Question about HP calculator traces.



#8

hello experts,

I was just asked, are the traces gold in HP. Specifically the 12C (see pictures) and subsequent models. I remember reading a brochure about the traces in the early 80's but cannot find and citations for the trace composition.

Geoff


#9

I don't know for sure but I'd say copper, plated with gold.
I don't think it's solid gold because I've seen total corrosion on some traces, and gold should be really resistant to chemical attacks;
on the other hand, if it wasn't gold at least externally, it wouldn't be so shiny after 20+ years (I've seen bare metal traces from old Russian calculators, they were dull and whitish, badly oxidised though still working).

Cristian

#10

Most PCB traces are made of copper covered with a coating of soldermask with very few were ever fully plated with gold. Those that were and not covered with soldermask are from the early history of etched PCBs. If what you're seeing (photos) is in fact gold it's only a light plating, otherwise it could just be very clean copper that hasn't oxidized.

The electronics industry realized it wasn't necessary to fully plate traces with gold for two very good reasons, (1) oxidation could be stopped with a light plating of nickel, and (2) solder-mask could be used to cover traces and in between pads to serve both purposes of reduced oxidation and eliminate bridging/shorting during the wave solder process.

Today the composition of a PCB trace (actually contact pads) that will need gold plating is usually:

1) Base copper, before scrubbing, of about 1/2 oz or 1.4 mils (thousandths of an inch) (12.7 um) (sorry ... my "u" is supposed to represent a micron or micrometer),

2) Plated copper of about 18 to 25 um,

3) Nickel of between 1 to 2.5 um, and finally

4) Gold of between 0.8 to 1.3 um.

Edited: 26 July 2011, 5:11 p.m.

#11

Hi, Geoff,

For much of that period (I was with HP's Instrument Group from 1979 through 1989) the standard HP circuit board process was what you show in the pictures. Specifically, the bare boards were coated with photoresist. The resist was removed where the traces were to be (*not* where they were to be removed as is usually the case). The boards were then gold plated. The resist was removed. Finally, the board was etched.

Note that the gold plating provided the etch resist function - i.e. the copper was etched where it was *not* gold plated. The resulting boards were very solderable (gold wets very easily with PbSn eutectic solder) and don't degrade over time. Connector pads are "free" too, as were key pads for snapdome switches which were popular in HP designs including top-of-the-line lab instruments.

As mentioned, solder resist, HASL (hot air solder levelled) tinning of the pads, screened solder resist, etc. have made this all obsolete. Today's high density surface mount packages (I've seen .25mm lead spacing) make newer techniques necessary. We won't be seeing many boards done this way any more.


#12

Jim,

many thanks for that description of the HP technique. Richard was looking for some answers and I am supplying the pictures.

Would I be correct in the assumption that a 1980 11C would have a similar trace pattern to the 12C, but of course, a different ROM set.

Cheers, Geoff

thanks for all the responses here, I was specifically looking at the HP technology.

Geoff


#13

The whole family (for that matter, all Corvallis machines) used that board process. So the 11C should be the same.

That said, my '11C (HP property - so I don't have it anymore) was, if I recall, a blob coated IC on the back of the display with no other ICs in the calculator. I believe I got it in the later '80s though, and don't believe they were out by 1980. So they may have changed the chip packaging from the earlier SMD parts to the bonded / 'blob' approach.


#14

Interesting. I've seen the innards of many Voyager-series calculators, and I haven't seen any of them with blobs prior to the 12C Platinum (introduced 2003). Maybe yours was a test of a cost-reduction that didn't get put into production?


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