New PCBs for electronic board sections

After getting our multiplexed hall sensor array working last week - and failing to demonstrate them at a Unity meetup in Brighton recently after wiring the one working prototype up incorrectly! - we made up some smaller sections, this time in a 7x7 grid.

This means we can use a single input port (PORTB) with pull-up resistors (since the hall sensors pull to ground when activated) and a single port to activate each transistor individually to turn on each row of hall sensors at a time. This means loads of discrete components:

For prototyping (and because of space restrictions on our breadboard) we've only actually wired up five of the seven transistors, but after proving the concept worked, went on to use all 7 in a test (we went on to wire up two of the SMT transistors at the top of the board).

Ok, not exactly loads. But quite a few. Enough to make wiring up each controller circuit for each board section quite fiddly. We etched up a few different circuit boards, using a variety of different mcus, surface-mount and through-hole components.

Even after making as many components as possible surface mounted, there were still a lot of discrete components to stick down on each PCB! Enough to make it worthwhile investigating an alternative, all-in-one integrated (IC) chip to do most of the work for us...

The A2982 may be relatively expensive, at about a quid a time when bought in bulk, but it does simplify the circuit a great deal.

The A2982 is a source current driver. It includes resistors on the transistor bases, to allow them to work at 5v logic levels, and each channel can source up to 500mA if needed. So all those discrete components are gone - and the entire circuit is now just two IC units: a single PIC 16F1829 and a single AS2982 chip.

Each panel is now much simplified from our earlier 12x12 design - both in terms of hall sensors and the controller PCB. Here we're using some thin hook-up wire just to get the board working (the final version will use ultra-thin magnet wire)

We're using 3.5mm stereo jack sockets to connect our PCBs to power/ground/data. So here's the final PCB design all soldered up and ready for the new firmware.

Right now we've got a couple of panels soldered up and working individually - so we're in for a marathon etching session tonight, to make up half-a-dozen or more controller PCBs for all the other panels. A few more like this and some updated firmware and we can create one, big, playing surface....

Creating electronic board game sections

Having sorted out the electronics for our electronic board game panels, we've tried a number of ways of building the hardware to embed them in. After lots of aborted attempts (using mixtures of card, foamex/foamboard and laser cut acrylic) we've settled on routed MDF.

Creating a consistent template for routing the channels, such that the pre-built PDBs will fit perfectly into the grooves, was a real pain, so we've come up with a simple router jig, out of offcuts of wood and scrap metal runners.

The rails are placed parallel to each other, at exactly 420mm apart (in truth, we used a piece of 420mm square cut wood to line up the rails, only to find they're about 420mm apart at one end and 418mm apart at the other, occasionally binding on the wood as it passes through!) But it's good enough for a first try.

The base is marked at 35mm intervals, so that we simply push our board through, under the router, then line up the leading edge with the marks. Set the router head spinning, plunge, then run the router from left to right. As our cutting head is 6mm wide, and the plunge depth set to 3mm, this makes 12 channels for our hall sensors to sit in.

Then we rotate the wood through 90 degrees and use a 16mm cutting head to create wider grooves for the PCBs at 70mm apart (though in the image above, our first board was cut exclusively with a 6mm bit, making the wider grooves with multiple offset passes).

We made a bit of a mess of the first offset (at both ends) for the PCB channel - but once we'd worked it out, the rest of the channels were cut quickly and easily.

The end result is a piece of MDF into which we can easily drop our pre-populated circuit boards. In practice, it may be easier to connect the wires between the boards before fitting (as we did with our earlier jig-based soldering) but we're still ironing out the exact manufacturing process!

The nice thing about this process is that the boards can all be routed independently of the circuit boards, which are then fitted quickly and easily into the grooves. All that then remains is to fit the main controller (that is used to connect multiple boards to each other) on the underside, and a thin cardboard layer fitted to the top, to cover the currently exposed circuit boards.

Getting jiggy with it (see what I did there?)

Using a jig and a stencil for applying solder paste has been quite a revelation for churning through our PCBs for the electronic board game sections - far more so than getting a rudimentary pick-n-place device working.

After just a few hours this evening, we had soldered and assembled a chain of eighteen boards for testing.

