HDI blind and buried via circuit boards have been widely used in many fields due to their characteristics, such as higher wiring density and better electrical performance. From consumer electronics such as smartphones and tablets to industrial equipment with strict performance requirements such as automotive electronics and communication base stations, HDI blind and buried via circuit boards are critical, and the line width and line spacing accuracy, as an important factor affecting its performance, has strict and detailed standards.

一、Importance of line width and line spacing accuracy
Electrical performance impact: line width is directly related to the resistance of the wire, the wider line width resistance is smaller, can carry more current; The line distance affects the capacitance and inductance between lines. In high frequency circuit, if the line width and line distance accuracy is insufficient, the change of capacitance and inductance will cause delay and distortion in the signal transmission process, which seriously affects the signal integrity. For example, on the HDI blind buried hole circuit board of 5G communication equipment, the signal transmission rate is extremely high, and the small line width and line distance deviation may make the signal unable to be accurately transmitted, resulting in a decline in communication quality.
Wiring density and space utilization: One of the advantages of HDI blind buried hole circuit boards is high-density wiring. The high-precision line width and line spacing can arrange more lines in a limited space to achieve more complex circuit functions. Taking the smartphone motherboard as an example, in order to accommodate a large number of chips, sensors and other electronic components, a large amount of wiring needs to be completed in a very small area. Only by strictly controlling the line width and line distance accuracy can we achieve efficient wiring in a small space, improve the integration of the motherboard, and meet the increasingly rich needs of mobile phones.

二、Common standard value of line width and line distance accuracy
Industry general standard: In the general HDI blind hole circuit board manufacturing, the common minimum line width can reach 3-4mil (0.076-0.10mm), and the minimum line distance is also about 3-4mil. For some less demanding application scenarios, such as non-core control boards in common consumer electronics, the line width and line spacing may be relaxed to 5-6mil (0.127-0.152mm). However, with the continuous progress of technology, the line width and line distance accuracy of high-end HDI circuit boards are developing in a smaller direction. For example, some advanced chip packaging substrates, their line width and line distance have reached 1-2mil (0.025-0.051mm) to meet the high-speed and high-density signal transmission needs inside the chip.
Standard differences in different application fields: In the field of automotive electronics, due to the high reliability requirements and complex working environment (such as high temperature, high vibration, etc.), the line width and line distance accuracy standards of HDI blind buried circuit boards are more strict. For example, the circuit board used in the automobile engine control unit (ECU), the line width and line distance accuracy are generally controlled at 4-5mil to ensure the stability and reliability of signal transmission in harsh environments. In the field of medical equipment, such as the HDI circuit board in magnetic resonance imaging (MRI) equipment, in order to ensure accurate signal acquisition and processing, the line width and line distance accuracy may reach 2-3mil, which puts extremely high requirements on the manufacturing process.

三、Factors affecting line width and line distance accuracy
Manufacturing process: lithography process is the key link to determine the line width and line distance accuracy. In the process of lithography, the accuracy of the exposure machine, the performance of the photoresist and the control of the development and etching process will affect the line width and line distance. If the accuracy of the exposure machine is insufficient, the exposure pattern may be biased, and the line width and line distance after etching will deviate from the design value. In the process of etching, the improper control of the concentration, temperature and etching time of the etching liquid will also cause problems such as too wide or too narrow line width and uneven line distance.
Material characteristics: The substrate material and copper foil material characteristics of the circuit board also have an impact on the line width and line distance accuracy. The thermal expansion coefficient of different substrate materials is different. In the manufacturing process, due to multiple heating and cooling processes, if the thermal expansion coefficient of the substrate material is unstable, it may lead to the deformation of the circuit board, which affects the line width and line distance accuracy. The thickness uniformity of copper foil is also important, and the etching rate of copper foil with uneven thickness will be inconsistent during the etching process, resulting in line width deviation.

四、Methods for detecting and controlling accuracy
Detection means: In the production process of HDI blind buried hole circuit board, a variety of detection means will be used to monitor the line width and line distance accuracy. Optical microscope is one of the commonly used inspection tools. By magnifying the surface image of the circuit board, the line width and line distance are measured manually or with the help of image analysis software to determine whether the standard is met. Electron