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!

858D hot air rework station

I just took receipt of a hot air rework station from eBay. It's one of the 858D hot gun things with a choice of nozzles.

Jason has one and when I used it a while back, it was great fun.
But there's something about the packaging that suggests the graphic designer was keen to get away, for an early dart on a Friday, when he knocked this one out...

Soldering speakers can be difficult

...unless you've got a big blob of blu-tack to hold the pesky thing in place and stop it sticking to your soldering iron!

(Ok, the Brighton Poundland only sells white-tack, but the idea's the same!)

Nail varnish as solder resist

We've been doing a lot of trying things out lately, and not a lot of actually getting much done. But sometimes that's ok, if only to satisfy your own curiosity. One thing that's been bugging me is how close we got to making some really useful pcbs with the laser cutter (cutting the etch resist from Halfords Car Paint applied to the copper board) and that it was the soldering, rather than the making of the pcb, that failed.

So what if we had some cheap form of solder resist - would the pcb have worked then?
I couldn't get the idea out of my head that an enamel paint should not only stop the solder from sticking to the copper board, but should also be pretty heat-resistant too. And the cheapest source of enamel paint...?

There's a peculiar fancy-dress shop on George Street, in Hove, filled with all kinds of outfits and make up and wigs, false eyelashes and nail varnish that's frequented by some rather burly looking fellas. Taking a lead from a strapping six-footer with a deep voice (who called the shop assistant by name, so it obviously wasn't his first visit) I wandered in and checked out the nail varnish. I'm sure the shop assistant thought I was looking for something that wouldn't clash with my eyes, but in all honesty I just wanted a matt, plain paint with no glitter or metallic flecks in it. And it had to be enamel, not acrylic. I was quite particular about that, which reinforced the shop assistant's view that I knew more about ladies nail varnish than I cared to admit to.

Just £1 later and I was the proud owner of some lime green, plain opaque nail varnish. It didn't paint onto the board very well, using the brush supplied in the top of the bottle, but well enough for a proof-of-concept attempt.

I deliberately painted over some of the holes I intended to solder, and sloshed it around over some tracks on an old pcb that had no use. The idea is to test for continuity across the tracks that have been painted over and hopefully will still return negative (which is why the nail varnish had to have no metallic flecks or glitter effect in it). Then I'll try soldering onto a painted area.

As you can see from the photo above, the solder took to the pcb where the pads were still exposed, but the last pair of pads appear scorched. This is because the solder refused to stick - it kept rolling up into a ball and coming away from the board as I lifted the soldering iron.

Also there's a patch where the two tracks run side by side which appears scorched too. This is where I tried to apply some solder to create a deliberate bridge between the two tracks. Once again the solder just balled together and fell off the board with a slight tap.

So all in all, it was a successful test. Our conclusion?
If you get non-metallic, enamel nail varnish, and paint it over your pcb, leaving only the required pads exposed, it does indeed act like a cheap solder mask.

Soldering fail

Our laser-cut pcbs were certainly quicker to make than using laser-etching (we converted the pcb image to vector and cut out from around the traces rather than using the laser to etch a rasterized bitmap image) and the ferric chloride made a great job of etching them.

But because the mask had been laser cut, the etched part of the board is very very narrow.
And this has made soldering rather difficult. The tiniest little movement of either iron or solder means it's really easy to create bridges across the tracks - even when using tons of flux

We made a right mess of soldering the pin-header connector too - ripping the traces from the pcb when removing the IDE ribbon cable we'd squashed on top of it. So it looks like we're going to need to re-think the pin header idea - and find a way of solder-masking the rest of the PCB to stop the iron from bridging the traces when too much solder is applied!

Soldering quarter pitch multi-core cable

Matt turned up at the last BuildBrighton meeting with two solder pots from DealExtreme. I'm a little disappointed with DealExtreme - they may be cheap, but you've got to be prepared for a long wait; these 'pots have been on order for about five or six weeks and there were no email updates to say where the order was up to (unlike PCBCart - another China-based supplier - who have excellent order status updates).