And with just one dry solder joint across all the boards, it was very encouraging indeed, to see each one responding the placement (and removal) of a magnet above each of the hall sensors

There are a few things come up with work today that may take a little time to resolve, so it's likely to be the weekend before we can get to the workshop and rout out some mdf boards to embed these in. But already things are looking very encouraging indeed. Can't wait to get a few sections daisy-chained together and actually see one of our board game apps responding to the  hardware for real!

Jig for SMT reflow soldering

After we got our PCBs and made a CNC pick-n-place machine from bits of junk, and created a solder-paste stencil from mylar, we were ready to start assembling our PCBs.

Except it's a multi-part operation. And one part was still a bit fiddly. Sure, getting the paste onto the boards was easy enough. And placing the components using our junk machine was quite painless (although quite a bit slower than we originally anticipated, thanks in part to the - relatively slow - speed of the geared scanner motor).

We even went crazy and bought a hot-air rework device to make soldering a bit easier (rather than have to hold each component exactly in place, we can blast it with hot air and let the paste pull the component into the correct position)

All this is fine for our surface mount components, that sit on top of the board. But we've also some hall sensors that not only sit on the board, but project out from the sides. And it's important that we place these so that their sensor parts are about 35mm apart (a few mm either side isn't critical).

Keeping the sensors raised while soldering the legs - particularly with a large hot air nozzle - while also ensuring they stayed the correct distance away from the PCB edge was proving tricky - so we decided to make a jig, to hold the board and the hall sensors during soldering.

In the fullness of time, we'd like to use a CNC mill and create the jig from some aluminium or similar material, but for now a few sheets of laser-cut mdf will suffice - at least to demonstrate whether the idea is worth pursuing or not.

The first - lower - layer contains slots for our PCBs to fit. This is cut from 2mm mdf (the boards are 1.6mm thick) and in the bottom of each slot we put some thick card so that the PCB is raised slightly. This means that the surface of the PCB is perfectly flush with the top of the mdf.

(the small rounded bits at the base of each pcb slot are to allow it to be easily lifted from the jig once soldered)

The top layer includes the pcb slots plus shapes that the hall sensors can be dropped into - ensuring that they are perfectly aligned with the connectors along the edges of each PCB, ready for hot-air soldering.

(a quick dry fit to make sure everything lines up)

Normally we'd just use some double-sided tape to stick our mdf pieces together, but since we're going to be subjecting this to some serious heat, it seemed sensible to use a generous amount of PVA to hold all the parts together. If the design proves successful, we'll probably invest in a slab of (soft) aluminium and have a go at routing the design on our CNC machine.

The final jig in use:

After trying the jig with a single board, we were very quickly soldering our PCBs in batches, up to ten at a time.

In practice, manually placing components with tweezers turned out to be quicker than using our junk-built cnc (and even worked out quicker/easier than manually placing using the vacuum pen). But the jig has proved invaluable for getting the hall sensors lined up and soldered in place quickly and easily.

Now all that remains is to embed them into the mdf panels that make up our electronic board game sections, and connect to one overall master controller. That's going to require a trip to the workshop, so for now, we'll just carry on soldering more of these PCBs up, until we can get down there!

Creating a solder paste stencil with ExpressPCB and Inkscape

Having got a few hundred PCBs ordered from 3pcb.com, it was time to think about how  we were going to solder these up when they arrived! Immediately, the obvious solution was to create some kind of solder mask.

We've tried making thin metal solder masks in the past with mixed success (the drinks-can etching approach didn't really work, whereas etching thin sheets of modeller's hobby brass using the same techniques as etching a PCB turned out quite nicely).

This time, we thought we'd try laser-cut mylar.

Creating a solder paste stencil from an ExpressPCB file is relatively easy - although it does involve a few steps, so pay attention: the first thing is to makes sure you have CutePDF installed. This great little program creates a "virtual printer" which allows you to create a PDF of just about any document that supports printing.

So in ExpressPCB, print the pads and silkscreen layer

Wait for the PDF to be generated, then open this in Inkscape. The first thing to notice is that the entire object is grouped - selected everything and click "ungroup" (ctrl +  shift + G) until there are no more groups in the selection. Now individually select the things we don't need - like the board outline, silkscreen shapes and text etc.

