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How does the military version with radiation shielding in the form of a metal cap not cause shorts? It looks like it makes contact with all the pins.. https:/

How does the military version with radiation shielding in the form of a metal cap not cause shorts? It looks like it makes contact with all the pins.. https://www.cpushack.com/2023/12/21/the-first-mass-produced-dram-of-the-soviet-union/img_20231216_091817/ https://www.cpushack.com/2023/12/21/the-first-mass-produced-...

dmsayer 2y ago
insulated on the underside is my only educated guess.
inkyoto 2y ago
Shielded on the inside and baked in an oven for a day or two. Surviving specimens were deemed of the military grade and passed QA.
adrian_b 2y ago
The metal cap does not do any radiation shielding. It is just for closing hermetically the cavity where the semiconductor die has been soldered, to prevent the entry of any contaminants from the atmosphere. The metal caps were usually cheaper to make, by stamping from a metal sheet, but there are package versions that used ceramic caps or glass caps. It does not make contact with any pins. The ceramic package has a depressed cavity with a grounded metallized base, on which the semiconductor die is soldered. So the ceramic body is made of at least two layers that are bonded one over the other. On the inferior layer, there is the large pad on which the die is soldered and many small pads that are connected to the pins by metal traces, which remain enclosed inside the ceramic body after its layers are bonded. The pads on the semiconductor die are connected to the pads on the ceramic body by wire bonding, typically with gold wires. The die, the pads that lead to the pins and the bonding wires remain at a lower height than the upper surface of the ceramic body. Then the metal lid is soldered on the ceramic body, closing hermetically the cavity, without touching the wires or the pads. All this is done because no plastic (i.e. polymeric material) is completely hermetic to gases, so if there are contaminants in the air they will pass slowly through the package, reaching the semiconductor die. Later, there have been developed methods of depositing protective layers directly on the semiconductor die, which have diminished the need for hermetic packages, at least in most milder environments.
- beAbU 2y ago
  If you look at the picture that GP linked, you'll see they are referring to what looks like a loose metal cap that fits /around/ the entire chip, and it clearly looks like it shorts out the pins all the pins. It's not the hermetic cap you are referring to.
  - adrian_b 2y ago
    That picture does not show any shorts. The actual DRAM integrated circuit is only the one from the top of the picture. The other 2 packages are like the one from the top, but they appear to have an additional plastic cover put over the package, to prevent touching the pins. That plastic cover might have been intended to avoid the damaging of the IC by technicians who did not use proper ESD protection measures when handling the IC. Perhaps the cover is not from conductive plastic, but from metal (though that would be an extravagant expense), but in any case it will be removed when the integrated circuit will be inserted in a PCB or in a socket. Its purpose is only avoiding electrostatic discharge damage during the previous handling. On the IC from the top, the only one where the metal lid and the pins are visible, it can be seen that the top of the ceramic body had a metal trace that formed a closed square, on which the metal lid had been soldered. The lid covers a cavity with the silicon die, the bonding pads and the bonding wires. As seen in the photo, the square trace used for soldering the lid is connected by a metal trace to one pin in the corner of the package, which is the ground pin, ensuring that the metal lid is grounded. In the middle of the ceramic body there are similar metal traces that connect the bonding pads to pads on the lateral side of the ceramic package. The pins of the IC are brazed to the pads on the lateral side of the package, before soldering the silicon die. The image is not clear, because the lateral side is in a shadow, but the lateral side has one metallized pad for each pin that will be brazed over it, and the metallized pads are separated by non-metallized ceramic surface, so there are no short circuits.
    - beAbU 2y ago
      I'm pretty sure that the "plastic cover" is in fact made from copper, and I'm pretty sure that the GP is asking about this cover is not shorting out the pins, which is sure looks like it does. I assume this cover is supposed to be removed after installation. So I think you are right in assuming it's purpose is there to avoid ESD damage during handling.
beAbU 2y ago
I wonder if that cap you are seeing is not a temporary measure, for storage and transport purposes. I agree, it does look like it's shorting out all the pins. Maybe it's intentional, to avoid any weird currents or whatever forming while the chips are not installed? Possibly an early form of ESD protection?

