Protective Device Assisting Layer Reflow Soldering Stability
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Solution Overview
Problem
Conventional protective devices for battery chargers fail to protect against excessive current or voltage due to flux evaporation or movement during the reflow soldering process, which prevents the low-melting point metal layer from melting and breaking, thereby failing to interrupt the circuit and protect the battery and charger.
Innovation Solution
A protective device with a substrate, first electrodes, a low-melting point metal layer, and an assisting layer, where the assisting layer has a liquidus temperature below that of the low-melting point metal layer but above the reflow soldering process temperature, ensuring it remains stable and effective in preventing oxidation and ensuring the low-melting point metal layer melts correctly to break the circuit when excessive current or voltage is applied.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If flux is applied on the low melting point metal layer to prevent oxidation and facilitate breaking, then the low melting point metal layer can be melted and broken effectively, but the flux evaporates or moves away during the reflow soldering process, causing the protective device to fail
Solution Approach 1:
The patent introduces an assisting layer as an intermediary substance between the flux and the low melting point metal layer. This assisting layer has a liquidus temperature lower than the low melting point metal layer but higher than the reflow soldering temperature, allowing it to remain stable during soldering while still enabling the flux to effectively melt and break the low melting point metal layer when excessive current or voltage is applied.
Solution Approach 2:
The patent changes the temperature parameter characteristics by selecting materials with specific liquidus temperatures for the assisting layer. The liquidus temperature of the assisting layer is controlled to be between the reflow soldering temperature and the low melting point metal layer's liquidus temperature, creating a temperature window that ensures stability during manufacturing while maintaining protective functionality during operation.
2Reliability
If the liquidus temperature of the assisting layer is set below the low melting point metal layer, then the assisting layer can facilitate breaking of the metal layer, but the assisting layer may melt during reflow soldering process
Solution Approach 1:
The patent precisely controls the liquidus temperature parameter of the assisting layer to fall within a specific range: above the reflow soldering process temperature but below the low melting point metal layer's liquidus temperature. This parameter optimization ensures the assisting layer remains solid during manufacturing while enabling effective breaking during protective operation.
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 assisting layer prevents evaporation and ensures the low-melting point metal layer melts reliably, effectively interrupting the circuit and protecting the battery and charger from damage by excessive current or voltage during reflow soldering.
Implementation Method 1
an appropriate amount of flux 26 is applied on the low melting point metal layer 23 for preventing oxidation occurred on the surface of the low melting point metal layer 23
Implementation Method 2
the heating member 113 of the protective device 11 generates heat for melting the current fuses 111 and 112
Implementation Method 3
the low melting point metal layer 23 will melt to break and form a breakage between the two first electrodes 22
Implementation Method 4
When the protective device 20 is being connected to a circuit board in a reflow soldering process, the high temperature over 200 degrees Celsius therein will immediately evaporate the flux or drive it to move away
Data Source
AI summary
A protective device includes a substrate, two first electrodes, a low-melting point metal layer and an assisting layer. The first electrodes are respectively arranged at two opposite sides of the substrate. The low melting point metal layer is arranged over the two first electrodes. The assisting layer is formed on the low melting point metal layer. The liquidus temperature of the assisting layer is below the liquidus temperature of the low melting point metal layer, and the liquidus temperature of the assisting layer is not below a predetermined temperature which is below the maximum working temperature of reflow soldering process by 25 degrees.