Why are ICs/Semis black?

[GlennSprigg]

This may be 'slightly' off topic, (but people that know me... well... you know...)...
Back in the 60's early 70's, when I first started playing with 'electronics', we often had BLACK painted Glass-Enveloped Transistors,
which from memory were in the 'OCxx' series...  Years later, I had a 'Debate' online with an old guy that had actually BUILT them !!
Now I told him how we would scratch the black paint off, to make them Light-Sensitive Transistors!!!  Was true, but he didn't believe it. 

Also around that time (and earlier!), certain old valves/tubes had their glass body spray-painted black, and sometimes gold...  Although
for different reasons!  although SOME clear-glass tubes actually had an external light-source close to them, in order to 'excite' the coated
plates, that's beyond the discussion here!!...

Anyway, their main reason for 'coating' in the above paragraph, was to HIDE what the manufacturers had done technology wise, inside.
But NOT to stop giving away 'good' secrets, but out of embarrassment when technology had 'improved', and a cheap manufacturer was
not using the latest science in their Plates/Coatings & 'Getter' technology, that was obvious without the Paint!

[james_s]

Also around that time (and earlier!), certain old valves/tubes had their glass body spray-painted black, and sometimes gold...  Although
for different reasons!  although SOME clear-glass tubes actually had an external light-source close to them, in order to 'excite' the coated
plates, that's beyond the discussion here!!...

Anyway, their main reason for 'coating' in the above paragraph, was to HIDE what the manufacturers had done technology wise, inside.
But NOT to stop giving away 'good' secrets, but out of embarrassment when technology had 'improved', and a cheap manufacturer was
not using the latest science in their Plates/Coatings & 'Getter' technology, that was obvious without the Paint!

I thought it was a conductive coating for shielding purposes? I don't see much value in trying to hide what's inside, the average person would never know the difference and anyone who actually wanted to see what was inside could just break it open.

There were also some tubes that had metal envelopes, not a metal shell over a glass envelope but the metal was actually the envelope.

[Gyro]

This may be 'slightly' off topic, (but people that know me... well... you know...)...
Back in the 60's early 70's, when I first started playing with 'electronics', we often had BLACK painted Glass-Enveloped Transistors,
which from memory were in the 'OCxx' series...  Years later, I had a 'Debate' online with an old guy that had actually BUILT them !!
Now I told him how we would scratch the black paint off, to make them Light-Sensitive Transistors!!!  Was true, but he didn't believe it. 

He may not have believed it, but Mullard certainly did!  The original OC71 had clear protective grease around the junction. People (we) would scrape the paint off and use them as phototransistors. Mullard got wind of this and started using an opaque white grease in them. They then sold the OCP71 phototransistor (clear grease) for a very significant markup! Irrc, it had a little plastic sleeve around the bottom with the part number on.

I even heard of people mounting the white grease transistors in spin dryers to centrifuge the grease down to the bottom of the package. Others would carefully file a groove around the bottom of the package, crack the glass top off and dissolve the grease away (I tried that one). Electronics magazines of the time (Practical Electronics) were full of similar reader suggestions.

[Gyro]

Also around that time (and earlier!), certain old valves/tubes had their glass body spray-painted black, and sometimes gold...  Although
for different reasons!  although SOME clear-glass tubes actually had an external light-source close to them, in order to 'excite' the coated
plates, that's beyond the discussion here!!...

Anyway, their main reason for 'coating' in the above paragraph, was to HIDE what the manufacturers had done technology wise, inside.
But NOT to stop giving away 'good' secrets, but out of embarrassment when technology had 'improved', and a cheap manufacturer was
not using the latest science in their Plates/Coatings & 'Getter' technology, that was obvious without the Paint!

I thought it was a conductive coating for shielding purposes? I don't see much value in trying to hide what's inside, the average person would never know the difference and anyone who actually wanted to see what was inside could just break it open.

