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HP Forum Archive 08

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good as GOLD circuits and classic HP's
Message #1 Posted by Michael Meyer on 13 Aug 2002, 8:55 p.m.

Hi. I was surprised at the dip in HP-45 prices and unexpectedly won an auction for a second one. Upon opening to repair a cracked switch, I observed what I'd only seen in HP-35s: really beautiful gold circuits. Even my office staff, who don't understand my hobby, pretended to be impressed.

My first HP-45 was built in the 50th week of 1973, though the circuit board is dated 1976. Either a swap or a repair, I figure. The new one was built in the 33rd week of 1973, and the circuits are all gold. Really pretty gold.

I've not seen it mentioned elsewhere in the forum or museum or in books, but I found it interesting. Was this when HP ceased using gold-plated circuits and went to copper-dipped-in-solder circuits? To me, it was a very important change in design. I once received, for free, a non-working HP-35. Upon sanding off the corrosion, which was extensive, there was enough gold circuitry remaining that the calculator easily came back to life. Amazing.

Anyone know anything more about the history of the use of gold plating or circuits in the early HP calcs?


PCB, gold plating facts
Message #2 Posted by Ellis Easley on 14 Aug 2002, 4:35 a.m.,
in response to message #1 by Michael Meyer

In the mid-1980's I had occasion to have some printed circuit boards made and I learned that the options available for gold plating on card edge connector fingers were 30 microinches, 15 microinches or "gold flash" which I think must be a contact plating, rather than electroplating. Also, gold isn't plated directly on copper, there is a layer of nickel plating in between.

One other thing - tin plating on connector surfaces (such as lower cost card edge connector fingers) is much harder than gold, and since tin develops an oxide coating like most metals (other than gold), to make a reliable connection it is necessary for the spring force pushing the two contact surfaces together to be quite high, so the oxide coating is scraped off during mating and a "gas tight", metal to metal connection can be made. Therefore, one of the worst things you can do with connectors is to mate a tin plated contact with a gold plated one - with the high spring force and greater hardness of the tin plated contact, the gold is scraped off the gold plated contact. Mating tin plated contacts together makes reliable connections, only the allowable number of mating cycles is much less than with gold plated contacts.

All PCB traces are copper, with or without other plating. The original way to make the traces was to start with a board covered with the full thickness of copper and print the desired pattern, in a positive image, with a coating resistant to the chemicals used to etch away the unwanted copper. The wider the traces, the longer you could leave the board in the etchant to make sure any stubborn bits of copper were dissolved, since there was some etching of the edges of the traces (between the resist and the substrate). As traces became narrower, the board had to be removed from the etchant as soon as possible, so copper shorts between traces became more of a problem. One solution was the "additive" process: starting with a board covered with a very thin layer of copper, the pattern was printed in a negative image and then additional copper was plated onto the board, making the exposed trace patterns thicker. Then the resist was removed and the board was etched until the original, unplated, thin copper between the traces was dissolved.

Many PCBs have their traces covered with solder to make it easier to solder components to their pads, but this step can cause shorts between traces. Also, solder shorts can be caused when the components are soldered to the board. "Solder mask" is the (usually green) paint which is applied to the majority of the surface of the board (except for the component pads) to prevent solder shorts between traces when the components are solderd in place, but it can lead to another problem: a board with solder plated on all the traces and then covered with solder mask can end up with solder shorts UNDER the solder mask, where they are harder to deal with. One solution to this problem is "SMOBC" - solder mask over bare copper. Sometimes with this kind of board the bare copper traces look black under the green solder mask (I imagine this is because the copper reflects mostly red light).

Some lighter colored solder masks with shiny copper or solder plated traces underneath make it look like the traces are gold plated when they aren't.

Re: PCB, gold plating facts
Message #3 Posted by Michael Meyer on 14 Aug 2002, 3:01 p.m.,
in response to message #2 by Ellis Easley

Thanks, Ellis.

So, does that mean that these early circuits aren't gold plated?


Re: PCB, gold plating facts
Message #4 Posted by Ellis Easley on 14 Aug 2002, 4:45 p.m.,
in response to message #3 by Michael Meyer

No, HP did heavily gold plate the traces of PCBs in a lot of equipment. Much of their old test equipment which I have is built this way. Lots of old electronic equipment has been thrown into acid baths (presumably the famous "aqua regia" - a mixture of nitric and hydrochloric acids, I think - which is able to dissolve gold) to recover the gold! When there is no solder mask, gold plating is unmistakable, and a lot of HP's pre-IC PCBs were hand soldered. Solder mask is primarily used to allow "wave soldering" with close traces, which would otherwise be hopelessly shorted together - back then, hand soldering was considered the most reliable method since every joint was inspected by human eyes in the process of soldering it. I don't know why HP went to the expense of gold plating all traces, but three possible reasons are (1)they wanted the protection from adverse environments that gold plating provides - they used to run a print ad featuring a piece of test equipment that was recovered after spending some years in a lake, which worked within calibration specs after just drying it out! and (2)they determined that the extra gold more than paid for itself by contributing to the high price people would pay for HP equipment! and (3)I believe the price of gold used to be kept artificially low in the US for monetary reasons - wasn't the price $32 per ounce for many years and weren't Americans not allowed to own bullion?

