Electronic – Why “substrate” is exposed through a “do not connect” pin at all

opto-isolatorsemiconductorstriac

I've were reading through MOC3041 opto-isolated triac when I noticed this part:

enter image description here

"Substrate". It's also repeated later on one page down further, but there's no explanation. I guess that might be the silicon substrate upon which the device was build, and I can imagine that actually connecting it to a wrong thing could make things soon pretty bad. I also know that in discreet MOSFETs the substrate is very often tied to the source pin, so if the substrate here wasn't internally connected to anything for some reason, and even the datasheet says "do not connect", then it seems quite obvious to me, that, well, it should be left not connected. OK.

But if that's the case, then I guess that leaving it internally not connected would be the easiest and safest way, and I saw that a lot in various devices that had to be packed in some standard casing that had too many pins.

Then why expose the substrate at all and then instruct designers to leave it floating?
What's the use for this?

I've also seen this question for a similar device, it could shed some light, but unfortunatelly it's unanswered.

Best Answer

The back of the silicon chip (the substrate) needs to be physically mounted (soldered) to some part of the lead frame as part of the assembly process, before the wire bonding is done and the epoxy is molded around it. Leaving it unconnected is not an option.

Sometimes the substrate connection is brought out to the ends of the package, and then sheared off flush as part of the final trim after molding. Note the vertical rails that support the central chip platform in this 16-pin DIP lead frame.

DIP lead frame

(source)

However, in an optocoupler, chip platforms are required for both the LED and the triac circuit, and having this kind of central rail would probably compromise the amount of isolation that's possible. So they bring it out in both cases on an ordinary pin. This works for the LED, because that's how LEDs are constructed anyway. And it works for the triac because they didn't need that pin for anything else. Here's one example:

optocoupler lead frame

(source)