It's prototyping again on the factory. This time around the PCB for the Micro:bit expansion has arrived. This is our PiicoDev adapter for Micro:bit that we were talking about last week. The PCB arrived, so I've had the opportunity to test the experimental footprint for this edge connector.

We also have this expansion board for Raspberry Pi Pico. I'll show you how I assembled this prototype and also a few features. Let's get started.

Now we're working on the version 1 prototype here. I secretly had a version 0 that I actually changed the footprint for for this version 1. Unfortunately, I forgot to order the stencil for the version 1. So here the stencil is for that earlier prototype, which is why the pads are a little bit too large and don't line up perfectly. However, I think it's going to be good enough to give us an idea of whether this soldering technique is going to work or not.

A neat trick that I got off Twitter with stencils was if you select a custom size for the stencil that still fits within the small standard shipping box that your PCB house uses, then the two can get shipped together. So the stencil will ship at no extra cost. This is a great trick because often stencils can be prohibitively expensive because they're just too large to ship.

Jumping over to the microscope, you can see that the stencilling of the full pad actually deposits some solder into the milled hole as well, which will give us some good solder coverage when we reflow. I do anticipate a bit of risk there though because if you fill the slot with solder and it can come out the bottom of the board, that means during the stencilling process you put your panel on, you stencil the panel, you take it away and you put the next panel on. If anySolder goes through and remains on the stencilling machine, you really have to make sure that machine stays clean because if you put the next panel on top and it lands on some waste solder, you've now got solder paste on the bottom side of your board where you don't want it and then you've ruined that board.

So I'm also going to experiment with taping over some of that aperture so that solder paste is only put onto the reservoir part of the pad. The point here being maybe if we just put solder onto that part of the pad, but not into the slot, we remove that risk.

So I wound up doing three variants. I put those through the oven and these are the results. So the first one is the full solder and actually this one is really the only one that has acceptable results. There's good wetting around both halves of each locking tab. I'm really happy with how this one's soldered.

For my little experiment with just partial stenciling, I'm not convinced that that's going to be up to standard. In fact on one of them I can actually pull that connector and hinge it back. So it really hasn't succeeded in soldering those locking tabs at all.

Okay, so that means that we have to do a full stencil. We have to get solder paste into that milled slot for it to work properly, which means that I need to come up with some other solution for stencilling and or soldering. It may just be as simple as having to wipe down the stencilling machine between each panel. I'd really like to get away from that if possible. I'd really like to maybe tune this so that we can guarantee that no paste comes out. It might be down to just manual inspection and cleaning.

At worst, maybe at best there's some kind of jig that we can use. Like maybe if instead of goingOnto a flat plate in the stencil machine, maybe we can create some kind of spoil plate with pins that support the boards and that way it doesn't really matter if a little bit of solder falls through. I think there's some experimenting on the horizon, but if you have any thoughts about this, I'd love to hear them.

Alright, on to the next prototype. Last week I mentioned these 1x20 headers that we were prototyping some trays for. These were actually intended for this guy. This is an expansion board for the Raspberry Pi Pico. So we're originally going to have four of these headers, two to seat the Pico and then two to provide breakout connections so that you can plug your jumper wires in and prototype easily.

By transitioning to 2x20 headers, we both halve the number of headers that we have to place, potentially by hand, and we can also use the second row as the prototyping connections. So the Pi plugs into the inner rows and the prototyping connections can be on the outer rows. So if I grab my battery and plug that into one end, we can power the board and we get a little charge indicator.

So this is a battery-powered expansion board, so you can take your projects on the go. And on the other end is a four-way PiicoDev connector, so you can plug in a PiicoDev module. This is a temperature sensor module that we've been working on. And now you can unplug USB power, you're on battery power, and you can take your project on the go.

And the assembly for this was pretty smooth as well. I used the same prototyping stencil trick with the small stencil in the small box, and I stenciled this with a rubber squeegee, and that might have been a mistake. I might roll back to using the steel squeegee. I could actually feel the rubber.Deforming into the apertures for these large pads, and I could feel it kind of gripping over each one. That might have contributed to putting a little too much solder paste on. Also, I allowed the stencil to bow a bit, so it wasn't the cleanest stencil, but it's still serviceable for this prototype.

And I was a little concerned about this one area in particular with the capacitor and the resistor side-by-side, so I'm keeping an eye on them during reflow, and you can see that resistor just slide up on its pads, being pulled, being drawn towards that capacitor. Not a problem for this prototype, but I might just move those components a little farther apart, and with a better stencilling as well, that won't be a problem.

Occasionally, I'm pushing down onto the hot plate in certain spots. If there's a little bow in the circuit board, if it's lifting up just slightly off the hot plate, then there won't be as much heat going into that part of the board, so I'm just making sure that heat's getting in wherever the solder is still not reflowing.

I've just busted out the screen render so I can show you some of the features. We have the battery connector up the top here, we have the PiicoDev connector at the bottom, the two headers for the Pico and for breakout connections, of course, and pin labels, and this is the battery charge circuitry here with the power selector.

I've included, it's almost a free feature, so I decided to include it, a little solder jumper so that you can select between two charge currents. Maybe you want to use really small batteries, and that's, you know, that's not appropriate for a very large charge current, so the device is, by default, set to a charge current of about 200 milliamps, and then by soldering.This jumper closed, you can double that to about 400 milliamps if you choose to use a large battery. So there you have it, two prototypes that are basically ready to panelize and send off.

I'm, in fact, I'm going to panelize this one now and send that off. This one, I'm just going to shrink those apertures a little bit to give it more of a press fit rather than a sliding fit.

Thanks for joining me for this week's episode of The Factory. If you want to see anything in a bit more detail, or if you have any questions, open a thread on the Core Electronics forums. I'll catch you next time.