Optical Module FPC Interconnect for Impedance and Reliability
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Solution Overview
Problem
The existing optical modules face challenges with thin lead terminals being easily bent during manufacturing, leading to poor electrical contact and degraded high-frequency characteristics due to impedance issues and thermal expansion coefficient mismatches between lead terminals and sealing glass, limiting design flexibility and productivity.
Innovation Solution
The optical module features a metal stem with a cylindrical portion and a base, a ceramic substrate with signal and ground wiring layers, and a flexible printed circuit connected via a metal wire, simplifying the structure and improving impedance management while maintaining excellent high-frequency characteristics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If thin lead terminals are used to transmit modulated electric signal and power, then the structure is simplified, but the lead terminals are easily bent during manufacturing and electrical contact becomes poor
Solution Approach 1:
The patent extracts the lead terminal function from the stem structure by using a flexible printed circuit board (FPC) that passes through the stem. The FPC carries signal and power lines externally, separating the transmission function from the structural support function of the stem, thereby eliminating the bending issue of thin lead terminals while maintaining structural simplicity.
Solution Approach 2:
The FPC acts as an intermediary between the external circuit and the internal components (semiconductor laser, photodiode). Instead of using thin lead terminals that bend, the FPC provides a flexible but robust transmission medium that maintains electrical contact reliability while allowing manufacturing tolerance.
2Reliability
If lead terminals pass through the stem to transmit signals and power, then electrical connection is achieved, but the portion inside the stem degrades high frequency characteristics by serving as an inductance component
Solution Approach 1:
The patent extracts the signal and power transmission paths from the internal stem structure and relocates them to the external FPC. This eliminates the inductance problem caused by lead terminals passing through the stem, as the FPC routes the high-frequency signals externally with controlled impedance, preserving high-frequency characteristics while maintaining electrical connection.
3Manufacturing precision
If the diameter of the lead terminal is increased to 0.2 mm to obtain 50Ω impedance with sealing glass, then impedance management is improved, but the lead terminal becomes more prone to bending and mass productivity decreases
Solution Approach 1:
The patent replaces the mechanical lead terminal system with an FPC-based transmission line system. The FPC provides controlled impedance (50Ω) through its structural design rather than relying on lead terminal diameter, eliminating the trade-off between impedance management and bending resistance. This substitution enables mass production through standardized FPC manufacturing processes.
4Manufacturing precision
If the dielectric constant of the sealing glass and thermal linear expansion coefficient difference are controlled to be appropriate, then impedance and thermal compatibility are improved, but the degree of freedom in design is reduced
Solution Approach 1:
The patent extracts the impedance control function from the sealing glass material properties and relocates it to the FPC transmission line design. This separates the sealing function (performed by glass) from the impedance control function (performed by FPC structure), allowing independent optimization of both. The FPC's controlled impedance design provides design flexibility without being constrained by sealing glass material properties.
Data Source
AI summary
A metal stem includes a cylindrical portion in which an FPC inserting portion is formed, and a base standing upright from one plane of the cylindrical portion. A tubular lens cap with one open end is fixed to a peripheral portion of the one plane of the cylindrical portion, and has a lens mounted on a bottomed portion. A substrate mounted on one plane of the base includes a signal wiring layer and a ground wiring layer. An optical semiconductor element is mounted on the substrate and has a signal terminal connected to the signal wiring layer of the substrate, and a ground terminal connected to the ground wiring layer of the substrate. An FPC substrate is disposed so as to pass through the FPC inserting portion and to face the one plane of the base. The FPC substrate includes a signal wiring layer connected to the signal wiring layer of the substrate with a metal wire.