate, leading to power losses and connection failures under extreme temperatures
Solution Approach 1:
The patent changes the material parameters by using a solder paste with a eutectic composition (96.5 wt% Sn, 3.5 wt% Ag) that has a specific melting point of 221°C. This parameter change enables the interconnection to withstand thermal cycling and extreme temperatures reliably, resolving the thermal resistance issue while maintaining connection integrity under harsh environmental conditions.
Solution Approach 2:
The patent employs a composite material system consisting of solder alloy particles (Sn-Ag eutectic) dispersed in a flux medium. This composite structure combines the high thermal and electrical conductivity of the solder alloy with the functional properties of the flux, creating an interconnection material that simultaneously achieves reliable thermal resistance and electrical conductivity, overcoming the limitations of conventional ECAs.
2Reliability
If silver-based electrically conductive adhesives are used, then electrical conductivity is improved, but the cost increases substantially
Solution Approach 1:
The patent modifies the compositional parameters by using a Sn-Ag eutectic solder paste with controlled particle size distribution (D10: 3-5 μm, D50: 8-12 μm, D90: 15-25 μm) and specific alloy ratios. This parameter optimization achieves excellent electrical conductivity and wetting properties while using a more cost-effective material system compared to pure silver-based ECAs, reducing fabrication costs while maintaining high electrical performance.
3Strength
If conventional solder alloy with high melting point is used, then mechanical strength is improved, but damage to the substrate occurs during the soldering process
Solution Approach 1:
The patent changes the thermal parameters by selecting a Sn-Ag eutectic solder paste with a melting point of 221°C, which is lower than conventional high-melting-point solder alloys. This parameter adjustment allows the soldering process to be conducted at reduced temperatures, preventing thermal damage to the PV cell substrate and encapsulants while still achieving adequate mechanical strength of the interconnection joints.
Solution Approach 2:
The patent utilizes the dynamic properties of the flux medium in the solder paste, which remains fluid at processing temperatures and solidifies upon cooling. This dynamic behavior allows the solder to flow and wet the substrate surfaces effectively during the brief heating cycle, then solidify to provide strong mechanical bonds without requiring excessive thermal energy that would damage the substrate.
4Ease of operation
If electrically conductive adhesives are used for shingling method, then interconnection flexibility is improved, but gel formation occurs after long storage leading to connection failure
Solution Approach 1:
The patent employs a composite solder paste material consisting of metal alloy particles dispersed in a stable flux medium. This composite structure provides long-term storage stability by preventing gel formation, as the flux is specifically formulated to maintain its properties over time. The solder paste remains pumpable and printable during storage, ensuring operational flexibility while achieving stable composition throughout the product lifecycle.
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 solder paste interconnections exhibit low voiding, enhanced reliability, and stability against environmental factors, with improved adhesion and conductivity, reducing power loss and increasing the durability of solar modules in harsh conditions.
Implementation Method 1
The impurities can be removed by mechanical cleaning or by chemical means
Implementation Method 2
The flux may help to prevent oxidation during the soldering process
Implementation Method 3
the elevated temperatures required to melt the filler metal (the solder)
Implementation Method 4
The solder paste interconnections exhibit low voiding, enhanced reliability, and stability against environmental factors
Data Source
AI summary
A method of fabricating a solar module by interconnection of a plurality of photovoltaic (PV) cells in which at least a first PV cell is interconnected to a second PV cell using an electrically-conductive adherent comprising or consisting of a solder paste. The solder paste comprises particles of solder alloy dispersed in a solder flux. The solder alloy comprises a Sn-containing solder alloy having a liquidus temperature of less than 225° C.