Coaxial Probe Fixturing

For high-frequency testing, coaxial spring probes provide electrical performance up to 3 GigaHertz (with shielding plunger) and 500 MegaHertz (without shielding plunger).

Coaxial probes are available in double-ended (compliant on both ends) or single-ended (compliant on one end) designs, and most have the option for a flanged contact barrel. Many single-ended coaxial probes come with a pre-attached coaxial cable and are terminated with an optional SMA or SMB connector (other connector options are also available). Other coaxial probe designs include a coaxial probe with a built-in SMB connector and a non-rotating shielding plunger. Most coaxial probe assemblies have signal conductors that are often replaceable with a standard probe (consult the manufacturer for probe replacement options). Additional options include smooth-faced or serrated shielding plungers.

Coaxial probes with a flanged barrel can be mounted via two fixture plates. The two fixture plates are mounted above and below the flange and bolted together at the fixture corners for retention. Caution should be used when using a pre-attached SMA connector or other large connector on the end of the coaxial cable. The diameter of the SMA connector is larger than the contact barrel of the coaxial probe, making it impossible for the coaxial probe with connector to slip through the bottom fixture plate.

An alternative to the two-plate method is a press-fit mount into one plate, whether the coax probe is flanged or unflanged. For single-ended probes, a cable may be threaded through a plate or plated-through hole. For double-ended probes, proper orientation needs to be confirmed before mounting the probes in place. Tooling may be needed to aid in the mounting process. For all probes, care must be taken to hold a tight tolerance on the mounting hole to ensure the required holding force. The size of the connector may need to be considered before using this method of fixturing. Proper positioning of unflanged coaxial probes and hole tolerancing are the responsibility of the customer.

Another method for fixturing coaxial probes is to epoxy the contact barrel to the fixture substrate. This method can be used for virtually all coaxial probes. This method may make probes more difficult to replace, but it is uncommon for entire coaxial probe assemblies to be replaced since the signal conductor of the assembly is often replaceable.

Similar to the epoxy method, solder can also be used for fixturing coaxial probes. Using a temperature-controlled application, solder can be used to hold the contact barrel to the fixture substrate. This method can be applied to virtually all coaxial probes and may offer better electrical conductivity than epoxy. Coaxial probes can also be surface-mounted using the signal pin tail for positioning in a mounting hole. However, like the epoxy method, replacing entire probe assemblies may be more difficult. Care is needed in the soldering technique to avoid damage due to prolonged and excessive heating. Standard soldering techniques should also apply.

These methods are based on previously observed applications. Customers should choose the method of fixturing coaxial probes based on their specific applications and experiences. The footprint of the test pad/via, signal test pad, and ground test pad should be laid out similar to the footprint illustrated in the figure. The test pad for the ground is only required for coaxial probe applications using the shielding plungers. Consult the manufacturer for alternate tip styles and other probe options.