Screen-Printable Polybenzoxazole Encapsulants for Embedded Capacitors
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Solution Overview
Problem
Fired-on-foil ceramic capacitors embedded in printed circuit boards face challenges such as porosity, microcracks, and chemical attacks from processing chemicals, leading to moisture penetration and insulation resistance degradation during accelerated life tests, which compromises their reliability and integrity.
Innovation Solution
A screen-printable composition comprising a polybenzoxazole polymer and a specific organic solvent is applied as an encapsulant, providing protection against moisture and chemicals, and is thermally dried at temperatures below 300°C to form a stable encapsulant that maintains insulation resistance and integrity during testing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Volume of moving object
If fired-on-foil ceramic capacitors are embedded in printed circuit boards, then circuit size is reduced and performance is improved, but the capacitors are exposed to processing chemicals and environmental moisture that degrade insulation resistance
Solution Approach 1:
A polybenzoxazole encapsulant layer is applied as an intermediary protective coating between the embedded capacitor and the external environment. This encapsulant shields the capacitor from processing chemicals (caustic stripping photoresist, brown/black oxide treatments) and environmental moisture, preventing chemical attacks and ionic surface deposits that would otherwise degrade insulation resistance.
Solution Approach 2:
The patent employs a thin film encapsulant made of polybenzoxazole material that conformally coats the capacitor structure. This thin protective film provides chemical and moisture barrier protection while maintaining the compact embedded configuration, preventing delamination and microcrack formation during thermal processing cycles.
2Strength
If the capacitor dielectric glass is exposed to caustic and acid solutions during oxide treatment, then adhesion of copper foil to prepreg is improved, but the dielectric glass is attacked and partially dissolved causing low insulation resistance
Solution Approach 1:
The polybenzoxazole encapsulant is applied to the capacitor surface before the oxide treatment process. This preliminary protective coating prevents direct contact between the caustic/acid solutions and the dielectric glass, shielding it from chemical attacks and partial dissolution that would create ionic surface deposits and reduce insulation resistance.
Solution Approach 2:
The encapsulant serves as a protective intermediary barrier between the aggressive processing chemicals (caustic stripping photoresist, brown/black oxide treatments) and the capacitor dielectric glass, allowing the oxide treatment to proceed for adhesion improvement without damaging the dielectric material.
3Ease of manufacture
If the fired ceramic capacitor layer is subjected to bending forces during handling, then processing is completed, but porosity and microcracks allow moisture penetration resulting in low insulation resistance
Solution Approach 1:
The polybenzoxazole encapsulant is applied beforehand to the fired ceramic capacitor layer, providing a protective cushioning layer that prevents the formation and propagation of microcracks during subsequent handling and processing. This encapsulant shields the porous dielectric structure from mechanical damage that would otherwise create pathways for moisture penetration.
Solution Approach 2:
The flexible thin film encapsulant conforms to the capacitor structure and provides mechanical protection during handling, preventing microcrack formation in the brittle fired ceramic layer while allowing the processing to continue.
4Reliability
If conventional encapsulants are used to protect embedded capacitors, then some protection is provided, but they fail to prevent chemical attacks from processing chemicals and moisture penetration during accelerated life tests
Solution Approach 1:
The patent employs polybenzoxazole material with specific chemical and physical parameters: low diffusion coefficients to moisture and gases, high degree of dimensional stability, high toughness, high glass transition temperature (Tg), low coefficient of thermal expansion, and low water uptake. These parameter changes provide superior protection compared to conventional encapsulants against chemical attacks and moisture penetration.
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 polybenzoxazole encapsulant composition effectively protects capacitors from moisture and chemical exposure, ensuring they pass 1000 hours of accelerated life testing at 85°C and 85% relative humidity under 5 volts DC bias, with improved insulation resistance and mechanical stress tolerance.
Implementation Method 1
PBOs may have utility as encapsulants since they generally possess low diffusion coefficients to moisture and gases
Implementation Method 2
removing the organic solvent by baking the printed electronic component at a temperature of less than about 300° C. to form a polybenzoxazole encapsulant
Data Source
AI summary
This disclosure relates to compositions and methods for using such compositions to provide protective coatings, particularly of electronic components. Fired-on-foil ceramic capacitors coated with a polybenzoxazole encapsulant which may be embedded in printed wiring boards are disclosed.