Backdrilling Via Stubs with Reduced Diameters on PCBs
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Solution Overview
Problem
Traditional backdrilling methods for printed circuit boards (PCBs) require larger drill bits to remove via stubs, leading to increased hole diameters that compromise signal trace clearance and via-to-via spacing, causing signal distortion and other issues.
Innovation Solution
A method involving drilling a via with a first diameter, depositing layers of materials like copper and tin, and selectively removing these layers through backdrilling and chemical etching to maintain a smaller passageway, allowing for tighter clearances and reduced backdrilling diameters.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If traditional backdrilling uses a larger drill bit to remove via stubs, then the via stub is effectively removed, but the hole diameter increases compromising signal trace clearance and via-to-via spacing
Solution Approach 1:
The via structure is segmented into multiple material layers (copper layer and tin layer) with different properties. The copper layer serves as the conductive via while the tin layer acts as a sacrificial mask that protects the copper during backdrilling, enabling selective removal of the stub portion without enlarging the main via hole.
Solution Approach 2:
The tin layer serves as an intermediary protective layer between the drill bit and the copper conductive layer. This intermediary layer is removed by backdrilling while protecting the underlying copper, allowing stub removal without direct mechanical contact that would enlarge the via hole and compromise clearance requirements.
2Reliability
If traditional backdrilling enlarges the via hole to remove stubs, then stub removal is achieved, but PCB density decreases due to reduced clearance and spacing
Solution Approach 1:
The via is segmented into functional zones: the main via hole containing copper for conductivity, and an outer tin layer that can be selectively removed. This segmentation allows the backdrilling process to remove only the necessary stub portion while preserving the core via structure, thereby maintaining higher PCB density.
Solution Approach 2:
The invention changes the material composition parameters of the via structure by adding a tin layer with different mechanical and chemical properties than copper. This parameter change enables differential removal during backdrilling, allowing stub elimination without proportional enlargement of the via hole, thus preserving PCB density.
3Reliability
If a larger drill bit is used for backdrilling, then via stub removal is more effective, but the minimum via-to-via spacing increases
Solution Approach 1:
The tin layer acts as an intermediary sacrificial layer that absorbs the mechanical action of backdrilling. This allows the use of drill bits sized to remove only the stub portion without necessarily enlarging the main via hole, thereby maintaining smaller via-to-via spacing compared to traditional methods that require larger drill bits for effective stub removal.
4Reliability
If traditional backdrilling enlarges via holes, then stubs are removed, but the clearance requirements for signal traces are compromised
Solution Approach 1:
The via structure is segmented into a core copper conductive path and an outer tin protective layer. This segmentation enables the backdrilling process to selectively remove the tin layer and stub material while preserving the core via hole dimensions, thereby maintaining adequate clearance for signal traces while still achieving effective stub elimination.
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 smaller backdrilled diameters, maintaining electrical connectivity while reducing the risk of cutting into signal traces and improving PCB density by allowing for tighter clearances and more precise interconnects.
Implementation Method 1
The chemically removing step may be performed with an etchant operable to etch the first material at a first rate and etch the second material at a second rate. The first rate may be greater than the second rate, thus, for example, enabling the selective removal of portions of the first material by the etchant.
Implementation Method 2
The depositing of the first layer may include electrically interconnecting with the first layer first and second conductive portions. The first and second conductive portions may be disposed at different locations of the PCB along a length of the via
Data Source
AI summary
Methods of backdrilling printed circuit boards (PCBs) to remove via stubs and related apparatuses. The method may include removing a via stub through a combination of backdrilling and chemical etching. The backdrilling may remove a masking layer from the via stub. Portions of an underlying layer may remain in the region of the via stub after the backdrilling is completed. The remaining portions of the underlying layer may be removed in a subsequent etching process thereby removing the via stub from the PCB. As the backdrilling step may be used for the limited purpose of removing the outer layer and portions of the underlying layer remaining in the via can be tolerated, the diameter of the backdrilling need not be as large as traditional backdrilling where all layers within the via must be ensured of being completely removed.