Anyway, the solder pot arrived and no sooner had I cut my finger on the sharp exposed edges around the pot than I had it up and running (careful of these cheaply-manufactured electronic tools - reminiscent of those nastily made PC cases that flooded the market in the early 90s, some of those edges can be razor sharp!). The moulded Australian style plug had to be cut off and a UK style plug fitted, but it didn't take long for the sharp tang of a new heater coil to fill the air.

The pot holds LOADS of solder - about quarter of a coil of regular solder wire and there was still only a small lump of molten solder in the bottom of the pot. The rosin/flux inside the solder didn't half smoke! And it created a tarry brown coating over the exposed parts of the pot. But once the solder was molten, it was easy enough to use.

 (picture taken after solder has cooled, to show the rosin tarring inside the pot)

First, cut some multicore wire to length and strip back to expose the ends.
(we use quarter-pitch IDE cable - the sort of stuff you find in "round" IDE cables)
Dip the exposed copper into the solder, leave for a second or two, then remove.

The ends are perfectly tinned with just the right amount of solder.
Place the cable onto the tiny-pitch PCB connectors and heat with the tip of a soldering iron. Easy peasy - every wire connected first time, with no bridges or lumps of solder like we usually get when trying to solder these by hand.

We're still having to connect each wire one-at-a-time, but it's relatively easy. Ideally we'd like to get a really wide (15mm) chisel tip on the iron, so all wires can be connected in one go. But so far, the solder pot looks like it's going to be a useful addition to our arsenal of tools!

Soldering 0.025" pitch cable

Our miniature guitar project uses a double-sided PCB for the fretboard which needs to be connected to the main board using a ribbon cable. We've tried "regular" IDE cable, but had trouble with the multi-stranded cores (they spread during soldering and can bridge with the wires/pads next to them).

Also, using 0.5" pitch IDE cable, the base of the neck-PCB needs to be quite wide to accommodate all the pads. Then we discovered some 0.025" pitch, solid core ribbon cable which seems perfect for the job.

Except, of course, that it's an absolute nightmare to work with.
Just look at how small this strip of 8-way cable is!



Although we have managed to make some workable guitars, each one took a loooong time to complete, and wouldn't be suitable for making on a regular basis. We've pretty much got assembling of the SMT boards down to quite a quick and simple method, using the solder-paste-and-hot-gun approach of assembly. Although fiddly, it's much quicker than using through-hole components.

We're still looking for a quick and simple way to connect our guitar's "body" and "neck" PCBs together. If there's any way we can quickly and repeatably solder this tiny cable to the PCB edge connectors, the electronic design will be complete, and we can put an order in with either quick-tech.co.uk or maybe PCBCart

We made up some test boards, to practice using different techniques for soldering the ribbon cable to the edge connectors. These boards don't actually do anything but allow us to practice connecting ribbon cables



With previous attempts at soldering this tiny pitch cable, we've had problems with it moving around as the solder paste melts and pulls the cable cores onto the traces. Holding or taping the cable to the bench causes it to lift up at the end touching the PCB which makes soldering particularly different. So before soldering, we prepare each ribbon cable:


the tiny bit of cable that will be connected to the board is trapped under a spare piece of PCB/copper clad board

After stripping and trapping one end of the ribbon cable, the remainder is bent vertically upwards and pinched between a second board to make a sharp crease...



...then bent along a second piece of board, away from the trapped end



The result is a "kink" in the cable, which allows it to lie flat along the bench, while the exposed fingers of wire sit perfectly along the PCB edge

Those horrible dirty fingernails are because they've been scraping solder paste off the kitchen table before anyone else noticed!

Using a small soldering iron we managed to solder an example bit of ribbon cable by applying solder paste onto the board and heating it with the soldering iron tip. The result was a technique that worked, but was very fiddly and difficult to do well. The solder paste bridged across the traces and was difficult to remove. Touching the soldering iron between the traces did remove the bridging, but in a lot of cases, also disturbed the other cores and bent them so they went out of alignment with the traces on the board.

So we prepared the board to try the hot air gun method of soldering



This was less successful than the first attempt!
The hot air caused the cable casing to melt. As the plastic melted, it caused the cable to buckle and lift off the PCB edge. With a bit of re-working, we got this bit of ribbon cable connected, but it too was tricky to achieve.



