As the saga of fixing my TDS3032 continues, I am now faced with the need to remove one of the relays from the ceramic board/hybrid front-end module of channel 1 in order to gain access to one of the laser-trimmed resistors printed underneath it on the ceramic substrate.
My question is:
what is the best way to desolder and re-solder the relay (as well as how to eventually replace the blown printed resistor) without causing the pads printed on the ceramic substrate to disintegrate and disappear?
So please guys, share your experience and best practices for reworking these ceramic hybrids!
My own experience can be summarized as follows.
The ceramic strips used in tube-era Tektronix instruments are not the subject of this post. Indeed, following the recommended use of silver bearing solder I have never had any problem with these. However, the one time I have worked on a modern ceramic hybrid was different. I had to replace an op-amp on an LEM current sensor, and desoldering this DIP-8 chip using a desoldering braid removed about 30% of the pads material printed on the ceramic substrate as well. After soldering the new chip in (using silver bearing solder), and removing it one more time (don't ask why I had to do that :-)) two of the 8 pads were completely gone from the ceramic substrate. Unlike a regular epoxy PCB where pads may delaminate if one applies too much heat, her the pads were simply slowly disappearing together with the removed solder.
I have no idea if the ceramic hybrid used in the TDS3032 front-end (and many other modern Tek scopes) are more robust than the LEM hybrid I have worked on, but I would hate to find out that they are the hard way.