Many folks have bench type LCR meters available and employ the usual general purpose Kelvin type clips or direct connect fixtures for most measurements. When encountering SMD components these measurement tools/methods can become difficult and frustrating for quality repeatable measurements, especially true for low Z components.
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The specialized SMD fixtures which utilize horizontal “plungers” perform well except with very low-Z components. The difficulty is due to the force and sense Kelvin connections being made at the small brass bolt that threads into the bottom of each plated brass plunger.
This leave the path from the small brass bolt through the plunger to the device under test (DUT) surface connection uncompensated and relies on the zero/short calibration for correction, which often leaves more measurement uncertainty than desired. Which is especially noticeable with very-low-Z SMD components such as resistive current sensing shunts, where one desires an accurate and repeatable low resistance measurement.
Having suffered with low-Z SMD components and measurement issues often, this created an opportunity to investigate other approaches outside the usual expensive OEM solutions (well out of budget). One idea came to mind was the technique of utilizing a lever toggle arm that worked well in the SMD adapter created for the Tek 577 Curve Tracer.
A custom PCB was developed to directly connect via 4 BNC connectors to the benchtop LCR meter, similar to the way OEM LCR meter fixtures behave. The SMD DUT would be held in place against the PCB exposed surface with the lever toggle arm similar to the concept with the Tek 577 adapter.
Both sides of the PCB were originally utilized to allow the lever arm to be located on the left or right and not interfere with the LCR meter controls and display as shown in Figure 1. This also shows the Tek 577 adapter along with another PCB version which doesn’t have direct BNC connections.
Figure 1 A custom PCB developed to directly connect to the benchtop LCR meter via 4 BNC connectors.
Figure 2 shows the LCR meter connection on a Tonghui TH2830 bench LCR meter.
Figure 2 The custom PCB connected to a Tonghui TH2830 benchtop LCR meter.
The LCR meter and SMD DUT fixture connections for meter Hcur (force) and Lcur (sense) contribute significant impedances, some meters can deliver over 100 mA, which can produce errors in the meters sense terminal Hpot and Lpot affecting results.
Using the PCB split-Kelvin technique developed where the DUT SMD makes contact with the exposed PCB surface but the force and sense connections are made separately by the DUT end conductive terminals because the PCB contact area is “split” between force and sense on the high and low sides. This allows the impedances “looking back towards the meter” and the highly variable DUT contact impedance to be within the meter Kelvin control grasp and significantly reducing DUT measurement uncertainty.
Another PCB version was also developed where the PCB doesn’t host the BNC connectors, but is smaller and fits onto a smaller supporting case with the BNC connectors that is a repurposed cheap LCR meter Kelvin cable/clips case as shown in Figure 3 and Figure 4.
Figure 3 Layout of another custom PCB version developed to directly connect to the benchtop LCR meter via 4 BNC connectors.
Figure 4 The alternative fixture developed, where the PCB does not host the BNC connectors. Instead, it is smaller and fits onto a smaller supporting case with the BNC connector.
Note in Figure 5 where a shield was added in the case between the high- and low-side BNC connectors to improve isolation.
Figure 5 Wiring inside the smaller, alternative fixture that directly connects to the benchtop LCR meter via 4 BNC connectors.
Various PCBs were investigated over the span of a few months and it was observed that the PCB surface contact with the SMD DUT could be improved by having gold-plated contact areas and/or by increasing surface contact roughness.
Surface roughness was improved by adding copper filings mixed with solder paste and flux and reflowed onto the DUT contact, see Figure 6.
Figure 6 Surface roughness was improved by adding copper filings mixed with solder paste and flux and reflowed onto the DUT contact.
A small piece of thin copper sheet cut to 2512 size makes a good zero/short calibration reference device. Caution with the so called “zero-ohm” SMD components, these were found to have significant impedance for most all sizes, and the thin copper custom cut proved the better reference.
Operation with the Hioki IM3536 LCR meter is shown in Figure 7.
Figure 7 Testing “zero-ohm” SMD components on the custom fixture for low-z components with the Hioki IM3536 LCR meter.
Anyway, these various custom fixtures have proven beneficial in daily LCR SMD measurements, and the latter version with the repurposed case (Figures 4, 5 and 6) especially useful and highly stable and repeatable. Hopefully others will find these custom DIY LCR Meter fixtures useful.
Michael A Wyatt is a life member with IEEE and has continued to enjoy electronics ever since his childhood. Mike has a long career spanning Honeywell, Northrop Grumman, Insyte/ITT/Ex-elis/Harris, ViaSat and retiring (semi) with Wyatt Labs. During his career he accumulated 32 US Patents and in the past published a few EDN Articles including Best Idea of the Year in 1989. All posts by Mike Wyatt below:
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