Flexible Circuit Cable Attachment for Overlapping RF Interconnects
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Solution Overview
Problem
Existing methods for attaching flexible circuit cables to a substrate, such as direct soldering, Anisotropic Conductive Film (ACF), and conductive epoxy, face challenges with overlapping cables, including wetting issues, co-planarity problems, and limited radio frequency (RF) bandwidth, especially in small assembly scenarios like optical sub-assemblies.
Innovation Solution
The method involves bonding multiple gold bumps on interconnection pads of the substrate to create columns that assist in solder or epoxy wetting and bonding, reducing bridging risks, and using a weight to maintain contact during reflow, while applying heat and potentially UV light to cure the epoxy, with a nonconductive underfill epoxy for mechanical strength.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If direct soldering is used to attach flexible circuit cables, then electrical connectivity is achieved, but heat reflow impacts pre-attached chips or flexible circuit cables and wetting is difficult when cables overlap
Solution Approach 1:
The patent introduces an underfill epoxy as an intermediary substance between the flexible circuit cable and substrate. This underfill serves multiple functions: it provides mechanical support, facilitates heat dissipation away from pre-attached components, and enables reliable electrical connectivity through the solder joints without requiring high heat reflow that would damage existing components.
2Reliability
If Anisotropic Conductive Film (ACF) or Paste (ACP) is used for attachment, then electrical connectivity is achieved through compression, but high thermal temperature is required to cure the film which impacts pre-attached components
Solution Approach 1:
The patent employs a low-cost, low-temperature curing underfill epoxy that can be cured at temperatures safe for pre-attached components. This underfill replaces the expensive and high-temperature requiring ACF/ACP materials, providing a more gentle curing process that does not thermally impact existing chips or cables on the substrate.
3Strength
If conductive epoxy is used for attachment, then bonding is achieved, but variance in planarity, under/over volume of epoxy, and limited reworkability are issues
Solution Approach 1:
The underfill epoxy is applied in a controlled manner and automatically levels itself to fill the gap between the flexible circuit cable and substrate. This self-leveling property eliminates the need for precise manual control of epoxy volume and planarity, making the manufacturing process more forgiving and easier to control while ensuring proper bonding.
4Productivity
If overlapping flexible circuit cables are attached, then high density signal routing is achieved, but wetting issues occur due to gaps created by pre-attached cables
Solution Approach 1:
The underfill epoxy is applied and cured before the soldering process. This preliminary action creates a stable, gap-free interface between the overlapping flexible circuit cables and the substrate, ensuring that subsequent soldering operations can proceed without wetting issues. The underfill prepares the surface in advance, eliminating the barriers that would otherwise prevent proper solder flow.
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 enables reliable attachment of overlapping flexible circuit cables with improved RF performance, increased mechanical strength, and reduced electrical resistance, addressing the limitations of previous methods by ensuring proper bonding and alignment without impacting pre-attached components.
Implementation Method 1
Appropriate heat is applied to reflow the solder or cure the epoxy
Implementation Method 2
If conductive epoxy was printed on the flexible circuit cable, then heat, ultraviolet (UV) light, or both can be applied to cure the epoxy
Implementation Method 3
Heat and capillary effect will draw the underfill epoxy in between the flexible circuit cable and substrate
Data Source
AI summary
A method and apparatus for multiple flexible circuit cable attachment is described herein. Gold bumps are bonded on interconnection pads of a substrate to create a columnar structure and solder or conductive epoxy is dispensed on the flexible cable circuit. The substrate and flexible cable circuit are aligned and pressed together using force or placement of a weight on either the substrate or flexible cable circuit. Appropriate heat is applied to reflow the solder or cure the epoxy. The solder wets to the substrate pads, assisted by the gold bumps, and have reduced bridging risk due to the columnar structure. A nonconductive underfill epoxy is applied to increase mechanical strength.