Recessed Connection Pads for Flat Fingerprint Sensors
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Solution Overview
Problem
Fingerprint sensing devices face challenges in achieving a flat surface design due to protruding wire bonds, which compromise aesthetic appeal and electrostatic discharge immunity, and thicker coatings to cover these bonds reduce capacitive coupling accuracy.
Innovation Solution
The design features recessed connection pads that are electrically connected to the sensing circuitry, allowing for a flat surface without protruding conductive parts, enabling efficient capacitive coupling and improved ESD immunity by ensuring the thickness of the device is determined by capacitive coupling needs rather than interconnect geometry.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Shape
If a layer of top coating is provided to cover protruding wire bonds and form a flat outer surface, then the aesthetic appearance and flatness of the sensing surface are improved, but the capacitive coupling between the finger and sensing element is weakened, leading to reduced sensing accuracy
Solution Approach 1:
The invention transitions from a planar surface solution to a three-dimensional recessed structure. Connection pads are formed in recesses within the substrate, allowing the sensing surface to remain flat while the wire bonds connect to pads at different depth levels. This dimensional change enables both flatness and strong capacitive coupling to coexist.
Solution Approach 2:
The connection pads are nested within recesses formed in the substrate material. This nesting approach allows the conductive pads to be embedded below the sensing surface, maintaining a flat outer appearance while preserving strong electrical connection capabilities for wire bonding.
2Ease of manufacture
If protruding wire bonds are used to provide electrical connection between sensing chip and readout substrate, then ease of manufacture through wire bonding is improved, but the electrostatic discharge resistance and aesthetic appearance are deteriorated
Solution Approach 1:
Connection pads are nested within recesses in the substrate, positioning them below the sensing surface. This nested structure maintains the wire bonding manufacturing process while protecting the conductive elements from electrostatic discharge by embedding them within the substrate material.
Solution Approach 2:
The recessed pad structure provides a robust, permanent solution that eliminates the need for protruding wire bonds, thereby improving ESD resistance without compromising the established wire bonding manufacturing process.
3Ease of manufacture
If protruding wire bonds are used for electrical connection, then ease of manufacture is improved, but the flatness of the sensing surface is worsened
Solution Approach 1:
The invention resolves the flatness issue by moving the connection pads into the third dimension - specifically, forming them in recesses within the substrate. This allows the sensing surface to remain perfectly flat while wire bonds connect to pads at different depth levels, maintaining manufacturing ease.
Solution Approach 2:
Connection pads are nested within recesses, embedding the conductive elements below the sensing surface. This nested configuration preserves the straightforward wire bonding process while achieving a flat sensing surface suitable for aesthetic and functional requirements.
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
This approach allows for a flat fingerprint sensor with enhanced biometric performance and ESD immunity, maintaining high accuracy while using established wire bonding technology for cost-effective high-volume production.
Implementation Method 1
each sensing element being configured to provide a signal indicative of an electromagnetic coupling between the sensing structure and a finger placed on the surface of the fingerprint sensing device
Implementation Method 2
contact pads of the sensing chip are provided for enabling an electrical connection via means of wire-bonding to corresponding contact pads of the readout substrate
Data Source
AI summary
A fingerprint sensing device comprising sensing circuitry comprising a plurality of sensing elements, each sensing element comprising a sensing structure arranged in a sensing plane and facing a surface of the capacitive fingerprint sensing device, each of the sensing elements being configured to provide a signal indicative of an electromagnetic coupling between the sensing structure and a finger placed on the surface of the fingerprint sensing device; and a plurality of connection pads electrically connected to the sensing circuitry for providing an electrical connection between the sensing circuitry and readout circuitry, wherein each of the connection pads is separately recessed in relation to the sensing plane such that each connection pad has a floor in a floor plane, and wherein each connection pad is separated from an adjacent connection pad through a portion of the sensing device being elevated in relation to the floor plane.