When you have only the pads remaining, select everything and remove the fill, while setting the stroke to 0.1mm. This should now show you the shape of your stencil-to-be.

With all of the pads still selected, choose Object -> Transform from the menu and set the scale to around 90%. This is because when cutting the mylar, we're allowing for the material to shrink back a little around the "burnt" edges. It's also creating a slightly-too-small-for-the-pad opening, so that each pad doesn't get fully flooded with solder paste.

It's important when re-scaling to apply the scale function to each object individually, not to the selected group as a whole (otherwise some of the pads will become out of line)

There are two ways to go forward from here. At first, we saved the document out as a DXF file, with polyline options. This makes the file compatible with most laser cutters (or even routers, if you're crazy enough to want to route this stencil from very fine metal - although you may want to reduce the pads even further, to allow for the thickness of the router cutting bit). This means we were (vector) cutting the lines from the mylar with our laser cutter.

It didn't really work - as the laser tried to outline each of the pads, the heat build-up caused the bits of plastic between the pads to simply disintegrate.




Instead, we printed our design to CutePDF, opened in Inkscrape, then exported as a bitmap at 600dpi. The result was a raster image that we could engrave, rather than cut. This gave a much nicer finish to the cut edges.

But even then, the finish was slightly off. It looked like the mylar was warping as the holes were being burned into it. So one last try, this time with a small piece of mylar taped to a piece of scrap mdf

This time the stencil looked just fine with very little sign of warping. When it came out of the laser, the holes lined up perfectly with the pads on the PCB.

Next we created a jig to hold our PCB, and a frame for the stencil/mylar to be mounted onto. Some locating holes ensure that the two line up properly during use.

Getting the stencil and the pcb to  line up with each other as well as the the alignment holes was a bit fiddly, but we got there in the end!

For some reason, our image contained a set of pads which are not actually on the manufactured PCB - so we're either using an old design, or a super-new one, including changes we've made since having these boards made up! Any paste deposited through these two holes on the right can simply be wiped off

The mylar film was fixed to the frame using good old fashioned double-sided tape, and a small amount of solder paste applied in a line across one edge. Using a bit of an old plastic from some food packaging, we swiped the solder paste over the face of the stencil, pushing it onto the PCB underneath

Whereas before it could take three or four minutes to apply solder paste to a board - with a pin or fine-tipped implement - it now takes just seconds. The end result isn't perfect - yet - but apparently, even with the best made, jet-cut, steel stencils, learning to apply the paste properly is something of a fine art.

Although the paste is a bit blobby around the hall sensor connector, the rest of the pads look just fine. It's much more precise than our current application method (which is to just lay a slug-trail of solder paste across all the pads anyway) so in the morning we'll place some components and give them the hot-air treatment, to see how the excess is handled.

Mylar film stencils are not as hardwearing as metal ones, but are generally good for about a hundred passes (maybe more). So, until we're churning out our PCBs by the hundred or thousands, this seems like an ideal way of creating stencils for applying solder paste to our prototype boards!

More PCBs arrived from 3pcb.com

...and don't they look smart?

A boxful of PCBs arrived earlier today. And they look great! Well packaged, well ahead of schedule, and no duty to pay!

This is something we're going to have to look into quite carefully - assembled electronics obviously have a VAT element to them when they are brought into the UK (the value-added element that attracts Value Added Tax is that someone has done the work of putting a load of components together for you). PCBs don't attract a VAT element (I remember reading about this somewhere online a number of years ago) but electronics components (such as the 250 shift registered just ordered from eBay) do.

I'm not quite sure where the value-added element of a bunch of shift registers is, to make them liable for VAT. But liable they are - according to DHL at least (and they deal with this kind of stuff all the time, so you've got to assume they're applying customs and excise duty correctly).

Any way, not only do you have to pay VAT on some imported goods, but the handling of that VAT also attracts a handling charge from DHL (to which they add their own VAT). So it suddenly makes a cheap online deal look not-so-cheap!

In future, we'll bulk buy from a single supplier in a single shipment - then we're not only paying carriage once (even if it's more expensive for a larger parcel) but only paying the VAT handling charge once too!

