PCB fixtures improve component measurements

Article By : Istvan Novak

Using PCBs as fixtures improves component characterization measurements over wires and connectors.

In the first two articles of SI/PI MasterClass, I showed how you can create simple fixtures to characterize small discrete components. In “Solder-wick trick characterizes bypass caps” [1], you can learn how to create a simple flexible fixture for quick, rudimentary measurements. In “Make simple fixtures from SMA connectors” [2], I showed how to make small fixtures with consistent geometry out of SMA connectors. In this article, I describe simple printed-circuit-board fixtures that you can either buy ready-made or create yourself.

The fixtures in references 1 and 2 have limitations because the shape of the current path isn’t controlled. In the solder-wick fixture, the shape of the flexible connections will vary depending on how you achieve the pressure-mount connection. With the SMA-connector fixture, you have a fixed geometry for the connector pieces, but there are no dedicated pads to solder down the parts. Thus, the actual current path depends on how you solder the DUT between the center pins and outer frame. To get more consistent results, use fixed-geometry fixtures with defined return paths. You have two options: a) generic fixtures, which can take a large variety of different DUT body shapes and sizes in the same fixture, and b) fixtures with dedicated footprints for specific DUTs. Here, I show you generic fixtures [3].

You can create generic PCB fixtures from small co-planar 50-Ω traces that have exposed trace and ground next to each other on the same side of the fixture. The DUT can be connected between the trace and the ground shape, letting you use the Two-Port Shunt-Through measurement topology.

Having a sufficiently large ground shape next to the trace, you can accommodate many different case styles and sizes. Having connectors at the ends of the through trace provides quick connections, though you could also use permanently attached (soldered) cables. Soldered cables eliminate the need for separate cables with connectors at both ends, but makes the calibration a little bit more difficult. Figure 1 shows the unassembled panel of fixture boards. One panel provides eight identical fixture boards (though they carry different labels) that you can break away.

Figure 1. This unassembled panel of eight fixtures contains three for calibration and five for component measurements. Source: SV1AFN

Three of the boards are labeled for calibration (OPEN, SHORT, LOAD) and five are labeled for measuring components (DUT). These fixtures also let you do a more comprehensive OPEN, SHORT, LOAD calibration, which I’ll explain and describe in later articles.

The panels are available with SMA male-female connectors attached, or with the loose male-female connectors shipped in the same package for you to solder them on. Figure 2 shows a female connector at one end and a male connector at the other end, which creates what is called an insertable piece—you can take a closed male-female connection, open it and insert the piece without the need of any adaptor.

Figure 2. Connectors let you insert a co-planar fixture in line with test equipment.

Alternately, you can get the fixtures unassembled and solder SMA female connectors to both ends, which will conveniently take cables with male connectors on both cable ends.

The lines of the fixture are coplanar waveguide (CPW) over ground. The printed circuit material is FR4 and the board thickness is 0.8 mm. The gold-plated nickel over copper is 35 µm (1 oz.) and the line width is 1 mm with 0.254 mm separation to ground.

These fixtures can take any component size from 0402 (1 mm long) to D-size (7.3 mm long). The DUT sample in Figure 3 is a 1210-size ceramic capacitor.

Figure 3. A DUT co-planar PCB fixture can accept several size components, including this 1210 size 47 µF MLCC soldered.

With these fixtures, you also have the option of connecting the DUT in different ways. Mechanically and electrically, you get the most robust and most reliable connection if you solder the part to the fixture. If you want to reuse the fixture and want to speed the swapping of components, you can opt to use simple pressure mount, like we did with the solder-wick fixture, and you can reduce the contact resistance and improve consistency of your collected data by applying a dot of silver paste each to the component terminals. If you solder the component, push down the parts on the pads during soldering to improve measurement repeatability.

[Continue reading on EDN US: VNA measurements]

Istvan Novak, PhD is Senior Principle Engineer at Oracle.

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