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....

SOIC Test clip

Our homebrew ICSP programming header didn't quite work out as planned.

We're not sure why, but we just couldn't get PICKit2 to recognise the device held under the programming header - maybe the pins aren't quite making contact, or perhaps they're routing to the wrong programming pins (we checked and double-checked the connections before making the PCB though!)



It all seems a bit convoluted anyway and if we're going to have to program a few SMT/SOIC PIC chips in place, we'll need something a bit more robust than a wonky home-made board, so we found this:



http://uk.farnell.com/3m/923660-24/test-clip-soic-24way/dp/178283



It'll take a day or two to arrive (Farnell are usually pretty good with delivery times, but recently we've had a few orders split into two or more parts, and they don't always all arrive on the same day!) but we've other stuff to be getting on with in the meantime....

Guitar PCB boards - SMT rules!

After initial reluctance, we're getting to quite like the idea of using SMT circuit boards for our miniature instruments. True, soldering them can be a bit fiddly - especially with a massive tipped soldering iron - but that can easily be fixed by using a smaller nib.

Even so, with a large tip we've managed to solder some pretty teeny little components.

Another benefit of using SMT is that everything is done on just one side of the board. With through-hole, all the components are on the "top" but on the underside are lots of little spiky bits where the component legs have been soldered to the copper board and trimmed. SMT gives us a nice flat surface on the underside to work with. A bit of double-sided tape on the unused side -



- and the board can be stuck to the acrylic "lid" of the guitar -



- and a nice flush finish, so threading the strings through from the top-side should be a doddle



Compare that to the original design, where getting the strings to line up with the holes, holding everything together AND soldering everything in place was a bit of a nightmare!


In fact, as part of our recent board edits, somehow we've managed to reduce the board length a little bit so the holes on the board don't line up with the holes on the acrylic. They're only a few millimetres out, but it will make quite a difference if we fix the board flush to the underside of the plastic. Then again, it's not a massive problem to sort that out.
There's a good chance we'll be making PCBs for a while yet, until we settle on a final design!

Laser cut solder paste stencils

Well, we've decided to give SMT a try again, for making our guitar PCBs. So the first thing to do is make an SMT version of our earlier (working) guitar board:

We tweaked it a bit - making some pads bigger and spacing a few out more. We also had problems with the first version of the board, connecting the guitar strings to the two rows of six pads at the top and bottom of the board. If we didn't take care soldering, the round pads would lift quite easily, so we've changed them slightly for this board.


This board was designed in ExpressPCB, printed to PDF using CutePDF then opened in Inkscape to flip it horizontally (mirrored) so the final transfer was the right way around - when you're working with through-hole components, you actually want the PCB to be mirrored on the underside so print it as you see it works fine. For SMT work, the image needs to be mirrored so that when it's transferred onto the copper board, it ends up the right-way around.

With our fancy new laser cutter (how longer before it stops being fancy and new, and just becomes "our laser cutter" I wonder?) we cut a stencil design into an acetate sheet. We won't do it again - acetate stinks! And it burns and browns and covers everything in a sticky goo. But here's the solder paste stencil we made anyway:



Maybe the stencil holes aren't fine enough, or maybe we didn't use it correctly, but after applying solder paste through the stencil, we just ended up with big blobs of paste on the board. Manually applying the paste with a small paintbrush may not give a better result, but is about ten times quicker to do!



So we cleaned the board off and applied the paste again using our tried-and-tested method with a small brush



Even with quite a large tip on our soldering iron we got quite satisfactory results. Simply touch the tip onto the pad to start the paste flowing. Any paste in the gaps between traces simply burns off (or flows onto a tinned pin). To be sure of a good contact, we touched the tip of the iron onto the pin to be soldered to make sure the solder doesn't just flow under it.

You may see solder on the bits of board between traces. On a professionally manufactured board, these bits would be covered with solder resist. This is where we just applied solder paste all over the pins then heated it to remove the bridges between traces. Obviously, where the solder paste settled on these "in-between bits" it's resulted in excess solder on the face of the board. We *could* have etched all this excess away and not bothered putting a "solid plane" between traces - but then the Ferric Chloride would take an age to etch the board fully.

Here's the finished board, with multi-core cables attached on one side.
It's not a complete working board, but we've proved that we can solder SMT components just as quickly (we're not going to say as easily!) as their through-hole counter-parts.



On a professionally manufactured board, with solder resist and only the pads exposed, soldering would be even easier - but we found hand-soldering SMT components onto a home-made board relatively straight forward (and pretty quick to do!)

