[time-nuts] OCXO housings - Why copper and not iron/steel?

[Bernd Neubig]

-----Tobias wrote-----
>Also an interesting material when high thermal conductivity is considered
is boron nitride. It is around  five times better than copper, comparable to
diamond and can be sintered to a ceramic material.
>Of course the Wiedemann-Franz-Lorenz law doesn't apply here as it is a very
good electrical insulator.
>A boron nitride OCXO would indeed be interesting! :-)

About 30 years ago, in my previous job as head of TeleQuarz oscillator
division (now Vectron->...Microchip), we had used AlN hybrid substrates
replacing the FR4 PCB in OCXO. We had built up a complete hybrid
manufacturing line for that. Aluminum Nitride has an extremely high thermal
conductivity while being an isolator. The thermal performance was excellent.
However it was nearly impossible to solder on it manually (e.g. to change
components for fine tuning purposes) because the solder literally was
freezing on the substrate and even a special soldering iron with temperature
regulation directly in thee tip (Metcal) was not able to deliver sufficient
heat fast enough. After a few years we stopped this line.

Have a nice Sunday.
Bernd