The final method of soldering was the "traditional" approach and worked the best of all (the latest cable is the one on the right with hardly any solder visible). This involved tinning the solid-core strands, placing them onto the PCB traces, then lightly touching each with a soldering iron tip. In this case, a larger tip works well as it joins two or three cores at a time.

The end result is a cable that is attached with very little excess solder and no chance of bridging. Soldering is as simple as holding the tip in place for a second or two. It's by far the easiest method so far for connecting the ribbon cable to the PCB.



The only downside is the preparation of the ribbon cable and tinning the individual strands. Using even a small soldering iron tip, this was fiddly and it took a few goes to get all strands tinned without any bridging between cores.

One solution may be to use a "solder pot" for tinning the wires.



Videos on YouTube suggest this may be the case:


So now we've got a solder pot on order from dealextreme.com. It'll be a few weeks before it arrives, but with a bit of luck, we'll be able to quickly tin/solder all 13 cores with a single dip - the plan being to batch-prepare a load of ribbon connectors and tin them all in one sitting. The solder takes about 10 minutes to heat up to a usable temperature so we don't want to be doing this for just one cable at a time!

Hot air soldering - it's like magic

Here are some before and after photos of our recent hot air soldering.
These are tiny SOT-23 package darlington transistors.
A blob of solder paste was put on each pad (manually using a small paintbrush). As you can see, the paste did spread onto parts of the board that we don't want soldering.



Apply some heat (300 deg, airflow about 1/3rd) and - ta-da!


Similarly, to solder the PIC 18F2455 SOIC SMT package neatly, first start by spreading solder paste all over the place:



Put the chip down with it's feet in the gunky stuff



Add the magic ingredient (heat) - and here's the finished job!



We tried to take a video showing the actual process, but fat fingers and moving parts close to the camera made the focus go all fuzzy. Here's a video from YouTube showing the same soldering process (but when heated from below rather than above).
Just like hot air soldering, it's pretty boring. Nothing much happens until about 1m:30 into the video. Then all of a sudden, the magic happens....

Hot air soldering

Jason brought along his hot-air rework tool to BuildBrighton this evening and, having worked with SMT stuff recently, we couldn't wait to give it a go.

He's the rework tool. It has a digital heat selection and a fan controller. The fan is actually housed inside the handle of the tool and controls the rate of flow of hot air onto your board(s).



Using our flux-and-solder-paste-everywhere approach, we blobbed some around on the pads of a SOIC package chip and let the hot gun do it's work.


The end result was quite impressive. It takes about a minute of heating before the solder paste goes noticeably dull then suddenly liquifies and heads off for the nearest bit of tinned copper trace it can find! The end result is nice shiny pads and no tricky pin bridging. Very impressive (and easy to do).

On these pads, we deliberately went crazy with the solder paste to see how the heat gun would deal with it. The result was the same, although there is noticeably more solder on these pads, they are still well soldered and no sign of bridging anywhere



Ribbon cable wasn't quite as successful.
Predictably the plastic casing melted and deformed. But this also caused the cable to flex and bend, so not all the cores were in contact with the board. Where they were, the wires soldered quite well. But where the ribbon cable lifted, the solder paste stuck to the undersides in blobs and bridged the fine 0.025" pitch traces and generally made a mess.



Further heating with the rework tool, to try to reflow this solder to remove the bridges caused the plastic to blister and burn in a series of small black bubbles. Prolonged use of heat caused the board to scorch (see above) and start to blister in places too.

In short, it looks like we're stuck with the soldering iron for connecting ribbon cables. But the hot air tool looks very promising for sticking down fiddly little SOT-23 sides transistors, and to get a lovely finish on the SMT chips.

We expected the hot air to blow the little tiny components all over the board, but the air flow isn't so strong as to make them move about too much. The only time we noticed any movement was as the solder liquified, the components "floated" on the hot solder. But sometimes this actually helped, as the surface tension automatically pulled the components in to line on the pads.

Now if only there was some easy way of converting a CNC machine into a pick-and-place machine, to simplify the actual board assembly.....