9
\$\begingroup\$

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.

\$\endgroup\$
7
  • 2
    \$\begingroup\$ You may be interested in this article: en.wikipedia.org/wiki/Semiconductor_package#Package_materials \$\endgroup\$
    – JYelton
    Jun 25 '13 at 6:48
  • 1
    \$\begingroup\$ What material would you suggest instead? Metals are great thermal conductors, but they're great electrical conductors, too. \$\endgroup\$ Jun 25 '13 at 6:58
  • 1
    \$\begingroup\$ Diamond would probably be the "ideal" material: great electrical resistivity, extremely strong, and excellent thermal conductivity. Of course, how are you actually going to package an IC using diamond, not to mention the raw material costs... \$\endgroup\$ Jun 25 '13 at 8:40
  • 2
    \$\begingroup\$ In general, heat generated internally is only a factor in certain very specific ICs, the main CPU or GPU of a modern computer for instance. For most other ICs, keeping the environment and heat out is a bigger issue. \$\endgroup\$ Jul 4 '14 at 21:46
  • 2
    \$\begingroup\$ Since ICs are covered in oxides + etc. it has poor heat conduction on the top. Heat is mainly removed through the silicon body and/or metal wires. Often then metal is used below as a thermal conductor, and you may make a hole under the IC to improve the heatsink capabilities. The ceramic cap can withstand much higher temperatures than plastic, which is one reason you might want a ceramic cap, or wire-bonding. \$\endgroup\$
    – HKOB
    Mar 20 '15 at 19:38
5
\$\begingroup\$

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.

\$\endgroup\$
3
\$\begingroup\$

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.

\$\endgroup\$
2
  • \$\begingroup\$ Interesting speculation, but it seems too narrow. It looks to me like older generations of ICs had higher frequencies of ceramic packages among non-EPROM types. Any ideas on the reasons for this and for the subsequent transition to epoxy in most cases? I wonder if it's relevant that pins often used to be gold-plated too, or if that was just showing off... So I wonder: was epoxy not available or cost-effective at first, were mfgs being paranoid or ostentatious by using the more resilient ceramic until they realised it wasn't worth it, did the industry force change, or was it something else? \$\endgroup\$ May 22 '16 at 11:42
  • \$\begingroup\$ @underscore_d: Ceramic is going to be more expensive to manufacture, but for high-value chips the cost might not have been high enough to matter, especially if the fallout in an epoxy-packaging line was any worse than for ceramic. I suspect also that early epoxy packages may not have been as heat-tolerant as later ones; given how hot many chips ran, that would have been a problem. \$\endgroup\$
    – supercat
    May 22 '16 at 13:44
1
\$\begingroup\$

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.

\$\endgroup\$
0
\$\begingroup\$

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.

\$\endgroup\$
0
\$\begingroup\$

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

\$\endgroup\$
1
  • \$\begingroup\$ Hi James, thanks for your answer, please make sure it answers the question asked though. \$\endgroup\$
    – jramsay42
    May 20 '20 at 7:46

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.