On Fri, 11 May 2012 05:07:20 +0000 (UTC), Cydrome Leader
wrote:

I love switching power supplies where every ****ing part is under or
completely surrounded by some sort of obnoxious glue or potting compound.

Unfortunately, the glue and goo are a necessary part of the design.
The problem is that there are many parts in a switcher that
mechanically vibrate at the switching frequency. If coupled through
the pins to the PCB, it will soon produce an intermittent connection.
I've seen transformers literally fall out of a PCB as the ultrasonic
vibrations cold flowed the solder. I could see an annular ring around
the pins. If I resoldered the pad, the problem would reappear in an
accelerated burnin. There are also noise specifications which were
difficult to meet with loose components, where the PCB acted like a
sounding board.

The glue and goo couple the component mechanical vibrations to the
PCB, heatsink, case, or whatever. Distributing the load from a point
source (wire leads) to a larger surface reduces the energy
concentration and eliminates the cracked solder connection problem.
RTV doesn't do this very well as it's elastic and does not couple
mechanical energy very well. Epoxy works well because it's rigid. It
also has a similar temperature coefficient to G10/FR4 PCB material,
which prevents breaking the glue joint with temperature cycling.

I've substituted power supplies inside various Cisco router in the
distant past (2500/2600 series). It wasn't difficult finding a
source. Make and model?

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558