PCB Via Bottom Profiling Using Interferometry and Segmentation
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Solution Overview
Problem
Conventional optical metrology methods fail to accurately profile the bottom surface of vias in printed circuit boards reinforced with preferentially oriented fibers due to obstruction by fibers, leading to incorrect measurements as the bottom surface is obscured and considered noise.
Innovation Solution
Utilizing a low-coherence interferometry method with a sufficiently high numerical aperture objective and taking advantage of surface roughness to diffuse light back to the objective, allowing for the processing of previously discarded fringe data to achieve precise measurements of the bottom surface, even when obscured by fibers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional optical metrology methods are used to profile vias, then the measurement process is simple, but the measurement precision deteriorates because the bottom surface is obscured by fibers and considered noise
Solution Approach 1:
The patent segments the via profile into distinct regions: top surface, intermediate fiber shelf, and bottom surface. By processing fringe data separately for each region and using different algorithms for obscured versus unobscured areas, the system achieves accurate bottom surface measurement while maintaining manageable system complexity through structured data processing
Solution Approach 2:
The patent introduces an intermediary processing layer that includes a processor to analyze fringe patterns and determine which regions are obscured by fibers. This intermediary processing enables the system to distinguish between valid measurement signals and obscured regions, allowing accurate bottom surface profiling without requiring complex optical modifications
2Measurement precision
If the numerical aperture of the objective is increased to illuminate the bottom surface under fibers, then the measurement precision improves, but the device complexity increases
Solution Approach 1:
The patent changes the parameter of numerical aperture to a sufficiently high value to illuminate the bottom surface under fibers. This parameter modification enables light to penetrate the fiber obstruction and reach the bottom surface, improving measurement precision while the patent manages the complexity by specifying a threshold rather than optimizing to extreme values
3Strength
If fiber reinforcement is added to PCBs to reduce flexing, then the structural strength improves, but the measurement accuracy deteriorates because fibers obscure the via bottom surface
Solution Approach 1:
The patent converts the harmful effect of fiber obstruction into a beneficial measurement approach by using the fiber shelf as a reference landmark. The fringe patterns from the fiber shelf are processed to identify obscured regions, and the bottom surface measurement algorithms are adapted to compensate for the obstruction, turning the fiber reinforcement feature into a useful measurement reference rather than purely a hindrance
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 more accurate and precise profiling of the bottom surface of vias, providing reliable and repeatable measurements by incorporating scattered light from the rough surface, exceeding previous measurement limitations.
Implementation Method 1
The signal captured in low-coherence interferometry (including structured light metrology) is fringes
Implementation Method 2
taking advantage of surface roughness to diffuse light back to the objective
Data Source
AI summary
The bottom surface of a via drilled in a fiber-reinforced PCB is profiled interferometrically with acceptable precision using an objective with sufficiently large numerical aperture to illuminate the bottom under the fibers. The light scattering produced by the inherent roughness of the surface of the via bottom causes diffused light to return to the objective and yield reliable data fringes. Under such appropriate numerical-aperture and surface roughness conditions, the bottom surface of vias can be profiled correctly simply by segmenting the correlograms produced by the scan and processing all fringes that correspond to the bottom surface elevation.