PCB Lower Pad Coating Structure for Black Pad Corrosion
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Solution Overview
Problem
Existing surface treatments for printed circuit boards, such as ENIG and ENEPIG, struggle to form protective layers thicker than certain thresholds, leading to hyper-corrosion or black pad issues, especially in compression attached memory modules where pads are vertically connected to a motherboard.
Innovation Solution
A printed circuit board design featuring a lower pad with multiple protective layers, including a first layer of nickel formed by electroless plating, a second layer of gold also by electroless plating, and a third layer of solder or silver formed by screen printing, with the third layer having a thickness ranging from 50 μm to 100 μm to enhance corrosion resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If ENIG or ENEPIG surface treatment is used, then the protective layer can be formed on the pad surface, but the protective layer thickness cannot exceed certain thresholds leading to hyper-corrosion or black pad
Solution Approach 1:
The protective layer is divided into multiple distinct layers: a first protective layer formed by electroless plating and a second protective layer formed by screen printing. This segmentation allows each layer to be optimized independently - the first layer provides base corrosion protection while the second layer adds thickness without causing hyper-corrosion, thus resolving the thickness control issue while maintaining reliability
Solution Approach 2:
The patent uses a composite structure combining different materials and formation methods - electroless plating material for the first layer and screen-printed material for the second layer. This composite approach enables the protective system to achieve both adequate thickness and corrosion resistance, overcoming the limitations of single-layer treatments like ENIG or ENEPIG
2Reliability
If a thicker protective layer is formed to prevent corrosion, then corrosion resistance improves, but hyper-corrosion or black pad occurs when thickness exceeds thresholds
Solution Approach 1:
By dividing the protective function across two separate layers formed by different processes, the patent achieves adequate thickness for corrosion protection while avoiding the hyper-corrosion threshold. The first electroless plating layer provides a controlled base thickness, and the second screen-printed layer adds protective thickness without triggering black pad issues
Solution Approach 2:
The first protective layer formed by electroless plating acts as an intermediary between the pad and the second protective layer. This intermediate layer provides a stable foundation that prevents direct interaction between the pad and the thicker second layer, thereby preventing hyper-corrosion while maintaining overall corrosion resistance
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 multi-layer protective structure effectively prevents or inhibits corrosion of the lower pad, ensuring reliable connectivity and durability in environments where pads are vertically connected to a motherboard.
Implementation Method 1
a first protective layer on the lower pad, a second protective layer on the first protective layer, and a third protective layer located on the second protective layer, wherein the first protective layer includes nickel and is formed by an electroless plating process
Implementation Method 2
wherein the second protective layer includes silver, wherein the third protective layer is formed by a screen printing process
Data Source
AI summary
A printed circuit board includes a body including a wiring pattern and an insulating layer at least partially surrounding the wiring pattern and a connector on a lower surface of the body, wherein the connector includes a lower pad located on the lower surface of the body, and a plurality of protective layers on the lower pad where one of the protective layers comprises solder.