dmsayer 2y ago

insulated on the underside is my only educated guess.

inkyoto 2y ago

Shielded on the inside and baked in an oven for a day or two. Surviving specimens were deemed of the military grade and passed QA.

adrian_b 2y ago

The metal cap does not do any radiation shielding. It is just for closing hermetically the cavity where the semiconductor die has been soldered, to prevent the entry of any contaminants from the atmosphere. The metal caps were usually cheaper to make, by stamping from a metal sheet, but there are package versions that used ceramic caps or glass caps. It does not make contact with any pins. The ceramic package has a depressed cavity with a grounded metallized base, on which the semiconductor die is soldered. So the ceramic body is made of at least two layers that are bonded one over the other. On the inferior layer, there is the large pad on which the die is soldered and many small pads that are connected to the pins by metal traces, which remain enclosed inside the ceramic body after its layers are bonded. The pads on the semiconductor die are connected to the pads on the ceramic body by wire bonding, typically with gold wires. The die, the pads that lead to the pins and the bonding wires remain at a lower height than the upper surface of the ceramic body. Then the metal lid is soldered on the ceramic body, closing hermetically the cavity, without touching the wires or the pads. All this is done because no plastic (i.e. polymeric material) is completely hermetic to gases, so if there are contaminants in the air they will pass slowly through the package, reaching the semiconductor die. Later, there have been developed methods of depositing protective layers directly on the semiconductor die, which have diminished the need for hermetic packages, at least in most milder environments.

beAbU 2y ago
If you look at the picture that GP linked, you'll see they are referring to what looks like a loose metal cap that fits /around/ the entire chip, and it clearly looks like it shorts out the pins all the pins. It's not the hermetic cap you are referring to.
- adrian_b 2y ago
  That picture does not show any shorts. The actual DRAM integrated circuit is only the one from the top of the picture. The other 2 packages are like the one from the top, but they appear to have an additional plastic cover put over the package, to prevent touching the pins. That plastic cover might have been intended to avoid the damaging of the IC by technicians who did not use proper ESD protection measures when handling the IC. Perhaps the cover is not from conductive plastic, but from metal (though that would be an extravagant expense), but in any case it will be removed when the integrated circuit will be inserted in a PCB or in a socket. Its purpose is only avoiding electrostatic discharge damage during the previous handling. On the IC from the top, the only one where the metal lid and the pins are visible, it can be seen that the top of the ceramic body had a metal trace that formed a closed square, on which the metal lid had been soldered. The lid covers a cavity with the silicon die, the bonding pads and the bonding wires. As seen in the photo, the square trace used for soldering the lid is connected by a metal trace to one pin in the corner of the package, which is the ground pin, ensuring that the metal lid is grounded. In the middle of the ceramic body there are similar metal traces that connect the bonding pads to pads on the lateral side of the ceramic package. The pins of the IC are brazed to the pads on the lateral side of the package, before soldering the silicon die. The image is not clear, because the lateral side is in a shadow, but the lateral side has one metallized pad for each pin that will be brazed over it, and the metallized pads are separated by non-metallized ceramic surface, so there are no short circuits.
  - beAbU 2y ago
    I'm pretty sure that the "plastic cover" is in fact made from copper, and I'm pretty sure that the GP is asking about this cover is not shorting out the pins, which is sure looks like it does. I assume this cover is supposed to be removed after installation. So I think you are right in assuming it's purpose is there to avoid ESD damage during handling.

beAbU 2y ago

I wonder if that cap you are seeing is not a temporary measure, for storage and transport purposes. I agree, it does look like it's shorting out all the pins. Maybe it's intentional, to avoid any weird currents or whatever forming while the chips are not installed? Possibly an early form of ESD protection?