Re: Best practices for working on a ceramic hybrid front-end board (TDS3032)

Brendan
 

On Tue, Feb 5, 2019 at 04:14 AM, benj3867 wrote:


Hi,

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.
On my TDS320 I cut the leads and ordered a new relay from Mouser. For the TDS320 the relays were under $2

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