There were also some tubes that had metal envelopes, not a metal shell over a glass envelope but the metal was actually the envelope.

Yes, it was usually for screening - they had to leave a clear circle around the anode / grid top cap. You'd find it on RF tubes rather than LF output ones.

[David Hess]

Optos BTW are usually made with, something like a silicone blob lens, between the emitter and detector, I think also with an explicit barrier such as a thin sheet of glass.

Some optocouplers include an electrostatic shield between the elements to reduce coupling capacitance.  I wonder if it is a metal screen or tin oxide film.

I think I've seen some blue thyristors, though checking, I only have gray and black ones in my bin.

I have seen plastic packaged thyristors and very rarely power transistors in various shades of red.

Back in the 60's early 70's, when I first started playing with 'electronics', we often had BLACK painted Glass-Enveloped Transistors,
which from memory were in the 'OCxx' series...  Years later, I had a 'Debate' online with an old guy that had actually BUILT them !!
Now I told him how we would scratch the black paint off, to make them Light-Sensitive Transistors!!!  Was true, but he didn't believe it. 

An early design that I did had to use glass diodes for input protection which also made it photosensitive.  So we ended up painting the glass diodes after assembly.

Old glass diodes were sometimes painted black.

[retiredfeline]

He may not have believed it, but Mullard certainly did!  The original OC71 had clear protective grease around the junction. People (we) would scrape the paint off and use them as phototransistors. Mullard got wind of this and started using an opaque white grease in them. They then sold the OCP71 phototransistor (clear grease) for a very significant markup! Irrc, it had a little plastic sleeve around the bottom with the part number on.

I once made remote xenon flash synchronisers using scraped (and scrapped, probably due to leakage) OC71 transistors triggering a SCR. Worked great.

[GlennSprigg]

Also around that time (and earlier!), certain old valves/tubes had their glass body spray-painted black, and sometimes gold...  Although
for different reasons!  although SOME clear-glass tubes actually had an external light-source close to them, in order to 'excite' the coated
plates, that's beyond the discussion here!!...

Anyway, their main reason for 'coating' in the above paragraph, was to HIDE what the manufacturers had done technology wise, inside.
But NOT to stop giving away 'good' secrets, but out of embarrassment when technology had 'improved', and a cheap manufacturer was
not using the latest science in their Plates/Coatings & 'Getter' technology, that was obvious without the Paint!

I thought it was a conductive coating for shielding purposes? I don't see much value in trying to hide what's inside, the average person would never know the difference and anyone who actually wanted to see what was inside could just break it open.

There were also some tubes that had metal envelopes, not a metal shell over a glass envelope but the metal was actually the envelope.

Hi James. Sorry about the delay!  Yes, there were various reasons for outer coatings... And there were (a lot!) with metal casings, (as you say, not
the clipped-on outer-shell for 'shielding'), and those were 'mainly' used in Military grade equipment, and mainly due to their physical robustness !   

However, some coatings were to 'mask' what's going on inside at the time!  I was 1st commenting just from memory, but I've found this now, that
may give further evidence... Below is one example from 'Wikipedia' talking about the 'Mullard/Philips' partnership...  https://en.wikipedia.org/wiki/Mullard
Not far from the top, on the right-hand side, you will find this photo, and underneath it a 'Description'...


A Mullard TDD4 valve. The gold spray coating served no purpose other than to hide the blackened interior,
as Mullard valves were still manufactured using the azide process, long abandoned by other makers.

[james_s]

But is that to hide it from prying eyes, or just dress it up and make it look fancy? They mention the blackened interior which makes me suspect they're referring to a getter flash which was black. It may be they wanted to avoid people thinking the tube was burned out or something due to a darkening resembling a well used light bulb.

[Canis Dirus Leidy]

More on the question of package colors. The old 237-series ICs with a "Kulon" case. Mostly dirty-white, but there are also colored plastic.