1
\$\begingroup\$

What was the advantage of having a golden heat spreader and bottom lid ?
Contacts make sense because of the electrical properties. This is still done today.
But what were the effects of using gold where no electrical signal passes through.

Bonus question: Why is this not necessary anymore ?

I could't find an answer to this, so thanks in advance.

\$\endgroup\$
5
  • \$\begingroup\$ I would guess that they're aiming to use the main benefit of gold, which is corrosion resistance. Corrosion on the heat spreader would cause a lot of added thermal resistance, so avoiding it is critical. Not sure why they dont use it now, probably some combination of cost/lack of necessity/better stainless allows, someone else probably has more info \$\endgroup\$
    – BeB00
    Commented Oct 13, 2021 at 2:16
  • 2
    \$\begingroup\$ @BeB00 I don't think they would use stainless for heat spreaders, though I could be wrong there. I would expect copper alloy, possibly plated, for a high-performance part. \$\endgroup\$
    – Hearth
    Commented Oct 13, 2021 at 2:25
  • 2
    \$\begingroup\$ Gold is almost never used for its electrical properties--the reason gold is used on contacts and such is for corrosion resistance more than anything else. Copper, silver, and aluminum (I think? maybe not aluminum) are all more conductive than gold, but all of those readily corrode in normal atmospheric conditions, given enough time. \$\endgroup\$
    – Hearth
    Commented Oct 13, 2021 at 2:26
  • \$\begingroup\$ The golden parts are always soldered onto the ceramic of the IC. Could the gold have been used to make the solid parts solderable ? \$\endgroup\$
    – Howie1337
    Commented Oct 13, 2021 at 3:12
  • \$\begingroup\$ @Howie1337 If it's a space-grade part maybe. \$\endgroup\$
    – DKNguyen
    Commented Oct 13, 2021 at 3:25

1 Answer 1

Reset to default
3
\$\begingroup\$

If you look at the construction of a present-day Kyocera hermetic ceramic lidded package, you'll see that there is quite a bit of gold plating in it:

enter image description here

The gold is over the die bond and wire bond areas as well as over the seal ring and the leads.

Here is a detailed drawing of such a package with a huge die cavity.

The purpose of the lid is just to (hermetically) close up the package via soldering, it's not primarily a heat spreader like in modern CPU packages.

In the latter, the heat spreader is actually in contact with the "back" of the die. In the lidded ceramic package, there is a big space between the top of the die (with attached bonding wires), and the inside of the lid.

\$\endgroup\$
4
  • 1
    \$\begingroup\$ Nicely presented information, but this doesn't seem to answer the question: why gold? \$\endgroup\$
    – TypeIA
    Commented Oct 13, 2021 at 4:39
  • \$\begingroup\$ @TypeIA The gold is there anyway for the leads, the die bond and wire bonding. Using it for the soldered seal ring and lid is a no-brainer if cost is not critical. Gold was cheap in relation to CPU cost in the 1970s and there's not a lot of it used. \$\endgroup\$
    – Spehro Pefhany
    Commented Oct 13, 2021 at 5:13
  • \$\begingroup\$ Thanks, that explains a lot. Is this kind of shielding not necessary with todays CPUs ? \$\endgroup\$
    – Howie1337
    Commented Oct 14, 2021 at 11:16
  • \$\begingroup\$ There is not usually electrostatic or magnetic shielding on individual chips in general. Most IC packages are plastic and the chip sits on a (typically grounded) metal tab that is part of the leadframe. The active area of the chip is a fraction of a mm away, close to the 'top' of the die. PC CPUs are made differently because they are physically huge, have a huge number of pins and can have enormous power dissipation and supply currents. \$\endgroup\$
    – Spehro Pefhany
    Commented Oct 14, 2021 at 12:22

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