Efficient Impedance Measurement of a Large Amount of Components by Using a Scanning System

Introduction

Today, component manufacturers need higher productivity and component yields in order to supply lower cost products with faster delivery and higher reliability to compete against very tough competition. Consequently, most component manufacturers require impedance testing using a scanner to improve productivity and to control the quality of their products. However, making measurements with a scanner is different from measuring with common test fixtures. It is necessary to reduce the measurement errors due to the residual impedance that exists in the extension cable and the scanner. This application note describes how to adopt measurement systems using an LF LCR meter and an LF impedance analyzer with a scanner and how to solve problems related to residual impedance, which can occur when using a scanning system. In addition, this note covers actual scanning system examples such as temperature characteristics evaluation and insulation resistance measurement for electronic components.

Design the Scanning System

This section initially introduces key points for designing your scanning system. Next, it explains system

configurations, special considerations, and solutions devised to deal with these issues.

Reviewing required specifications

An example of the scanning system is shown in Figure 1. This scanning system allows one measurement instrument to measure many devices under test (DUT) by switching from one to another. When you design a scanning system, you have to select the scanner and cable configuration. To do this, you should clarify your measurement requirements (specifications) for the system. The specifications to review include measurement accuracy, impedance of the DUT, test frequency, and the number of scanner channels. The required measurement accuracy will determine the maximum residual impedance and stray capacitance allowed in the scanning system. In addition, the measurement value and test frequency requirements are determined by the cable configuration. The number of channels determines the type of scanner you should select.

Selecting the cable configurations

The cable configuration used for a scanner depends on the impedance value to be measured. As shown in Table 1, two cable configurations are available: the four-terminal pair (4TP) and the shielded two-terminal (2T). Using the 4TP configuration will achieve the highest accuracy over wide impedance and frequency ranges. However, the hardware cost increases because it requires many cables and switches for each measurement channel. On the other hand, the shielded 2T configuration can be used if the impedance of the DUT is larger than 100 ohms and the measurement frequency range is higher than 100 kHz. The shielded 2T configuration allows you to have the maximum number of measurement channels at a lower system cost.