Was this a thing?  Looping diodes on vintage card assembly - Page 1
Forum section: electronics / general

author: snuci
author_rank: Newbie
timestamp: July 13, 2018, 11:15:08 pm
content: I recently bought a loose MITS Altair 8800 CPU board plus a couple of other related boards. These boards, when bought in a kit, were assembled by the purchaser. These boards would have been assembled in 1975 and I have never seen this type of assembly. Was this done at the time or a flourish? I've never seen this before and I have lots of vintage computer equipment of the era. Is there any advantage or disadvantage of doing this? I am guessing it's not necessarily a good thing. This was done with diodes on two boards.

author: tsman
author_rank: Frequent Contributor
timestamp: July 13, 2018, 11:18:19 pm
content: Only reason I can think of is that they didn't want to risk cracking the glass body *shrug*

author: David Hess
author_rank: Super Contributor
timestamp: July 14, 2018, 12:52:27 am
content: No, it was not common. I do not remember ever seeing that except on prototypes.Preventing cracking of the glass package is what occurs to me also; I had that happen before I made it a habit of using a lead bender.I have occasionally done the same thing though to make test points.

author: mariush
author_rank: Super Contributor
timestamp: July 14, 2018, 12:59:56 am
content: I suppose it could be also to make leads longer and let them act as a heatsink.Also thought maybe they used some plastic or wood something to keep diodes in place by having some stick or needle through the loops to keep diodes at certain height, while circuit board goes through wave soldering machine.... but there are lead forming machine which can form leads to keep diodes at certain height.Another thought would be those tiny loops will act as a couple of very tiny inductors reducing component count.

author: Stray Electron
author_rank: Super Contributor
timestamp: July 14, 2018, 01:05:28 am
content: Making the leads shorter not longer would help the diode cooling since the PCB would act as a better heat sink than the excessively long leads. I built a number of kits from MITS back in the day and also owned two Altairs, one factory assembled, but I've never seen the leads looped like that. Did they put loops in any other leads than the diodes?

author: snuci
author_rank: Newbie
timestamp: July 14, 2018, 01:12:52 am
content: Quote from: Stray Electron on July 14, 2018, 01:05:28 amI don't have enough light to take pics but in a word, "no". Just diodes. This was done the the Alir 8800 CPU board and the Altair RAM board. There is a third Tarbell Cassette Interface board but it doesn't have any diodes so no loops.

author: David Hess
author_rank: Super Contributor
timestamp: July 14, 2018, 02:39:52 am
content: Quote from: mariush on July 14, 2018, 12:59:56 amThe leads need to be as short as possible to lower the thermal resistance to the printed circuit board.QuoteOn a slow 1 watt zener diode? Not a chance.

author: @rt
author_rank: Super Contributor
timestamp: July 14, 2018, 03:03:19 am
content: Being scared to break the diodes was on my mind as well.It still crosses my mind with 1N4148.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 14, 2018, 03:41:14 am
content: Quote from: David Hess on July 14, 2018, 02:39:52 amI've seen it done on 5 & 10 watt resistors on PCBs .These devices, if mounted close to the board will "cook" it, as they are designed to dissipate heat in free air.The PCB looks to have a greater surface area, but the heat is closer to the solder joints which will degrade over time.Eventually a point of "runaway" is reached, ultimately causing delamination of the board.This isn't just theory, I have had to repair many such "cooked" PCBs over the years.The loops radiate heat into free air, & do help, but the biggest help is to get the big hot devices clear of the board.Used on diodes, it was probably due to concern about cracking the bond between the lead & the glass envelope.

author: TERRA Operative
author_rank: Super Contributor
timestamp: July 14, 2018, 04:38:28 am
content: I remember being shown this technique in trade school.We were told it was used for strain relief, so if the board flexed due to vibration or g-loading, the leads wouldn't break so easily (Instead of the bending being confined to a 90deg bend, it is spread over the 360deg loop) or pull on the component and cause damage.

author: Mr. Scram
author_rank: Super Contributor
timestamp: July 14, 2018, 04:44:48 am
content: It reminds me a lot of the old looped brass petrol tubes on ancient motorbikes. Those were intended to mitigate vibration damage too.

