Automated 2D Metrology for Impedance Control

March 26, 2018

Automated 2D Metrology for Impedance Control

AOI-integrated tool enables accurate PCB line top and bottom measurement

Acceleration of electronic data transformation

It’s clear that 4G technology will not be able to keep up with digital data volume and speed requirements of the next decade. That’s why the development of 5G – which is predicted to connect at least 3x more devices and handle 10x larger data volumes than 4G – is in high gear.

As new methods and tools supporting the acceleration of electronic data transformation continue to be developed, it’s also clear that PCBs will play a major role in this revolution. With the transition to higher data volume transmission and processing, there will be a growing need for new PCB metrology tools that efficiently control line speed and dimensions.

Transmission lines in high-frequency electronics

The length of lines connecting components of electronic devices that work at low frequencies is not an issue; in these cases, the voltage on the wire is assumed to be constant at all points at any given time. However, for devices working at high frequencies, the signal cycle time is comparable to the time it takes the signal to travel along the interconnection. In these cases, the lines must be designed and controlled as transmission lines.

Electronic applications requiring wide bandwidth

The following applications require high frequencies and controlled impedance design attention:

• Cellular base station antennas and power amplifiers
• High-speed serial communications (e.g. PCI Express, InfiniBand)
• Interconnection between high speed memory and controller (e.g. DDR3)
• Automotive radar and sensors
• Real-time critical control systems in aerospace and military applications

PCB interconnect dimension and transmission speed

The speed and attenuation of the line signal depend on line inductance and capacitance, which are determined by the conductor line’s dimension, dielectric layer thickness, and dielectric factor (εr).  

Frontline impedance solver tool as part of InPlan engineering tool

High-frequency PCB design requires dedicated CAD simulation tools to calculate the required line structure and materials. However, production variations induced by etching, lamination pressure, plating, and other processes lead to line width and line profile shape changes, which in turn, alter line attenuation and impedance. Therefore, the speed and timing of critical lines must be properly controlled throughout production by ensuring that design dimensions fall within defined tolerances.

Copper line shape and impedance value at high frequencies

Several factors impact the electrical properties of a PCB transmission line.

Line width accuracy and stability

The impedance characteristic is influenced by line width and shape. Decreasing the width of a trace will lead to higher inductance and lower capacitance, which will significantly increase line impedance per unit of length. Therefore, measuring and controlling the line width is critical when high impedance accuracy is required.

Copper line profile and signal integrity

At very high frequencies, signal loss and time delays are influenced by line inductance and capacitance, which in turn, are influenced by line dimension.

In fine line PCB patterns, variations in the dimensions of the line’s cross section occur due to etching and plating processes as well as differences in the copper density level throughout the panel.

Controlling and maintaining the shape of the line’s cross section is very important for high-frequency applications for several reasons:

1. Impedance increases when the line cross section area decreases
2. Straight walls bring the characteristic impedance of the trace closer to theoretically calculated values
3. The tight proximity between the lines, particularly in dense patterns and flat trapezoid shapes, causes significant crosstalk where the signal in one line adversely impacts the signal in another line

Impedance control in PCBs today and over the next 5 years

Currently, impedance control tolerance is typically ±10%, and only several lines per design are defined as critical. Impedance is measured via impedance control coupons on the panel, which is located out of the active PCB area. A special electrical impedance test probe is used to measure impedance by touching the PCB coupon pads, and the test is conducted on several panels in the final stage.

In addition, In-Process Quality Control (IPQC) is a common procedure in which a sampling of panels from each production lot is manually measured for line and space width size. The measurement takes 30-60 seconds per single spot and with many test points per panel it is quite costly. Therefore, only several spots on 1-2 panels only per lot are measured, which is insufficient for mass production of very high frequency boards.

Manual line width measurement

Orbotech’s 2D metrology - a new dimension for AOI

Automated 2D Metrology tool with AOI video microscope

Today, all PCB panels, inner and outer layers are inspected for critical and quality defects. However, AOI machines do not measure line width and shape at the resolution and accuracy required for impedance control. A new 2D Metrology tool is offered today that measures line width and shape within the AOI operation flow. 2D Metrology offers the following capabilities:

• Accuracy of ±2µm for line width of <100µm
• 1.5 seconds per line measurement – move to measurement point, image acquire and measuring time
• Line profile measurement – top and bottom line widths

Orbotech’s unique 2D metrology utilizes UV illumination and fluorescence imaging of the laminate material, ensuring a highly accurate measurement of the copper line bottom.

In addition, the measurement can be implemented on vertical, horizontal and diagonal lines, corners, and circular and rectangular pads.

These measurements are then automatically logged and displayed, and a report is sent to the process engineer and central SPC system.

2D Metrology benefits

PCB manufacturers using Orbotech’s AOI with integrated 2D Metrology enjoy the following major benefits:

• Automated and consistent measurement
• Reduced handling (less manpower and damage)
• Top and bottom width measurement of the conductor
• Faster, easier and higher sampling – seconds/panel rather than 10-15 minutes/panel
• Higher repeatability compared to manual measurement
• Online results and statistics – better traceability tools

2D Metrology reporting as part of Industry 4.0 \ Smart Factory tools

The top and bottom line measurements collected by 2D Metrology on each panel are uploaded into the Orbotech Data Server (ODS), which is part of the Orbotech Smart Factory solution.

The measurements are then analyzed, and statistical reports can be extracted according to the PCB manufacturer’s needs.

Summary

In the near future, we will see a ramp-up in the production volume of PCBs that will support high RF digital mobile electronics featuring fast electronic design and tight impedance-controlled transmission lines.

Common methods of impedance control such as coupons or line-dimension measurement of several spots per panel on 1 panel per batch will need to be replaced by new solutions. These automated solutions will measure multiple spots on all panels in a batch more quickly and accurately than ever.

2D Metrology is an automated measurement tool that can be integrated with most Orbotech AOI solutions featuring top and bottom line measurement in an accurate operator-independent measurement mode. The measurement is fast, fully automated and supports high sampling rate and automated reporting tools that meet the needs of PCB manufacturers and OEMs.

2D Metrology has been tested and accepted by leading PCB manufacturers and OEMs worldwide, and is available to most Orbotech AOI users.

Author: Micha Perlman, Senior Marketing Manager, PCB Division, Orbotech