BGA Pad Stray Capacitance Reduction via Shielded PCB Design
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Solution Overview
Problem
Current information handling systems face challenges with high stray capacitance in ball grid array (BGA) pads, which becomes unsuitable for modern high-speed data interfaces due to impedance reduction, especially at frequencies above 8-10 giga-transfers per second, leading to inadequate signal performance.
Innovation Solution
The implementation of a printed circuit board (PCB) design that includes a first and second BGA pad, a metal layer with a shield pad directly underneath the first BGA pad, and a plated via connecting the second BGA pad to the shield pad, effectively reducing stray capacitance by ensuring the shield pad is driven by the same signal, thereby minimizing net voltage difference and capacitance between the BGA pad and the ground layer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional BGA pad design is used, then the structure is simple and easy to manufacture, but the stray capacitance is high which causes impedance reduction and inadequate signal performance at high speeds
Solution Approach 1:
The shield pad structure is segmented into multiple portions (first portion directly below first BGA pad, second portion coupled to first portion and directly below second BGA pad) that are distributed across different locations. This segmentation allows the shield pad to effectively reduce stray capacitance for multiple BGA pads while maintaining a manageable PCB structure that can be manufactured with standard multi-layer PCB techniques.
Solution Approach 2:
The solution moves the shielding function from a two-dimensional surface layout to a three-dimensional structure by placing the shield pad on an internal metal layer directly below the BGA pads on the top surface. This vertical arrangement in the third dimension (through the PCB thickness) enables effective capacitance reduction without significantly increasing the horizontal footprint or manufacturing complexity.
2Reliability
If the shield pad is placed directly below the BGA pad, then stray capacitance is reduced, but the PCB requires additional metal layers and plated vias increasing manufacturing complexity
Solution Approach 1:
The shield pad structure serves multiple functions simultaneously: it reduces stray capacitance for signal integrity, provides electromagnetic shielding, and can be integrated with existing ground planes or power planes in the PCB. The same shield pad structure can serve multiple BGA pads, reducing the need for separate shielding structures for each pad, thereby simplifying manufacturing while achieving capacitance reduction.
3Device complexity
If multiple BGA pads share a common signal, then the signal routing is simplified, but the stray capacitance accumulates and degrades signal performance
Solution Approach 1:
Multiple BGA pads that share a common signal are merged with a common shield pad structure below them. The shield pad portions are coupled together through plated vias, creating a unified shielding structure that collectively reduces the stray capacitance for all connected BGA pads. This merging approach maintains simple signal routing at the top layer while providing enhanced capacitance reduction through the combined shield structure.
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 configuration significantly reduces stray capacitance to 1-10 pico-Farads, resulting in an impedance reduction of only 1-2 ohms, providing adequate signal performance for both current and upcoming high-speed data interfaces.
Implementation Method 1
a plated via that couples the second BGA pad to the shield pad
Implementation Method 2
effectively reducing stray capacitance by ensuring the shield pad is driven by the same signal, thereby minimizing net voltage difference and capacitance between the BGA pad and the ground layer
Data Source
AI summary
A PCB includes a first BGA pad on a first surface of the PCB, a second BGA pad on the first surface, a metal layer within the PCB that includes a shield pad with a first portion directly below the first BGA pad and a second portion coupled to the first portion and directly below the second BGA pad, and a plated via that couples the second BGA pad to the shield pad. The first BGA pad is collocated with a first BGA ball of a BGA device and is associated with a first signal of the BGA device. The second BGA pad is collocated with a second BGA ball of the BGA device and is associated with a second signal of the BGA device. The first signal and the second signal are a common signal.