author: David Hess
author_rank: Super Contributor
timestamp: July 14, 2018, 02:17:23 pm
content: Quote from: vk6zgo on July 14, 2018, 03:41:14 amI have run across that many times with power resistors but diodes should not be operating hot enough to scorch a printed circuit board.Unlike resistors which can operate reliability at higher temperatures, axial power diodes depend on conduction through their thick copper leads into the printed circuit board for power dissipation. It almost never matter but for where it does, there are application notes discussing junction-to-ambient thermal resistance of diodes based on lead length and copper pad size.

author: coppice
author_rank: Super Contributor
timestamp: July 14, 2018, 02:19:19 pm
content: I've seen boards like that where all the signals you are likely to probe with a scope have been looped to make it easy to attach a probe clip.

author: snuci
author_rank: Newbie
timestamp: July 14, 2018, 08:24:43 pm
content: Here's a part of the second board. A MITS Altair memory board. Same thing. Diodes only. Three are shown with a red arrow.

author: rdl
author_rank: Super Contributor
timestamp: July 15, 2018, 03:47:45 am
content: I vote for heat dissipation.Did those older boards even have the massive ground planes that more modern devices have? As I recall, mounting parts such as diodes and resistors away from the board to improve air flow and heat radiating ability was common practice in those times and a reason for those parts to have heavier leads in the first place.Style and convenient attachment points for probes would be a side benefit, but I doubt that was a primary purpose.

author: duak
author_rank: Super Contributor
timestamp: July 22, 2018, 02:42:22 am
content: I built and debugged an Altair 8800 in 1976 and remember the various boards but not having to loop the leads like that. I do something like that when prototyping circuits so I can probe, trim or snip more easily but I've never seen any lead kinks on production equipment except for power components.BTW, I don't know if the CPU board will work without a front panel board as a few signals are sourced there. Certainly the front panel is needed to write to memory unless you can find a PROM board to get some sort of monitor running. It took about four months to find a defective chip that prevented the CPU from reading the front panel switches. The assembly instructions were OK, but the test instructions and circuit description were, um, lacking. I learned a lot though.Many people had trouble with the original 4K DRAM board. The layout wasn't so good (Ground plane? We don't need no stinking ground plane!) Event timing was determined by one-shots (monostable multivibrators) and wasn't stable. MITS came out with a synchronous version of the board later on so they must have been aware of problems.Cheers,

author: johnwa
author_rank: Frequent Contributor
timestamp: July 23, 2018, 11:40:01 am
content: I see people have mentioned heat dissipation, and mechanical stress relief, but it is my understanding that these loops are basically for a combination of these two effects: reduction of stress from thermal expansion.Imagine if the circuit is switched on, the diode starts dissipating power, and warms up. Due to thermal expansion, the diode and the horizontal portions of the leads will try to elongate. However, they cannot, as the ends are anchored to the PCB. The PCB will be at a lower temperature, so the pads will still be the original distance apart. The difference in length must be taken up by bending of the diode leads, the solder joints, and/or the PCB. Repeated temperature cycling caused by powerup/powerdown cycles may eventually result in a fatigue failure.With the loops in place, any elongation of the diode simply results in the loops getting slightly larger, and the resulting mechanical stresses are considerably reduced.That is the theory, anyway; I don't know of any quantitative data on the generated stresses. The thermal expansion coefficient of FR4 is likely significantly different to that of the diode, so it may be that, depending on temperature gradients, the PCB actualy grows more than the diode. The loop should work in either case though.

author: langwadt
author_rank: Super Contributor
timestamp: July 23, 2018, 12:25:16 pm
content: Quote from: Stray Electron on July 14, 2018, 01:05:28 amon a modern multilayer pcb with solid planes sure, on a single/double layer pcb with just traces not so much

author: capt bullshot
author_rank: Super Contributor
timestamp: July 23, 2018, 01:11:07 pm
content: I've seen such kind of diode assembly in a Datron 1061 DMM. It is used on the reference diodes to reduce mechanical stress on them. This was for sure done to keep them from drifting due to mechanical / thermal stress, but I don't see a reason to do so on computer boards or vanilla zener diodes.