So what's with all these double-sided boards anyway?

We're not normally a fan of double-sided PCBs.
The problem with them is they're a pain to make - not if you're a cool PCB fab house, but if you're a bunch of nerds with nothing more than a laser printer and some ferric chloride, getting a decent double-sided PCB can be a real pain. Normally we stick to single sided designs, use manual routing (rather than some, let's be honest, pretty crappy auto-router) and every now and again stick a surface mount 0R resistor in, to act as a jumper over single trace tracks.

That works fine for most projects.
Even when we get PCBs manufactured professionally, we try to stick to single sided designs. It means we get a chance to prototype the board before committing the design and getting a batch made (Steve knows only too well how often we end up with a ground plane missing, or a single connection - usually to somewhere important like the regulated 5v supply - completely forgotten about, because on the prototype we just cobbled in a bit of wire).

Single-sided boards also mean that if the layouts are published here, anyone else can have a go at creating the same thing - not everyone is willing to waste hours and hours and quite a bit of relatively expensive material, trying to get a double-sided design to work (and who can blame them!)

But this time we're adamant on a double-sided design. So why is that?

Well, we're back to our electronic board game idea. We've tried any number of different manufacturing methods, including (but not exclusively limited to)

• each board section being made up from smaller PCB sections (makes the squares on the board too small or close together, and altogether very expensive to make a decent sized playing surface)
• one massive PCB per playing surface (very, very expensive!)
• multiple PCBs connected on the underside of a large playing area (no less expensive than a massive PCB, with the added hassle of having to connect everything together)
• using large membrane switches instead of PCBs for the playing surface (too expensive)
• using multiple small membrane switches (where the connectors go to the underside of the board, the playing surface is distorted) 
• eliminating PCBs and wiring hall sensors to a number of microcontrollers on the underside (not nearly as expensive as having PCBs or membranes manufactured, but very time-consuming to construct)

From these main approaches, the pcb-free idea (running wires up and down the board and attaching hall sensors to them) is the cheapest. It's also the most complicated and time-consuming to build.

So we're looking for a short of half-way house. We're trying to build the smallest possible PCB to hold the maximum number of hall sensors and a shift-register (so that the PCBs can be daisy-chained to a single, central microcontroller). This should reduce the build cost (since we don't have large, vast expanses of PCB "real estate" standing empty) as well as the shipping costs (for the same reason). But also simplify the construction.

Of course, each piece still needs to be wired to the next in the chain, so it's not completely work-free. But it's a compromise between the massive amount of work needed working with just wires and the massive cost of working with just one big PCB.

Perhaps the answer lies somewhere in the middle.........

When making PCBs...

....do you drill first, or etch?
Interesting to see that 3pcb.com do all their drilling first, then etching. We tried that when making double-sided boards and 50% of the time it worked ok, and 50% of the time it was a massive fail.

3pcb.com run a 24-hour PCB factory. Check out those manufacturing start times - they're running right through the night. And what a nice touch, showing where you board manufacturing is up to, at any point, through their website.

Then again, I guess a professional PCB fab house isn't lining up sheets of toner-transfer paper, by hand, to make their circuit boards. Or at least, I'd rather hope they weren't!

More circuit boards from 3pcb.com

After proving that our simple double-sided board design does actually work, it was time to "pull the trigger" and get some on order from 3pcb.com

3pcb is just another name for pcbway.com and both websites use what looks like the same calculator to give the same price (the pcbs arived last time in pcbway.com branded boxes, but 3pcb is just easier to remember!)

I don't know if it's deliberate, but - like those 3 for 2 deals at the supermarket, or those 24-packs-of-crisps-for-the-cost-of-15 - their calculator seems to encourage you to over-buy. For example, 100 PCBs costs the rather reasonable price of $53 (including delivery don't forget).

But when you see prices like that and realise that set-up charges and delivery make up a large part of the total order cost, it's tempting to see how much more you can get for your money. 200 pcbs is only $78 - that's $25 more for double the volume....

So what if we rack up the numbers? A thousand units costs $260. That's a lot of money - but that's a helluva lot of boards! In the end we settled on 500 pcbs for $159. It's probably more than we're going to need initially, but since we're using them in multiples of 20 at a time, they won't last too long!

