Back-Drilling Depth Detection Using Segmented Test Patterns
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Solution Overview
Problem
The challenge in manufacturing printed circuit boards lies in accurately controlling the depth of back-drilling to remove overlong plated stubs, which causes impedance mismatching and signal delay, especially in high-frequency boards, due to variations in layer thickness and the complexity of existing detection methods that increase material costs and may not be applicable to all holes.
Innovation Solution
A method involving the formation of test pattern layers that are not overlaid when viewed from the drill entrance, with a surface conductor layer overlaying all test pattern layers, allowing for precise back-drilling by measuring currents when the drill contacts these layers to determine the depth of the conductor-wiring layer, enabling high-precision drilling without increasing material complexity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional detection patterns are used to detect drill depth, then back-drilling depth can be detected, but the detection pattern becomes complicated and the number of layers increases, resulting in increased material cost
Solution Approach 1:
The detection function is segmented from the main conductor-wiring layer and implemented through separate test pattern layers. These test pattern layers are positioned at specific depths and contain test patterns that can be detected electrically, allowing depth detection without complicating the main circuit design.
Solution Approach 2:
The test pattern layers use simple conductor patterns that serve solely for detection purposes. These patterns are temporary in nature, used only for depth detection during manufacturing, and do not need to maintain long-term structural integrity or complex functionality like the main conductor layers.
2Measurement precision
If conventional detection patterns are used to detect drill depth, then back-drilling depth can be detected, but there may be holes around which a detecting portion cannot be formed at all
Solution Approach 1:
The test pattern layers provide a universal detection mechanism that can be applied to all drilling locations on the printed circuit board. The test patterns are distributed across the board in a manner that ensures detectability for any hole position, making the system universally applicable rather than location-specific.
3Reliability
If the plated portion is not trimmed by back-drilling, then impedance mismatching and signal delay occur, but back-drilling requires precise depth control which is difficult due to layer thickness variations
Solution Approach 1:
The system uses electrical detection of test patterns to provide real-time feedback on drill depth. When the drill contacts a test pattern layer, the electrical connection is detected and used to terminate the drilling operation, providing automatic feedback control that compensates for layer thickness variations.
Solution Approach 2:
The test pattern layers are preliminarily formed at predetermined positions within the board structure before back-drilling occurs. This preliminary placement of detection targets allows the drilling process to use these pre-positioned layers as depth reference points, enabling precise depth control without requiring complex real-time measurements.
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 allows for simple and accurate back-drilling, reducing material costs and ensuring precise removal of plated portions, thereby improving signal integrity and impedance matching in multilayer printed circuit boards.
Implementation Method 1
performing drilling toward one of the selected test pattern layers by use of a drill for the drilling, and detecting a current produced when the drill comes into contact with the relevant test pattern to measure the depth of the layer (D1)
Implementation Method 2
a through-hole is formed and conductive plating is applied to the through-hole, to provide a terminal for connecting with a predetermined inner conductor-wiring layer
Data Source
AI summary
A method of manufacturing a printed circuit board is provided. The method includes preliminarily forming a plurality of test pattern layers for detecting the depth of an inner layer in a multilayer printed circuit board such that at least a part of a lower test pattern layer is not overlaid with any upper test pattern layer when viewed from a drill entrance side, and preliminarily forming a surface conductor layer; applying a voltage between the surface conductor layer and the test pattern layers; performing drilling toward one test pattern layer, and detecting a current produced when the drill comes into contact with the test pattern to measure the depth of the layer (D1); performing drilling toward the other test pattern layer, and measuring the depth of the layer (D2); and performing drilling up to just before the conductor-wiring layer based on a depth calculated from D1 and D2.