author: emece67
author_rank: Frequent Contributor
timestamp: July 23, 2018, 01:36:38 pm
content: .

author: joseph nicholas
author_rank: Frequent Contributor
timestamp: July 23, 2018, 01:50:56 pm
content: This was done to keep the electrons happy. Kind of like a Six Flags amusment park ride.

author: ruffy91
author_rank: Regular Contributor
timestamp: July 23, 2018, 02:21:16 pm
content: It's used for the same reason that 90° corner traces in high speed PCBs are forbidden.The electrons would fly off the sharp corner of the bend!

author: tooki
author_rank: Super Contributor
timestamp: July 23, 2018, 03:45:34 pm
content: Quote from: ruffy91 on July 23, 2018, 02:21:16 pmAnd here in Switzerland flying off the bend could mean flying off the side of a mountain!!!

author: SpanishVikingNorCalEE
author_rank: Newbie
timestamp: August 01, 2018, 12:20:27 am
content: This thing you ask about is a method mostly used in flight HW and some other high-reliability systems assembly back in the late 70's through the advent of SMT devices. SMT is so much more reliable than through-hole that all the flight or life critical kind of systems went to that as soon as they could, so this kind of thing died out much more quickly than did the "rest" of the through-hole PCB assembly market.It was indeed (as pointed about above by someone who said they saw it in tech-school) for strain-relief. Flexing of the board, thermal expansion, etc. was prone to cracking of the leads for resistors (esp. power resistors), diodes, and other axial lead through-hole components.You see this a lot in 70's-80's era test equipment (esp. RF test equipment) from quality manufactures like HP. It came about during space-program testing, and from field repair learnings from early jet fighters, etc. It was a mil. spec. when I first learned of it. I used to have a copy of that mil. spec. (on actual slaughtered tree media, not electronic). :-)Gee. Does that date me? Ha ha.Well if not, the fact that the first computer I used (and coincidentally the first one I owned) was a Kaypro-II made by Non-Linear Systems. Precursor to the modern lap-top. It was "portable" in that the keyboard snapped onto the machine's front-panel and formed the base when in transit (there was a handle on the back) and that covered the floppy drives and the 9" green phosphor mono-chrome monitor to protect those and the keyboard while lugging the 25 lb. boat-anchor around with you. :-) (It had all the RAM one could address in a CP/M OS machine running on the 8-bit Zilog Z-80 CPU... 64KB That's right KB, two digits. :-) And folks thought 640K was all the RAM you'd ever need. That's because only a few years before, all they could get was 64K at a time. There were machines that had 128K, but they implemented a clunky form of paging and could only address the upper or lower half at a given time.https://en.wikipedia.org/wiki/KayproEnjoy

author: coppice
author_rank: Super Contributor
timestamp: August 01, 2018, 12:47:39 am
content: Quote from: SpanishVikingNorCalEE on August 01, 2018, 12:20:27 amEarly SMT device had a terrible record for reliability, and most people needing high reliability stayed away for several years. The exception was the military, who sponsored most of the early development of SMT. They lived with the reliability issues, worked on them, and things matured. The key issue was differential thermal expansion rates, for which there were some bizarre short term solutions, like multi-layer ceramic PCBs. These were made from the same ceramic as high reliability IC packages, which avoided differential expansion issues at the cost of a ludicrously brittle board. Fun days.

