Capacitive Fingerprint Sensor Bond Wire Embedding
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Solution Overview
Problem
Capacitive fingerprint sensing devices face challenges in achieving a flat sensing surface due to protruding wire bonds, which reduces accuracy and requires thicker coatings that weaken capacitive coupling, and existing solutions either compromise on sensitivity or involve costly manufacturing steps.
Innovation Solution
A capacitive fingerprint sensing device with a sensing chip, adhesive, and protective plate configuration where the bond wires are embedded in the adhesive, reducing the distance to the sensing surface and enhancing capacitive coupling, and using a flexible adhesive film with a dielectric intermediate carrier layer for mechanical stability and ESD protection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Shape
If a thicker protective coating is provided to cover protruding wire bonds and achieve a flat sensing surface, then the flatness and aesthetics are improved, but the capacitive coupling between the finger and sensing element is weakened, reducing sensing accuracy
Solution Approach 1:
The harmful protruding wire bonds are removed from the sensing surface area by embedding them in the adhesive layer. This extraction of the problematic element allows the protective plate to be positioned closer to the sensing elements without needing to cover the wire bonds, thus maintaining both flatness and sensing accuracy.
Solution Approach 2:
The adhesive layer serves as an intermediary medium that embeds the wire bonds and positions the protective plate. By using this intermediate layer, the wire bonds are concealed without requiring a thick protective coating, and the protective plate can be optimally positioned for capacitive coupling.
2Ease of manufacture
If wire bonds are left protruding to maintain simple manufacturing, then manufacturing complexity is reduced, but the sensing device cannot achieve a flat surface, requiring additional coating layers that reduce sensing performance
Solution Approach 1:
The adhesive layer serves multiple functions simultaneously: it bonds the protective plate to the sensing chip, embeds the protruding wire bonds to create a flat surface, and positions the protective plate at the optimal distance for capacitive coupling. This merging of functions achieves both manufacturing simplicity and sensing performance.
Solution Approach 2:
The adhesive layer is designed to perform multiple roles: mechanical bonding, wire bond embedding, and spacing control for capacitive coupling. This multi-functional approach eliminates the need for separate components for each function, maintaining manufacturing simplicity while improving sensing performance.
3Measurement precision
If the protective plate is positioned closer to the sensing elements to enhance capacitive coupling, then sensing accuracy is improved, but the wire bonds would be exposed and the surface would not be flat
Solution Approach 1:
The wire bonds are extracted from the sensing surface area by embedding them in the adhesive layer. This allows the protective plate to be positioned closer to the sensing elements for enhanced capacitive coupling without the wire bonds being exposed, thus achieving both accuracy and flatness.
Solution Approach 2:
The adhesive layer acts as an intermediary that conceals the wire bonds while allowing the protective plate to be optimally positioned close to the sensing elements. This intermediate structure enables close spacing for accuracy while maintaining a flat surface appearance.
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 improves capacitive coupling and sensor sensitivity, eliminates the need for spacer layers and molding, allows for higher surface roughness of the protective plate, and provides efficient area utilization while maintaining accuracy and uniformity, thus enhancing the overall performance and reducing manufacturing complexity.
Implementation Method 1
enhancing the capacitive coupling between a finger on the surface of the plate and a sensing element located underneath the protective plate
Implementation Method 2
using a flexible adhesive film with a dielectric intermediate carrier layer for mechanical stability and ESD protection
Data Source
Figure 1
Figure 2a~2b
Figure 3~5
AI summary
The present invention relates to a fingerprint sensing device and to a method of manufacturing such a device. The device comprises a substrate with readout circuitry, a sensing chip arranged on the substrate. The sensing chip comprising a plurality of sensing elements having a surface defining a sensing plane, each sensing element being configured to provide a signal indicative of an electromagnetic coupling between a sensing element and a finger placed on the sensing device; bond wires arranged between bond pads located on the sensing chip on the substrate, respectively,to electrically connect the sensing chip to the readout circuitry. A portion of the bond wire protrudes above the chip and an adhesive is arranged on the sensing chip to covering to cover the chip so that the portion of the bond wire protruding above the chip is embedded in the adhesive. A protective plate is attached to the sensing chip by the adhesive. The protective plate forms an exterior.