Ceramic Capacitor Electrode Design for Ag Migration Control
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Solution Overview
Problem
Ceramic capacitors face issues with silver (Ag) migration due to electric fields generated between internal electrodes and conductive adhesives, leading to degraded insulating resistance, especially when using Pb-free solders or conductive adhesives, and variations in capacitor capacity when reducing internal electrode sizes to mitigate these issues.
Innovation Solution
A ceramic capacitor design with specific electrode configurations and dimensions, including rectangular shapes and dummy electrodes, to manage electric fields and prevent Ag migration, ensuring consistent capacity and resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the size of the front end portion of the internal electrode is decreased to reduce electric field and suppress Ag migration, then Ag migration is suppressed, but the shape of the internal electrode becomes unstable due to oozing and blurring of conductive paste, causing considerable variation in capacity
Solution Approach 1:
The patent applies different properties to different parts of the internal electrode. The front end portion (exposed at the end surface) is designed with larger dimensions than the rear portion (connected to external electrode), creating a stepped structure. This local differentiation allows the front end to have sufficient size for shape stability during manufacturing while the overall electrode configuration still reduces electric field intensity at critical points, thus suppressing Ag migration without causing paste oozing and blurring issues.
Solution Approach 2:
The patent changes the dimensional parameters of the internal electrode, specifically setting the front end portion dimensions (length L1 and width W1) to be larger than the rear portion dimensions (length L2 and width W2). This parameter change ensures that the conductive paste maintains stable shape during printing and firing processes, preventing oozing and blurring, while the overall electrode design still achieves reduced electric field intensity to suppress Ag migration.
2Reliability
If a dummy electrode is provided outside the internal electrode to reduce electric field intensity at the end of the external electrode, then Ag migration is suppressed when the primary surface faces the mounting substrate, but the electric field intensity is not reduced when the side surface faces the mounting substrate
Solution Approach 1:
The patent segments the internal electrode into distinct portions: a front end portion exposed at the end surface and a rear portion connected to the external electrode. This segmentation creates a stepped structure where each portion serves a specific function. The front end portion with larger dimensions stabilizes the electric field distribution regardless of mounting orientation, eliminating the need for dummy electrodes and ensuring consistent Ag migration suppression whether the primary surface or side surface faces the mounting substrate.
Solution Approach 2:
The patent addresses the mounting orientation issue by modifying the electrode structure in the thickness direction rather than adding dummy electrodes on the same plane. The stepped configuration with different length and width dimensions at different portions of the internal electrode creates a three-dimensional electric field distribution that remains effective across various mounting orientations, providing adaptability without additional components.
3Object-affected harmful factors
If Pb-free solder is used instead of lead-containing solder for mounting ceramic capacitors, then environmental load is reduced, but the low melting point of Pb-free solder makes it difficult to use in vehicle applications requiring high temperature resistance
Solution Approach 1:
The patent introduces an intermediary barrier layer between the internal electrode and the mounting substrate. This barrier layer prevents direct interaction between the electric field and the conductive adhesive or solder, thereby suppressing Ag migration even when Pb-free solder with low melting point is used. This allows the use of environmentally friendly Pb-free solder while maintaining reliability under high temperature conditions through the protective barrier mechanism rather than relying on high-melting-point materials.
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 design effectively reduces Ag migration and maintains capacitor capacity consistency by controlling electric field intensities and adhesive interactions, even under high voltages, thereby preventing structural defects and maintaining insulating resistance.
Implementation Method 1
because of an electric field generated between an internal electrode located closest to the substrate when the ceramic capacitor is mounted thereon and the electrical conductive adhesive adhering to the ceramic base, Ag contained in the electrical adhesive moves into the ceramic base
Implementation Method 2
When a high voltage is applied to the ceramic capacitor, Ag contained in the external electrode may move therefrom to the other external electrode through the surface of the ceramic capacitor, so that migration may occur in some cases
Data Source
AI summary
When a voltage two times a rated voltage is applied between a first external electrode and a second external electrode of a ceramic capacitor, the electric field intensity generated at portion connected between a first internal electrode and an end of a portion of a second external electrode at a side of a first side surface by a shortest distance FS is about 0.34 kV/mm or less.