author: SpanishVikingNorCalEE
author_rank: Newbie
timestamp: August 01, 2018, 04:56:04 am
content: You point out an importatnt detail I left out...Early SMT mfg. lines tended to be extremely unreliable. My take on the whole thing was that if the line engineer for a given line understood what it really took to properly flow paste, and how smooth the pic-n-place line, as well as any conveyance between there and the re-flow oven running smoothly enough that it didn't bounce the heavier IC's out of their less than perfect adhesion to the land provided by the paste, then the solder would flow and whet properly to the lands and leads, and it would be reliable as the PCB itself generally. But wave solder and stuffed through-hole components were tolerant of a large number of PCB assembly sins that SMT isn't.If the boards get fingerprints all over them going into the line, and the ovens aren't clean enough, or the line bounces components off their pads (even out of registration too far), or the oven's temp profile isn't right, then the solder quality suffers in ways that just didn't matter even on large scales for TH assembly lines (remember the 100% hand stuffed, often in some house-wife's kitchen a few hours each day, then going back to the assembly shop for wave-solder in their hatchback the next day. they generally worked, or at least the solder generally worked.).Ah the good 'ol days. SMT was thought to be really hard in the beginning, but mostly that seemed to be the opinion of experienced TH assembly folks. Only some of them could see past their years of status-quo methods to understand that things like finger-prints, dust, hair, oil, smoke, or pretty much ANY other contaminate on the parts, the PCB, or in the oven's internal environment really mattered. They matter at any large scale weather or not anyone in QC noticed anything wrong or not.That's how the military got away with their early ramp of reliable SMT assembly technology in the high reliability segment. They understood the science behind what they were doing, and had the discipline to follow that, and make sure their suppliers did as well.I'm feeling a little nostalgic now for those 'good 'ol' days when assembly was easy, but computing was hard. :-)

author: coppice
author_rank: Super Contributor
timestamp: August 01, 2018, 11:09:14 am
content: Quote from: SpanishVikingNorCalEE on August 01, 2018, 04:56:04 amEarly SMT was all vapour phase. Reflow came later.

author: Daixiwen
author_rank: Frequent Contributor
timestamp: August 02, 2018, 09:23:19 am
content: I remember some electronic kits from the 80's where they said you needed to do that to all the diodes to prevent destroying them while soldering, so I agree with emece67.I'm not sure if diodes were more fragile at that time or if they were overthinking it. I've never destroyed a diode with a soldering iron.

author: rrinker
author_rank: Super Contributor
timestamp: August 02, 2018, 05:41:23 pm
content: Heck, I've soldered some glass diodes with the leads cut off rather short (model railroaders are famously cheap buggers - so what if I can buy 1000 new ones for $1, I'll use the old ones I cutout of a previous layout!) and had no problems. I'd probably buy new ones in most cases, but the guy I was helping had a bin full of cut off ones removed from old layouts so we used them up. For soldering purposes, I'm not so sure - but for long term thermal change and vibration - I'd buy that. Though they had one of those diodes on the old Radio Shack kits - the first one I had had each component on a plastic block, with L shaped metal pieces you linked under the spring terminals to complete the circuits and plastic connectors to physically attach each block and that thing certainly got tossed around a bit and none of the semiconductors ever failed - and that kit would have been about the same vintage as an Altair micro. The later ones I had of course were in those wooden cases so individual components didn't get tossed about. I uncovered a part bag of the 50 pack of glass diodes Radio Shack used to sell that was carelessly tossed in a box full of random junk and while I haven't checked them all, I used a couple and they were fine. Stored, they were subject to fairly extreme temperatures (not sure of the actual manufacturer so no idea if outside the data sheet specs) plus they had stuff piled on top, and the whole box was rather tossed about since in general there was nothing fragile obviously in it.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: August 02, 2018, 05:44:16 pm
content: I've seen it occasionally. Reduces stress, both dynamic when inserting and static/thermal cycling etc. also provides useful test points

author: David Hess
author_rank: Super Contributor
timestamp: August 02, 2018, 07:36:34 pm
content: The DO-41 package in glass is not very common anymore; most are now plastic. I remember we used to get 1N4004s in the glass DO-41 package.

author: duak
author_rank: Super Contributor
timestamp: August 04, 2018, 04:13:54 am
content: In the mid-70's I built myself a digital clock that used a bunch of loose LEDs for the display. Until then about the only semiconductors I fried while soldering were germanium signal diodes so I wasn't super worried about heat sinking the LED leads. Sure as shootin', about half of the LEDs were dim or dead. When I assembled the Altair front panel, with its forest of LEDs, I made damn sure to use a heat sink while soldering.

author: David Hess
author_rank: Super Contributor
timestamp: August 04, 2018, 01:50:27 pm
content: I have had the same problem with LEDs and I suspect the problem is not the heat itself but the packaging and strain if the leads are bent during soldering.

author: Nerull
author_rank: Frequent Contributor
timestamp: October 08, 2018, 07:12:24 am
content: From the IPC-A-610 standard on lead forming: