One of those useful things to have around on your bench is a decade resistance box, essentially a dial-a-resistance instrument. They used to be quite expensive in line with the cost of close-tolerance resistors, but the prices have come down and it’s within reach to build your own. Electronic design consultancy Dekimo have a nice design for one made from a series of PCBs which they normally give out at trade fairs, but now they’ve released the files for download.
It’s released as Gerbers and BOM with a pick-and-place file only, and there’s no licence so it’s free-as-in-beer [Editor’s note: the license has been updated to CC-BY-SA], but that should be enough if you fancy a go. Our Gerber viewer is playing up so we’re not entirely sure how reliable using PCBs as wafer switches will be long-term, but since the pictures are all ENIG boards we’d guess the gold plating will be much better than the HASL on all those cheap multimeters.
We like this as a conference giveaway, being used to badges it’s refreshing to see a passive take on a PCB artwork. Meanwhile this isn’t the first resistance box we’ve seen with unconventional switches.
No license means no license.
The page does link to CC-BY-SA-4.0, but I agree it would be clearer if it was marked on silkscreen or in a text file in the zip.
there’s one in the zip file now
Licenses don’t matter, do as you please.
Right – a surprising error in the post assuming
“No license” = “free as in beer”
while there is a CC-BY-SA mentioned on the website.
The ENIG coating is very thin and soft gold. It will not last long when used as switch contacts. Once it wears away, the copper will oxidize and the switches will make bad connections. The contacts must have a hard gold plating to be reliable.
I don’t know much about the finer details of PCB fabrication. Is there a coating that would work better for this?
As Cody said you’re better off with hard gold plating (sometimes the pcb manufacturer will refer to it as card edge connector plating or something similar). If unsure talk with your manufacturer to see what they suggest.
Ah, right. I didn’t realise that hard gold plating was the name of the coating. I’ll check for that if I ever do something like this, thanks.
Here’s a little background on ENIG (Electroless Nickel / Immersion Gold) and hard gold plating.
The purpose of ENIG is to provide a nickel barrier layer over top of the copper, so the intermetallics are formed with the nickel instead, producing a stronger joint. The gold is there to protect the nickel until the solder joint is formed.
As Cody mentioned above, the gold layer is very thin – only a few atoms thick at most. It exists only to serve as an oxygen barrier, to keep the nickel from building up a passivation layer through atmospheric oxidation (which would impair its solderability). The gold dissolves into the solder immediately on contact, and is such a minuscule amount to have a negligible impact on the alloy of the solder joint.
On the other hand, hard gold is electroplated onto the surface to form a much thicker and more durable layer. This is usually a selective process because economical quantities of gold are deposited. All of the surfaces to be plated must be electrically joined together during the plating process.
For card edge gold fingers there is usually a runner attached to the mating edge, having traces joining all of the contacts together. After plating, this runner is cut off and the edge chamfered to complete the card edge connector. Sometimes you can see remnants of the connecting trace at the tip of each of the contact fingers.
Hard gold plating is also used when the PCB itself is a functional element of a switch. This can be found with conductive carbon pill silicone buttons, tactile dome overlays, and rotary (wafer) switch rotors. Hard gold plating is also used with chip-on-board (COB) attachment (the infamous black epoxy blob, but also for LED dice), particularly for the wire bonding process. In this case you may see junctions between adjacent traces (2-3 at a time) scattered across the PCB that have been drilled or punched out. These were added to the design to facilitate the plating process, just like the runner does for card edge contacts.
Thanks a lot for this summary/explanation!
ENIG lasts a looooot longer than you might think.
Nice idea for a freeby, and probably got more publicity being fearured on HaD than they ever got giving them away physically!
What are they using for wipers? Some kind of spring contact is needed. It seems fake lol.
A harwin surface mount contact. In the BOM. Stock at mouser and digikey.
Where is the spring wiper to make contact with the pads? Previous comment was deleted sigh.
If these were on amazon, I would buy a few of them. At the moment I have some with jumpers and they work, but this is nicer.
I always think ‘a decade box would be a ‘handy thing to have’ but then immediately after think: Uhm, potentiometers exists and are easier surely.
Basically what you need is a potentiometer and a scale around its knob to know what you set it at.
Or maybe a digital pot and a rotary encoder and some SMD LED to fancy it all up?
I use to think and do that with pots, but the issue I ran into for me, is I might have a 100K pot and try to thum twist it to a 100ohm resister is difficult, so I would use a low 1k pot, then find myself needing a 4.7k value… I ended up having a few pots and, just easier with a decade box.