The 3pcb.com website is dead easy to use. Enter some values, if you like the price, upload some gerbers, pay by paypal. Bang - done in less than ten minutes.

There's even a detailed beakdown of the fabrication process available on the website. Because this order has only just gone in, there's not much there at the minute. But as the boards get built, you can track them in realtime and see which part of the manufacturing process they're up to. So you can see when they've gone in for etching, when the soldermask has been applied, when they've been routed and so on.

It's not really very useful information (you can't influence what's going on!) but it's nice to see so much effort has gone into keep the customer informed. We'll report back here in about a week or ten day's time when they finished boards arrive....

Etching with ferric chloride - hot versus cold

Our etching heater packed up today. In fact, it's not the first time it's happened; the previous time, it turned out to be the bimetallic strip was getting stuck and not turning the heater element on when it was needed.

This time it looks a little more serious - we've not been able to get it to come on at all. Which means etching with "cold" ferric chloride.

Now ferric chloride doesn't need to be heated in order to work. It'll still etch copper off a copper clad board. But it does take a lot longer - it's amazing the difference the heater makes. We're only etching a couple of SOIC-to-DIP breakout boards (albeit with a slightly unusual pinout)

Usually, a simple board like this would take less than five or six minutes in a 3L tank of (heated) ferric chloride (at about 40 degrees C). But this board has taken more than 20 minutes - and it's not even "fully etched" it's just about usable (we couldn't be bothered waiting for it to finish properly, to get nice neat edges).

The problem with cold etching, and leaving the board in the etchant for a longer period of time, is it's more likely to suffer from undercutting. So we've made the traces on this board a massive 0.5mm (instead of the usual 0.3mm which still feels quite chunky, even when etched "hot").

While cold etching works for very simple boards, there's no way we'd have been able to make our double-sided efforts, with their tiny-pads-for-through-hole-vias if the boards had to stay in the ferric for more than twenty minutes at a time.

We're not sure if this heater can be salvaged and repaired. But if not, it'll mean another trip to eBay - there's no way we can etch many more boards using the cold solution and expect to get usable results. Already the odds aren't in our favour - we're using a less-than-ideal printer and cheap chinese press-n-peel-alternative paper. Add into the mix a less-than-ideal etching method, and it's a wonder than any of these circuit boards work at all!

Double sided PCB with toner transfer (chinese press-n-peel alternative) success!

Well, success, sort-of.
Maybe that should be a caveat on homebrew PCB manufacturing. Everything "sort-of" works. Or, at least, on this blog anyway, with all the short-cuts and penny-pinching and not-always-using-the-best-tool-for-the-job, but making do with the cheapest....

You get the idea. It's taken two evenings and a few sheets of copper board and toner transfer paper to get the result, but we got there in the end. As Steve might say, it's a bit pikey - but it works! Of course the way to get the best result would be to email the design files over to a PCB fab house, wait anywhere between 5 and 15 days, and try out the professionally produced, exceptional quality circuit board.

But that's not what home PCB manufacturing is about. At least, not here at Nerd Towers anyway! We're about getting something that works. Soldermask? Pah! So what if it makes it a nightmare to solder? That's part of the fun, right?!

Anyway, thanks to some suggestions from Jason (and later, Steffen) and after discovering some cool functionality in the Copper Connection software we use for making Gerbers, we managed to successfully etch a double-sided board which lined up enough to be usable. So in our eyes, that's a success, right there.

First up, getting rid of the printing problems:
It was actually quite straightforward. Instead of just dumping ExpressPCB to Gerber using Robot Room's Copper Connection, we downloaded the latest version and were delighted with the printing options.

The transfer etch option prints out two sheets (for a double-sided board) with the top-most layer mirrored. This means that after transferring, the top layer is the "right way around" and the bottom layer is flipped - exactly as it needs to be. No need for exporting to PDF then flipping in Inkscrape and trying to work out which layer needs to be flipped, and messing about to get it to print properly by converting to a bitmap and all that nonsense - just hit print and let the software do the thinking for you!

Jason's suggestion was to draw a border around the PCBs on both top and bottom layers (obviously making sure they're perfectly aligned in the drawing!). Then, from the carrier sheet - after printing - cut a template and place onto the double-sided copper board. Using the template as a guide, drill holes in each corner, outside of the border.

This does mean that the PCBs are in the centre of the copper clad board, with quite a wide border around their edges (something we try to avoid, in an attempt to reduce waste and get our designs as close to the edge as possible). For now, we'll just take the extra waste as a hit, in preference for nicely aligned boards!

After drilling the marker holes, place the press-n-peel on both sides of the copper clad board at the same time, carefully aligning the edges of the paper with the guide holes.

We're using paper-based masking tape to hold the paper in place; Jason recommended using an iron to "tack" the paper in place before sending through the laminator.

Laminate as normal, fixing the toner transfer paper to both sides of the copper clad board at the same time, and etch. Keep everything crossed until the very last bit of excess copper has gone, shut one eye, stick your tongue out to one side, chant a mantra and do a little dance. Then drill through the holes and hope they line up.......

Don't look too closely, else you'll see a lot of holes are off-centre. In fact, before drilling it was possible to see that the back and front didn't quite line up. So instead of drilling exactly in the centre of the holes on one side and potentially missing the centre of the pad on the other (or, worse still, drilling through the middle of an adjacent trace) we actually deliberately drilled off-centre. The result is a board that looks pretty crappy on both sides - but functionally does what it needs to do!

And there we have it. 

Success. Sort of.

Well, not perfect, but good enough to be usable.

And, as stated at the top of this post, for us, that means success.

Double sided board fail

Creating double-sided PCBs using toner transfer is never easy.
Getting both sides lined up exactly, while etching one side and protecting the other during the first part etch is a real pain in arse. At best, you can hope for boards that are "basically useable" but very rarely (only twice in my experience of making PCBs over about a hundred years I think) do you get a "perfect" board.

But this time, we've had a spectacular fail.

It's probably related to the Inkscape printing fail earlier, because this time not only did the two sides not line up (we could live with that at put it down to ham-fisted sausage fingers) but they're not even the right size!

Here's the first side of the board. Compared to the actual 16-pin soic component, it's the perfect size

(using the ExpressPCB-to-PDF-to-Inkscape-to-PNG-to-DTP-to-printer method, finally managed to get the pcb design onto the copper board via cheap chinese press-n-peel alternative (the bad etch in the far corner is where the board was handled before it went into the ferric chloride - d'oh!)

But on the reverse....

The print just about lines up with the first set of holes. Not perfect, but useable.
But by the time we get to the other end of a 100mm long PCB:

(the bad trace leading to the first hole is where I wiped the board with my wet finger to see if the print was misaligned, or just some debris on the board. It turns out it wasn't crud on the board!)

Not even close!
Frustratingly, the offset between the holes and the printout aren't even the same distance apart. So if the whole image was shift (as you look at the image above) slightly from the right to the left, then the holes at the other end (in the image before the one immediately above) would be too far out the other way.

If the whole thing was shifted, there's no way we can see of making this a "perfect" match. It's almost like the image needs to be printed at 101% to space the holes further apart.

Of course, it's not just these holes we need lining up. Throughout the board are much smaller, 0.5mm holes, to act as vias (in the event of a perfect line-up, or even a "good-enough" match we would normally just solder wires through these instead of messing about trying to plate through from one side to the other).

There's no way we'll be able to get our double-sided board working if this is as close as the two sides line up! So is the problem with Inkscrape, converting pdf to bitmap, or something else? Maybe one of the BuildBrighton lot have some ideas about double-sided boards........

SMT LED fail

While making some double-sided PCBs, we had quite a few left-over press-n-peel copies of our PCB design. It seemed a shame to waste them, but we didn't want to go through the trials of making more double-sided PCBs, so thought we could make single-sided versions of our sci-fi scenery walls.

Making single-sided PCBs is a doddle compared to double-sided (though quite why double-sided proved such a headache, we're still uncertain). It didn't take long to get a few boards etched and soldered up with some SMT LEDs.

For this particular design, we're using just a 74HC164 shift register to sink current through the LED to make it light up. Even without the shift register in place, we can test the board so far, by simply applying a low voltage (up to 3v) to the rails of the PCB.

