Plastic or ceramic ICs?
 

I've recently been repairing an item of equipment I built about thirty years ago. This makes use of several HEF-series ICs and I thought it might be a good idea to obtain some spares in case they become more difficult to obtain with time.

The data sheets indicate that these were available in ceramic DIL packages as well as plastic although there doesn't seem to be any difference in electrical or thermal specifications. Nowadays you can buy mil-spec ceramic ones for a pound or two if you so desire. Does anyone know whether there's any particular advantage in specifying the (originally presumably much more expensive) ceramic version? I could understand it in the case of something like a microprocessor, in which better heat dissipation might be an issue, but a 4049 hex buffer is hardly likely to get into thermal distress!.

Re: Plastic or ceramic ICs?
 

Mainly reliability - ceramic for e.g. military/aerospace or other applications requiring high reliability. Also ceramic has higher thermal conductivity so higher power / faster clock rates.

For 7400/4000 series, plastic is plenty good enough for consumer applications.

Re: Plastic or ceramic ICs?
 

No, plastic was very bad for 4000 CMOS logic.

Traces of moisture entered where the plastic was not hermetic with the legs. It dissolved phosphorus traces out of the glass passivation, and the phosphoric acid rotted the aluminium metallisation layers.

Look up 'CMOS purple plague' It's insidious in that it progresses even in stored parts.

DAvid

Re: Plastic or ceramic ICs?
 

When I was using them in the early 1970's, as well as a wider temperature range, the military TTL devices were specced for a wider supply voltage tolerance. From memory, standard 74xx series was 5V +- 0.25V, 54xx series 5V +- 0.5V.

Re: Plastic or ceramic ICs?
 

Hi David,

My memory is not as good as it was but I recall that 'purple plague' was an issue caused by the formation of a weak Au / Al alloy in the wire bonding process (bond wires were originally gold, the pad metal aluminium), not by moisture. When the aluminium bondwire process was mastered the problem went away.

But yes plastic was generally less reliable and hence ceramic preferred for mil spec parts.

Keith

Re: Plastic or ceramic ICs?
 

For the princely sum of £1.50 apiece I've just acquired a couple of Motorola 4049s which are allegedly mil-spec. The complete designation is "14049A/BEAJC 883 CGG QY 8604U"

I've seen designations such as BEAJC and BCAJC on ICs previously but never understood them. Motorola data books suggest that the "B" is the level of MIL-883 to which the device is qualified and that "EA" is something to do with the packaging but quite what a 4049A is eludes me. I assume it's unbuffered and that 8604 is the date code but can anyone shed further light?

No amount of Googling seems to help, naturally.

Re: Plastic or ceramic ICs?
 

I've only got reference manuals for the CMOS 4000B series made by Fairchild (some of which were designed in the Bristol office where I worked in the late 70's) and the RCA 4000A/B series.

4049A - the A series were the originals, the B series came later and it appears had a higher voltage rating, plus came in B (buffered) or UB (unbuffered). The idea of the buffered one was they had an output buffer which meant the output high/low resistance was independent of logic state of the inputs; the penalty was they were slower.

The books say the MIL spec parts had suffixes of D, K, F, H and E. From what I can make out D were welded-seal packages, E were plastic, F were ceramic frit-seal, K was ceramic flat-pack. H was bare chip.

The 4049A by the way was a hex inverter. If you need I can copy the datasheet for you.

Re: Plastic or ceramic ICs?
 

Yes, ceramic is generally better than plastic although ironically the ceramic material is more radioactive than plastic so putting the chip more at risk to random failure due to ionising radiation.

Re: Plastic or ceramic ICs?
 

>4049A - the A series were the originals, the B series came later and it appears had a higher voltage rating, plus came in B (buffered) or UB (unbuffered). The idea of the buffered one was they had an output buffer which meant the output high/low resistance was independent of logic state of the inputs; the penalty was they were slower.<

Ah, that's interesting. I noticed that in my particular application (a control unit for a VHF linear amplifier) the 14049A is noticeably slower to switch as the HV sample comes up than an HEF4049B and wondered whether it was a buffer issue.

>The 4049A by the way was a hex inverter. If you need I can copy the datasheet for you.<

That's very kind but I do have an original Philips LOCMOS data book and a few other useful items. It was amusing to see from a contemporary STC components catalogue that ordinary plastic HEF4049s were £4.60 each in 1985 whereas posh mil-spec ceramic ones nowadays cost about a quarter of that!

Re: Plastic or ceramic ICs?
 

On another tack, does anyone know what "BSS2" appended to the part number (e.g. "4050E/BSS2" on a NatSemi example) might imply?

Re: Plastic or ceramic ICs?
 

Just checked a data sheet for the basic SN5400/ 7400 as published in a TI book from 1973. The only differences in the electrical parameters are the supply voltage tolerance previously mentioned and the slighly higher short circuit output current of the 5400 at its max supply voltage, no doubt accounted for by the fact that the 5400 has a slightly higher max supply voltage than the 7400.

Re: Plastic or ceramic ICs?
 

That's interesting since no such differences seem to exist in between the plastic (B) and ceramic (BD) versions of the HEF4000 series. The electrical and temperature ratings are identical.

That said, there was allegedly a slightly different LOCMOS family (the HEC series) which also had ceramic packaging and were specified as having a slightly wider temperature range. However I've never seen any of these in the wild and an erstwhile colleague suspected that none were ever actually made.

Re: Plastic or ceramic ICs?
 

The datasheet also has the military flatpack version, same electrical spec as the 5400 version. The custom ICs I was designing at that time were mounted in that type of military flat pack, which I believe pre-dated the DIL package and were possibly the first type of SMD. AFAIR the empty packages (ceramic frame and gold plated base and cover) cost 50p each in 1973 money, about the same as the silicon chip that was mounted in them, subsequently increasedy by the savage inflation of the time. One of our team had a resin encapsulation kit and we used it to turn some of the failures into ear-rings and pendants!

My first encounter with TTL was in 1967 as a student at Plessey, when I was given the task of hard wiring a dozen or so ICs with point to point wiring that provided time multiplexing for a 28V avionics warning indicator: guess who got the 5V and 28V lines transposed! All the ceramic 54xx TTL (SGS-Fairchild and TI) survived the more than 500% overvoltage, all the plastic 74xx ones (mostly Ferranti) died. Fortunately sockets were used so replacement was simple.