Saturday, July 15, 2017

Using PCB rivets vias for homemade double sided PCB

You may remember in a former article the super tiny rivets I bought for double sided prototyped printed circuit boards (these but check those, they are way cheaper!). I realize that I never documented how I used them. So here it is.

The "raison d'être" of these costly rivets is specifically to help doing the vias that route the signals from one side to the other side of the board. The industry does plated holes with a chemical process that deposits a layer of conductive metal on the inside walls. It both conducts the signal but it also really helps making the board sturdier, as the copper rings (well square, here), are bonded together. Without it, the risk to tear apart one of them is much more important. Actually, a few makers do it also but it is both difficult, lengthy, messy and specially risky with a lot of nasty chemicals... not for me since I mill my PCB ;)

So in general, making double sided PCBs at home means there is no such plated holes. So we usually solder a thin wire through the hole on both sides. It is time consuming and it stresses the copper pads a lot, especially when the damn wire falls down when both sides melt again when you just want to solder the other side ;)

Top and bottom layers: making vias by means of PCB rivets on a double-sided milled PCB.
The milling job is quite bad here: the traces were not cut properly on the top layer (left side, check why here),
and the layers were mis-aligned after the board was flipped over to mill the top side... I need more practice.
Also, there are better and expensive tools to fix the rivets, but this prototype worked fine in the end anyway ;)
Now, rivets still help to make more robust single sided PCBs, most notably for the connector holes that will be subject to mechanical stress. Rivets will protect and keep the small copper "rings" tightly bounded to the PCB support material, so they will be less likely torn away.

Most notably, dirt cheap bakelite boards give no second chance in this respect, while fiberglass (FR4) endure more abuse and re-soldering. I guess the bonding is much stronger with the latter.

Which PCB rivets to buy?

I recommend 0.8mm outer diameter (0.6mm ID), because there their heads leave just enough room to route a trace between them. Hence in EAGLE CAD, I make sure to configure my vias to be 0.8mm accordingly, so they fit tightly.

Wednesday, July 5, 2017

Measuring micro amps, the easiest way

You can build yours (see my former review), but you can buy them already made: these $10 sub micro ampmeters on ebay work pretty well from 50mA down to 0.001mA. This range matches nicely the currents of many sensors, including when they are in deep sleep mode. This range is very convenient for IoT autonomous projects and sensors, and the 5 digits makes it unnecessary to switch between milliamperes and microamperes like on a multimeter.

You lose one digit when plugged backwards. The feature is not really interesting except that it makes it safer (you will not burn it by accident).

$10 for these micro current meters is a bargain!
They really are straightforward to use, and they do measure very low currents: the display goes as low as 0.001 mA, ie. 1µA, but I am not sure it is reliable at that level -- anyhow, below a few micro amps, the self-discharge rate of a battery usually becomes not negligible, so there is often no point in fighting further to reduce the sleeping currents. A CR2032 coin cell can run for years at this level, so you would better start optimizing also the wake-up consumption (make sure to read this impressive in-depth review on the matter!).