As someone with no background in electronics whatsoever, I wonder: why are ICs packed inside ceramic or plastic? I thought we wanted the heat to go out as fast as possible, and ceramic is a good thermal insulator.
In IC packages it certainly is desirable to dissipate heat with the lowest possible thermal resistance.
However, at the same time, electrical insulation and protection from oxidation / corrosion are also desirable, at least for discrete components that are likely to be handled or exposed to the environment.
An insulating packaging such as ceramic or plastic allows this insulation and protection, while permitting heat dissipation through controlled paths, such as integrated heat sinks or heat sink tabs in some packages, or just through the pins in others.
Many IC packages are also sold as bare die, or wafer level chip scale (WLCSP) packages, for the circuit assembly process to directly connect to the PCB. The bare chip is then environmentally protected using epoxy potting or similar protection coatings, after soldering or bonding of the lead bumps to the circuit board.
Such bare packaging of course requires more sophisticated assembly equipment than the much larger IC (and larger contact pitch) packages do, so they aren't for everyone.
The type of chips most commonly seen in ceramic packages are those with UV-erasable memory. In order to allow such memory to be reused after it is programmed, it must be possible to expose the die to a considerable quantity of UV light. This requires that the chip have a quartz window, and installing a quartz window on a chip in turn requires that the chip's package be made of something whose thermal expansion characteristics reasonably match those of quartz. If a quartz window were installed in an epoxy package, thermal expansion and contraction of the package would likely cause the seals to fail, allowing atmospheric air (including water vapor) to reach the part and destroy it. I saw one chip once which looked like it was made from epoxy with a plastic window which looked a bit "milky"; I didn't examine it closely enough to confirm that, though. If it was a plastic window, it would probably have been usable for a few UV-erase cycles, but many plastics degrade relatively quickly UV exposure. Perhaps someone figured that making EPROM chips with plastic cases would save enough cost that even if they would fail after a few uses, they'd be reusable enough to justify using them instead of non-windowed parts, and cheap enough to justify using them instead of ceramic parts. I don't think they ever caught on, though.
The main other place I've seen ceramic parts was in places where they had a metal top which would be heat-sinked. There again, the dimensional stability of the ceramic was necessary to prevent the seal from failing under changing temperature conditions.
Ceramic is used in RF and microwave applications because they have insulating and impedance properties crucial to radar, and cell phone base stations towers. Many plastics and epoxies absorb moisture from air. Changing characteristics with humidity change. This affects frequency tuning. They can be sealed well enough to slow infiltration and damage by hydrogen and oxygen in orbiting satellites.
For thermal conductivity actually BeO is extremely good but the manufacturing process presents hazards. Aluminum nitride is fairly good thermally and can be used song as the design fits. Lastly another trouble with plastics is that some chips will run hot enough to melt it or break it down. There are applications using ceramic coated metals where it doesn't affect RF frequencies.
Oldschool parts came in the less popular metal can. Cans aren't common for mass produced parts. Ceramic and plastic packages are engineered to have fairly high thermal conductivies (~20W/m∙K), and they come at a fraction of the cost of a metal package. Ceramic packages are usually white because they are a high alumina material. Plastic packages are black because they contain carbon black and/or graphite to dissipate both heat and static charge.
Early nineties military memory die were in ceramic packages with gold plated ceramic packages. I worked in a backend assembly clean room maintaining eutectic dieattach machines aluminum wedge binders and bevel saw. Process was gold silicon eutectic die bond aluminum wedge bond. Die bond subjected to over 20gs post in burn in along with lifetime testing