PCB high-speed signal impedance test skills sharing

For low-frequency boards, PCB is to interconnect the components in the design drawings , but with the increasing frequency, PCB has begun to play the function of transmission line in electronic equipment, such as mobile phone wireless communication , radar communication, satellite Communication, the higher the frequency, the higher the performance requirements of the PCB board , and its performance can determine the performance of the entire device, so the performance test of the PCB is particularly important in all aspects of product production.

Using TDR ( Time Domain Releflectometry ) time domain reflectometer to test the characteristic impedance of PCB boards, cables and connectors is the characteristic impedance measurement method specified by the IPC (American Electronic Circuit and Electronic Interconnection Industry Association. It has been widely used and popularized. However, TDR is also an electronic device. Generally, like other electronic devices, the higher the bandwidth, the more sensitive it is to electrostatic discharge (ESD) / electrical overload stress (EOS), so most of the use of high bandwidth TDR (such as above 18GHz) Industry customers may have experienced ESD/EOS damage from TDRs. Especially in the PCB manufacturing industry, due to the high frequency of use of TDR, static electricity damage, whether or not EOS is included, such damage often has a greater impact on production efficiency and cost control, so some customers even start to consider using other equipment instead. TDR. However, considering the time domain characteristics of the TDR impedance test method and its advantages of high test efficiency, test accuracy and test consistency, it has always been specified by the IPC-TM-650 specification and industrial organizations such as HDMI,USB - IF , VESA Displayport, etc. , Authoritative characteristic impedance test standard.

Most experienced engineers know that most ESD/EOS are generated instantaneously, and due to the influence of accidental factors such as changes in laboratory ambient temperature, the experience of operators, and static electricity in the product under test, ESD/EOS is It cannot be completely avoided. All electronic measurement equipment such as oscilloscopes , signal sources and VNAs have clear and specific anti-ESD/EOS requirements. The higher the performance of electronic measurement equipment, the higher the corresponding anti-ESD/EOS requirements. . At the same time, the impact of ESD/EOS on electronic measurement equipment can be accumulated continuously. The accumulation of static electricity caused by a large number of repeated ESD/EOS will gradually damage the measurement equipment, and eventually cause its measurement function to deteriorate.