PCB Contact Finger Plating to Reduce Galvanic Etching
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Solution Overview
Problem
The Galvanic Effect between dissimilar metallic materials in printed circuit boards (PCBs) leads to over-etching of internal connection points, causing weakening or increased resistance due to corrosion during the cleaning process of electronic devices like hybrid flip chip products.
Innovation Solution
The solution involves configuring the PCBs with a contact finger and via connection pad plated with a common material, and separating the exposed portion of the contact trace into a via connection pad and an outer connection pad, which are physically separated by a distance, to reduce galvanic etching. This configuration ensures that the contact finger and via connection pad are plated with the same material, and the exposed portion is connected via a solder joint after the cleaning process.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If dissimilar metallic materials are used for contact finger and internal connections, then electrical conductivity and signal integrity are improved, but galvanic etching occurs during cleaning causing increased resistance and connection weakening
Solution Approach 1:
A plated pad structure is introduced as an intermediary between the contact finger and the internal trace. The pad is plated with a material compatible with the contact finger material, preventing direct galvanic interaction between dissimilar metals. The trace connects to the pad through a via, creating an indirect connection that eliminates the galvanic cell formation while maintaining electrical connectivity.
Solution Approach 2:
The connection path is segmented into distinct components: the contact finger, the plated pad, the via, and the internal trace. This segmentation allows each component to be optimized for its specific function and material compatibility, preventing the continuous dissimilar metal contact that causes galvanic etching.
2Strength
If trace width is increased to compensate for galvanic etching, then connection strength is maintained, but impedance control accuracy deteriorates
Solution Approach 1:
The protective plating is applied to the pad in advance, before the cleaning process occurs. This preliminary protective action prevents galvanic etching from occurring in the first place, eliminating the need for oversized traces as a compensatory measure and allowing precise impedance control.
3Reliability
If contact finger surface area is increased, then connection to host device is improved, but galvanic voltage differential increases causing more severe etching of internal connections
Solution Approach 1:
The pad plating material is selected to be homogeneous or compatible with the contact finger material. This material matching eliminates the dissimilar metal interface, preventing galvanic cell formation regardless of the contact finger's surface area, thus allowing large contact areas without increased etching risk.
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 effectively reduces etching due to the Galvanic Effect, allowing for precise control of trace width and thickness, maintaining accurate impedance and preventing corrosion, thereby enhancing the reliability and performance of electronic devices.
Implementation Method 1
The contact finger and the via connection pad are plated with a common material to reduce galvanic etching of the first contact trace, and the exposed portion of the contact trace is electrically connected to the impedance trace by way of a solder joint
Data Source
AI summary
Devices and methods are described for reducing etching due to Galvanic Effect within a printed circuit board (PCB) that may be used in an electronic device. Specifically, a contact trace is coupled to a contact finger that has a substantially larger surface area than the contact trace. The contact finger is configured to couple the electronic device to a host device. The contact trace is electrically isolated from the rest of the PCB circuitry during a fabrication process by a separation distance between an exposed portion of the contact trace and an impedance trace. The contact finger and the exposed portion of the contact trace are plated with a common material to reduce galvanic etching of the contact trace during fabrication. The contact trace is then connected to the impedance trace using a solder joint.