Since gold became very expensive, the trend in all interconnect systems (including PCBs) is "selective gold" which means using the minimum amount of gold plating, only where contact between parts takes place. An excellent example is connectors for ribbon cable (as distinct from "ribbon connectors" which is the official term for what are often called "Centronics" type connectors, so named because the actual mating contact is cut from a ribbon of metal). Connectors for ribbon cables actually have two connection points per pin: the point where the connector mates with another connector (for example a .025" square pin or a PCB finger or a "D" connector pin) AND the point where the connector pierces the insulation of the individual ribbon cable wires ("insulation displacement") and provides a narrow channel into which the stranded conductor is forced, forming a "gastight" electrical connection. In the early days, the entire piece of metal forming an individual contact of such a connector would be plated with gold. Now, only the portion that actually touches the mating connector surface is gold plated - the mating surface is often a curved section of spring metal and only a small area on either side of the actual point of contact will be plated. The rest of the contact - including the insulation displacement end - will be tin plated. This is actually an improvement, since the wires in the ribbon cable are also tin plated.

On my HP25's, the logic PBC's have solder plated traces (wave soldered, with solder mask only on the solder side) and the gold plated contacts soldered in place that connect to the keyboard and display. The keyboard PCB's are completely gold plated although the only critical points that benefit from gold plating are the key switch contacts, the slide switch contacts, and the holes ("vias") that mate with the pins in the logic PCB. (On the display PCB, HP appears to be mating the gold plated pins with silver plated vias.) My HP97 is basically built the same way. I've seen pictures here of HP97 logic boards that have solder mask on the component side and appear to have gold plated traces. While I can't say for sure from a picture, this looks to me like the kind of light colored solder mask that makes shiny traces look like they are gold plated.

Re: PCB, gold plating facts
Message #5 Posted by Meyer Michael on 15 Aug 2002, 12:38 a.m.,
in response to message #4 by Ellis Easley

Thanks. It's been years, but I used to make my own circuit boards. I'd always admired the HP-25's gold connectors. One note I've discovered about these circuits: We all admired the HP circuits which were covered with shiny solder compared to the "cheap" TI copper traces. Nearly 30 years later, many of the HP circuits are hopelessly corroded from outgassing, where many of the plain copper boards held up better!!

So, my original observation stands: my HP-45 is one of the prettiest darn circuity I've ever seen. Gotta cover your eyes due to the glare of gold. <grin>


Re: PCB, gold plating facts
Message #6 Posted by Ellis Easley on 18 Aug 2002, 10:19 p.m.,
in response to message #5 by Meyer Michael

I opened up my HP45 and found that the logic board is completely gold plated and has solder mask on the solder side. It makes sense that they would wave solder a board with so many component pins. I guess HP's rule was that if any gold plating was needed on a PCB, they plated all the traces. This would save steps, since if they had only plated the vias that mate with the pins from the keyboard (which IMHO is the only place they needed gold plating), it would have been necessary to mask off the areas not to be plated, then remove the mask afterward.

When only the PCB card edge connectors are to be gold plated, as is frequently the case with PC option boards, the PCB fabricator builds the board with additional traces (outside the final outline of the PCB) connecting the card edge connector fingers together, to use as a common terminal for electroplating. Then after the plating is done, the extra material is trimmed off. Also, some tiny PCBs like handleld games, the little music boxes that come in greetings cards, etc. are made with the different networks shorted together with additional traces that are then drilled out. I assume this is done to facilitate plating the traces. I wonder what method HP used to make contact with all the traces on PCBs in order to gold plate them? One possibility that occurs to me is a variation on the "additive process", since the whole board is covered with thin copper and the desired traces are built-up by electroplating copper, additional electroplating could apply the nickel and gold layers before the resist is removed and the thin copper is etched away.

Re: good as GOLD circuits and classic HP's
Message #7 Posted by Tom (UK) on 15 Aug 2002, 3:36 a.m.,
in response to message #1 by Michael Meyer

I have an early HP41C (made in 1979), the PCB can be seen when the battery tray is removed. The tracks on the PCB are a very shiny gold colour, there does not seem to be any coating on the PCB - are these tracks gold plated? If so when did HP stop plating the tracks with expensive gold?

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