Hello - I am faced with installing a 0402 package SMD resistor into an existing HP RF assembly (called a test port coupler, part of a vector network analyzer test set) whose existing thin-film resistor was destroyed due to application of excess RF power at some point in the past. The coupler is too expensive to simply replace, so I'm seeking to repair it, at least enough to get it to be serviceable. The HP part number is 5086-7489 and there are images online (if you remove the top of the gold cover which is attached with ten Torx screws, inside near the main (large) test port connector is a small white circuit board.)
The small circuit board in the coupler near the test port connector uses a ceramic (or some other exotic material) substrate. The traces on the substrate are gold (or gold plated). In a few places a resistor is actually printed onto the substrate (I think this is called a "thin film" resistor), and the dimensions of the (former) thin-film resistor which was destroyed and the adjacent gold/plated traces (visible under a toy binocular microscope) will accommodate a 0402 package thin film resistor, but with little clearance. I attached such a resistor (of approximately the correct value, which is known) with a tiny dab of (regular) epoxy onto the end of a wooden toothpick, temporarily pressed the resistor into position, and verified that the RF assembly functions (at least sufficiently) so that I'm now considering how to mount the resistor permanently.
The confines of the component around the resistor are too small to permit soldering, and the small circuit board cannot practically be removed from the test port coupler housing, so it needs to be repaired in place. For these reasons, I am thinking attachment with silver-based conductive epoxy might be a suitable solution. MG Chemicals 8330S claims "extreme" thermal and electrical conductivity (that would be good to minimize further electrical impairments from this fix) and long working time (before hardening), but the material itself is expensive, so I don't want to head down that path without some confidence I'm approaching this properly and with some practice on a piece of perf or prototyping board and some cheap/low cost conductive epoxy, so I can get the feel of the material and components before delving into the coupler for permanent installation.
So, I am looking for advice from someone that has hands-on experience working with conductive epoxy and tiny components in confined spaces, preferably around RF/microwave devices.
Thanks very much for any advice, pointers or comments,
Dave
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