So we did exactly that - applied 3v to the common power rail that is running to one side of every LED, and grounded the opposite sides of the LEDs, though the traces on the board. As each one lights up, we at least know that the LED has been properly soldered to the board, and that there are no breaks in any of the traces.

Except nothing lit up. Not a thing.

We checked our 2xAA batteries and they were working fine - the problem seemed to be with the PCB. Which, given it's only a few LEDs and some traces, seemed unlikely.

Reluctantly, we tried with the polarity reversed.
And every LED lit up, one at a time. Every LED had been mounted backwards!

Now the odd one might be forgivable - but to solder every single LED back to front it just plain crazy! We were quite diligent in checking the markings on the underside, to make sure that they were being soldered the right way around - but somehow, we'd managed to mis-read the markings on every one of the LEDs.
Then we came across this article online:
http://blog.screamingcircuits.com/2013/01/how-not-to-mark-a-diode.html

And it turns out that we're not the first ones to have had this problem. It turns out that some suppliers use the same markings to indicate different things!

In fact, sometimes the same manufacturer uses the same markings to indicate different polarities in the LEDs! So you can't always just assume that the markings on the underside of the LED will always indicate which way around they need to be mounted on the board.

In short, we're going to have to double-check every LED before placing it, to ensure it's the right way around. Our SMT LEDs have been dispensed into little multi-compartment boxes; we've no idea which are which, who made them, whether their markings indicate the anode or the cathode.... even a simple thing like an LED has suddenly just got a whole lot more complicated!

Double-sided PCB fails

It feels like ages since the Nerd Unit Laser Cutter was fired up, so we put that right this weekend. There are a number of projects on the go (as always) but it was good fun to fire up the laser and etch some boards, instead of messing about with laminators, press-n-peel, and the frustratingly awful Brother laser printer (the old Xerox was a joy to make PCBs with - this Brother laser printer makes every board a hit-and-miss affair).

Anyway, one thing we thought we'd try was some double-sided boards.
It seemed easy enough in principle - firstly, etch our PCB designs onto some MDF and drill out a few holes.

MDF?!
This is just going to be a template for etching; the idea is to place the copper boards on top of the template and when we hit "etch" again, it should carve the same pattern in exactly the same place on the board.

So after etching one side of our double-sided copper board,we drilled the same holes as we had done in our MDF template. Then simply push some pins (we used 0.8mm copper wire) through both the board and the MDF, to get the reverse side to line up

Tape the copper board to the template, spray with matt black paint, etch the reverse pattern, and.... ta-da!

Not exactly successful. The holes are probably a good 3mm away from where they should be!
Undeterred, we went back to the trusty old method of press-n-peel.
There are a few articles online about how to make double-sided boards with press-n-peel, and many use this same approach:

The idea is to print the design twice, one flipped (and mirrored if necessary) with a line exactly half way between the two designs. This is where we're going to crease the press-n-peel. To ensure that we got a perfect line up, we cut our press-n-peel along the very top of the design, and made sure that, after folding, the two top edges were perfectly aligned.

Insert the (double-sided) copper clad board, affix in place and laminate to secure the image

This time the alignment was better - but still not perfect (or, to be honest, not even good enough to be used!)

[photo here]

So in a last-ditch effort, we scrapped all the fancy-shmancy ways of making double-sided boards and went back to the method we know works best - etching just one side at a time.
This involves placing one design on one side of the board (using the "traditional" press-n-peel method) coating the reverse with paint (to protect the copper during the ferric chloride etching) and etching the board as if it were a single-sided PCB.

Then we drill a few alignment holes and clean the back of the board...

...and manually align a second design onto this reverse side.

Alignment is still not perfect - in hindsight, we should have image-transferred and etched the boards before cutting them down to their final size, so that the masking tape holding the press-n-peel in place had a bit more material to stick to!

Although not perfect, at least the alignment of the holes means this board will actually be useable. So the last thing to do is coat the front side (the bit that's already been etched) with paint (to protect it from the ferric chloride during the second etching) and to etching the reverse of the board.

And there we have it. A double-sided PCB that sort-of lines up.
Today's lesson - if you want to make something as quickly as possible, slow down, take your time, and don't try to do anything clever.