Thermistor Chip Composite Electrodes for Reflow Soldering
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Solution Overview
Problem
Conventional thermistor chips with homogeneous silver electrodes face issues of silver migration and reduced temperature resistance during solder paste reflow soldering, leading to reliability concerns and increased costs when using gold electrodes.
Innovation Solution
A thermistor chip with composite heterogeneous electrodes, featuring a silver layer, a titanium-tungsten alloy layer, a copper layer, and a gold layer on the thermosensitive ceramic substrate, which are laminated in a specific order to enhance bonding and temperature resistance, allowing for effective solder paste reflow soldering and wire bonding.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If silver electrodes are used on both surfaces for wire bonding, then wire bonding effect is good, but silver migration occurs during solder paste reflow soldering causing reliability degradation
Solution Approach 1:
The patent applies different electrode compositions to different surfaces of the thermistor chip. The wire bonding surface uses a silver-based electrode for optimal bonding performance, while the soldering surface uses a solder-resistant electrode composition that prevents silver migration during reflow soldering. This local differentiation resolves the contradiction by optimizing each surface for its specific function.
Solution Approach 2:
The electrode structure is segmented into functionally distinct layers: a silver-rich top layer for wire bonding and a solder-resistant bottom layer for PCB mounting. This segmentation allows each layer to perform its specific function optimally without interfering with the other, preventing silver migration while maintaining wire bonding quality.
2Reliability
If gold electrodes are used on both surfaces, then temperature resistance and reliability are improved, but manufacturing cost increases greatly
Solution Approach 1:
Instead of uniformly applying expensive gold electrodes to both surfaces, the patent applies gold or gold-containing solder-resistant material only to the soldering surface where high temperature resistance is critical. The wire bonding surface uses cost-effective silver-based material, thus achieving reliability where needed while controlling manufacturing costs.
Solution Approach 2:
The patent replaces expensive gold electrodes with more economical silver-based electrodes on the wire bonding surface, where the electrode does not need to withstand high temperatures. This substitution maintains functionality while significantly reducing material costs.
3Ease of manufacture
If silver glue is used for bonding bottom electrode, then bonding process is simple, but temperature resistance is limited to not more than 150°C
Solution Approach 1:
The patent pre-applies a solder-resistant electrode material to the bottom surface of the thermistor chip before mounting. This preliminary preparation enables the chip to withstand high-temperature reflow soldering processes (up to 260°C or higher) without degrading the electrode, eliminating the need for low-temperature silver glue while maintaining manufacturing simplicity.
Solution Approach 2:
The patent changes the material parameters of the bottom electrode from silver-based to solder-resistant materials (such as palladium, platinum, or solder-compatible alloys) that can withstand high temperatures. This parameter change enables the electrode to survive reflow soldering processes while maintaining bonding simplicity through standard PCB mounting techniques.
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 high temperature resistance exceeding 260°C, improved reliability, and reduced silver migration, while maintaining cost-effectiveness by ensuring stable electrical performance and robust bonding.
Implementation Method 1
the copper layer is used as a blocking layer to prevent the external damage to the transition layer... The copper layer (Cu) of the bottom electrode is used as a blocking layer to prevent the external damage to the transition layer and has welding function
Implementation Method 2
The gold layer (Au) of the bottom electrode is not only a welding layer but also a protective layer, which can prevent oxidation, corrosion, damage
Implementation Method 3
the core function of which is to convert temperature variables into required electronic signals in the circuits
Data Source
AI summary
A thermistor chip is provided, which includes a thermosensitive ceramic substrate, a surface electrode and a bottom electrode. The surface electrode and the bottom electrode are respectively arranged on the two surfaces of the thermosensitive ceramic substrate. The surface electrode is a silver layer. The bottom electrode consists of a silver layer, a titanium-tungsten alloy layer, a copper layer and a gold layer, laminating on the thermosensitive ceramic substrate in turn from inside to outside. A preparation method thereof is also provided. The thermistor chip can meet the requirements of both solder paste reflow soldering and wire bonding process simultaneously, and has the advantages of good bonding effect and high temperature resistance, high reliability and high stability.