Gregory's ServerIntel's 386 processor (1985) was an important step toward today's computers, moving the x86 architecture to 32 bits. The chip came in a 132-pin ceramic package. There's more going on in this package than you'd expect. Let's take a look. 1/13
Ken Shirriff
28 comments
Ken Shirriff
Historically, Intel had been very stingy with pins, using the "God-given 16 pins" for the 4004 processor even though it made the chip slower and harder to use. When Intel was forced to use 18 pins, Federico Faggin said it "was like the sky had dropped from heaven". 3/13
Ken Shirriff
Eventually, Intel moved to 40-pin chips (photo: 8086). But by the 1980s, these "Dual Inline Packages" (DIPs) weren't good enough: cheap but not enough pins and much too big compared to the tiny silicon die inside. Small-outline ICs and other new packages became popular. 4/13
Ken Shirriff
The 386 needed 132 pins, so Intel designed a custom ceramic pin grid array package. The ceramic package contains 6 layers of wiring: two layers for signals, two for power, and two for ground. 40 pins are used for power and ground to minimize electrical noise. 5/13
Ken Shirriff
The 386's logic circuitry is powered separately from the I/O pins. Changing an output takes a lot of current, resulting in power fluctuations. These could cause unreliability if logic circuits used the same power. So I/O has separate pins and wiring on the die. 6/13
Ken Shirriff
This X-ray shows the layers inside the 386 package. Thick traces go from the signal pins to the die. Power/ground have metal planes, perforated so the ceramic sticks together. Thin tungsten(?) wires to the package edge are used to electroplate the pins. 7/13
Ken Shirriff
This closeup shows how tiny bond wires connect the die to the package. The package has two tiers of golden pads that are connected to the tiny bond pads on the silicon die. 8/13
Ken Shirriff
The power and ground connections use multiple wires in parallel. This allows more current and also reduces inductance (and thus noise). Intel's package was much better than other 132-pin packages with 1/4 the inductance. Resulting noise was under 0.5 volts. 9/13
Ken Shirriff
Intel's cost to produce 386 dies rapidly dropped. Eventually, the package cost more than the die itself. Intel redesigned the 386 so it could fit in a cheaper plastic package. 10/13
Ken Shirriff
Packaging is underappreciated, but it is essential to making chips small and reliable. In particular, getting power to a chip and then distributing power on the chip is harder than you might think. Nowadays, packaging, power, and removing heat is an even bigger issue. 11/13
Ken Shirriff replied to Ken
For more on the 386, see my previous tweets:
Dave Warnock replied to Ken
@kenshirriff thanks for this thread. Brought back fun memories. Oh I also remember when I was able to upgrade the office NetWare server from a 286 to 386 again a real game changer. 
Anthony 🦘🐨🪃🕯🧙♂️🔆🇦🇺 replied to Ken
@kenshirriff wow! An interesting tour through a bit of cpu history. 😃
ES Michelson
@kenshirriff I think this is a 386SX chip. A little sibling chip with, I think, 16 bit external data path. Maximum supported RAM was 4MB and I bought such a PC with 1MB RAM as a 'smart purchase.' The PC ran MS-DOS, then Windows 3.1 It had a 40MB HD which, the seller said, would never fill up. With the Windows upgrade the RAM and HD had to be upgraded. The next PC after that was a PowerPC chipped Macintosh. That was an excellent computer in its day but hobbled by increasingly terrible OSs.
tTh
@kenshirriff This is a SX 386, who have an only 16bits address bus and a smaller data bus than the vanilla 386. 
Lee Cremeans
@kenshirriff AMD made a PQFP 386DX-40 in a package like this one, and it showed up on a bunch of late-model (circa 1993-1995) cheapie motherboards.
Steve Loughran
@kenshirriff 386SX was 16 bit memory bus, wasn’t it. So needed fewer pins at the cost of memory access times. And the double sigma on some of the packages means “gets 32 MUL eax right” so safe for 32 bit apps….  
varx/social
@kenshirriff It always kills me when there's a connector or something with like 30 pins and I look up what they do, and it's 2 for signal and 28 for power. 😆 
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@kenshirriff 
Ross of Ottawa
@kenshirriff I worked intimately with the design and analysis of these PGA packages in my early career then. Interesting psychology anecdote – when you had a sample and you showed it to people (eg upper management) and told them explicitly do not touch the interior, they would find it irresistible and stick their finger into the inner die-attach surface and bonding pads anyway, ruining the part. It was very weird how they couldn't control themselves. Must. Jab. Finger. In. Package. |
Here's an X-ray of the 386 package. The pins are bright white. The gray square in the middle is the silicon die. Inside the package, the grayish traces connect the pins to the contacts surrounding the die. Thin bond wires connect these contacts to the die. 2/13