Composite Fusible Element for Lead-Free Circuit Protection
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Solution Overview
Problem
Existing over-voltage and over-current protection devices face challenges in preventing burnout or explosion of batteries during charging or discharging, particularly due to the use of lead-containing solder which is restricted by the RoHS Directive, and struggle with effective blowout during high-temperature reflow processes.
Innovation Solution
A protection device featuring a composite fusible element with a high melting point first metal layer and a low melting point second metal layer, where the second metal layer constitutes 40-95% of the fusible element's thickness, allowing for effective blowout even at high reflow temperatures, and the second metal layer erodes the first metal layer to speed up the blowout process.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If lead-containing solder is used for the fusible element, then the melting point is higher than 300°C preventing blowout during reflow, but it violates the RoHS Directive restriction on hazardous substances
Solution Approach 1:
The fusible element uses a composite structure with a first metal layer (higher melting point) and a second metal layer (lower melting point). This composite material achieves both RoHS compliance (using lead-free metals like Sn, Ag, Cu, Ni, Zn) and appropriate melting characteristics for protection functionality.
2Reliability
If the second metal layer with lower melting point is used, then effective blowout is achieved during over-voltage events, but the fusible element may be deformed during high-temperature reflow process
Solution Approach 1:
Different regions of the fusible element have different melting characteristics - the second metal layer (lower melting point) provides blowout functionality while the first metal layer (higher melting point) provides thermal stability during reflow. This local differentiation of material properties resolves the contradiction between blowout effectiveness and reflow stability.
Solution Approach 2:
The composite structure combines metals with different melting points, where the higher melting point first metal layer prevents deformation during reflow while the lower melting point second metal layer enables effective blowout during over-voltage events.
3Productivity
If the second metal layer is made thicker to improve blowout effectiveness, then the blowout speed increases, but the manufacturing precision requirement increases to maintain proper thickness ratio
Solution Approach 1:
The patent specifies a thickness ratio parameter where the second metal layer is 40-95% of the total fusible element thickness. This parameter definition provides clear manufacturing guidance while ensuring both effective blowout performance and manufacturability.
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 provides reliable over-voltage, over-current, and over-temperature protection by ensuring the fusible element is not deformed during reflow and achieving timely and efficient blowout, while complying with lead-free material regulations.
Implementation Method 1
When the IC device detects an over-voltage event, the IC device enables the switch to 'on'. As a result, current flows through the heating layer to generate heat to melt and blow the low-melting metal layer
Implementation Method 2
The second metal layer has a lower melting point than that of the first metal layer... the second metal layer erodes the first metal layer to speed up the blowout process
Data Source
AI summary
A protection device comprises a substrate, a fusible element and a heating element. The substrate comprises a first electrode and a second electrode on its surface. The fusible element is disposed on the substrate and connects to the first electrode and the second electrode at two ends. The fusible element comprises a first metal layer and a second metal layer disposed on the first metal layer. The second metal layer has a lower melting point than that of the first metal layer. The heating element is disposed on the substrate. In the event of over-voltage or over-temperature, the heating element heats up to melt and blow the fusible element. The second metal layer is 40-95% of the fusible element in thickness.