Mystery compound in non encapsulated RF transistors

I do a little repair work for my friends on the side and I was recently working on a 148 Mhz transceiver.

The final power module is bad so, I successfully removed the lid to this power module and found the 2 RF transistors in side how ever, these units do not have any encapsulated case but are exposing the silicone wafer and electrical connecting wires. I noticed the use of a compound injected into the body of these units which had debris in it, most likely from when I was removing the lid. Under the microscope with my precision tweezers I removed the debris and noticed the compound is still solidified.

My question I guess is, what is this compound and how does it stay in place ? I can visualize the unit being stored up side down and having this compound oz out how ever, this apparently isn't the case and I know this unit has had some abuse being placed in lots of different angles for long periods of time.

"noticed the compound is still solidified."

Q: do you mean "un-solidified" above ?

This is a silicone gell with no 'flow rate' characteristics at normal or 0G.

It is used to prevent the ingress of gas or fluid to the specific wafer and its wire bonds.

RL

Whatever happened to good old Beryllium Oxide?

BeO is a ceramic material with low thermal impedance. It has nothing to do with the inherent hermeticity of a package construction that it may be used in.

It is highly carcinogenic, if its dust is inhaled. Parts are often identified by bright colouration (pink or blue being most common), added solely for purposes of identification. You can still buy commercial insulators made from BeO, but they are generally long lead-time, as no-one wants to hold inventory.

It's not used often, due to the hazards inherent in the workplace and possibly because too many Jamies are inadvertently hurting themselves by farting about with it and inhaling dust from (the resulting) cracked or broken after-market assemblies. Warning labels covering safe disposal methods often become disassociated from the actual parts in the field.

Aluminium Nitride may be a suitable alternative.

RL