In the example above, we tried two different types of wire - what we're calling "half-pitch" (0.025") IDE cable and "regular" 0.5" IDE cable. We thought that the bigger cable would be easier to solder to the board but thought we'd try the two side-by-side to compare them. Although the regular cable is easier to solder (the traces can be bigger and slightly wider spaced) it's also more likely to fail when working on a large number of boards - because each core is multi-stranded, you have to take great care when soldering as the strands can fan out and easily create bridging with the other contacts. The smaller cable is a bit more fiddly to work with, but because each core is solid, once it's in place, there should be no worries about bridging with other wires.

We've yet to settle on a final design but whichever we choose, will probably be a compromise between easy of soldering and likelihood of failure - we want something that's easy to put together, but if we're likely to get a high failure rate, we'd be better off spending a bit more time on something a bit more fiddly, but be confident that it'll work once it's done!

We're through with drilling

See what we did there? Through with drilling? Because we're only drilling the PCB to allow us to use through-hole components. Clever huh?

Honestly, when you have to explain a pun it makes it really lame.
Anyway, in the search for an easy-to-make circuit board that can be soldered quickly and repeated easily, and since we're trying out old ideas once more, we've decided to give SMT a go again.

This time, things will be different.
For a start, we can tin-plate the PCB before soldering. We already know that this makes soldering easy, to the point where we're able to solder tiny 0.025" pitch cable to an edge connector on the board.

In fact, using the tin-and-solder-paste method, we attached the multi-core cable much quicker than we ever could if we were using the older method of splitting the cable and soldering each strand through a series of 0.1" pitch holes.

With all this in mind, we figured we give SMT a go again.
We've already got a selection of SMT components in our tool box - mostly 1206 sized resistors and capacitors, and some SOT23 darlington transistors. The SMT crystals from Farnell are the same size as the through-hole versions, only with tabs instead of legs.

The only thing we haven't got is an SMT version of the PIC 18F2455 microcontrollers we've been using. But we thought we'd print out an SMT version of the body PCB and see if the components we had were suitable.



In this example, the ribbon cable(s) to connect the main board to the neck is our tiny 0.025" pitch IDE ribbon cable. We already know we can solder this to the edge connectors, quickly and relatively easily.



Another benefit of using SMT components is the non-copper side of the PCB will be completely empty and we'll have no nasty spiky bits of wire poking through the bottom. So we'll have a nice, low-profile board, a flush reverse side that we can stick straight onto the underside of the guitar top, and no nasty messy tangle of wires like we had on our first attempt.

Of course we'll still need to keep the two rows of six holes along the top and bottom edges of the PCB, since the strummer strings still need to pass through the board from the top of the guitar. But this new layout now requires only 12 holes instead of the seventy or so in the original design.

Inspired by Crazy Dave in this eevblog video (below) we reckon soldering a whole load of SMT components might actually turn out quicker than using their larger, chunkier, through-hole versions.



In fact, if the prototype board works we might just head over to http://www.quick-teck.co.uk and get some double-sided boards professionally made up. Check the spelling to get your browser over there!

Soldering made easy

The key to good soldering has always been to tin everything properly before starting. If you're not too confident with your soldering, you can also use flux/rosin to help the solder to flow (we flood everything with flux before soldering!)

The problem with fine-pitch soldering is getting the components tinned, and especially tinning fine multi-core cable. Tinning PCBs can be a problem, but with plenty of flux and a tiny amount of solder on your tip, this should be possible, even with a big tip. We've always found tinning multi-core cable to be really tricky!
The tiny thin strands in each core separate, the solder creates bridges between the different cores, separating the cores makes the wires heat up and melt the plastic coating and generally everything gets a bit messy!

While we're not quite ready for SMT soldering again yet, being able to pre-tin PCBs creates some exciting possibilities (though only really if you're excited by this sort of thing). Here's an example of how tinning can make soldering really easy (which is great if you're still having problems with big chunky through-hole components and massive 2mm pads!)



The job here is to solder the 0.05" pitch IDE cable to the traces on the edge of the PCB. Normally we'd use standard 0.1" pitch holes, split the cable at the ends and thread the individual wires through the holes and solder to the underside of the PCB. By using this new method, we don't have to drill as many holes on the board!

[EDIT: see last photo in this later post for an example of the "old way" of soldering IDE cable to our PCBs]

As always, we begin be covering the PCB traces with plenty of flux. Then just splodge a lump of solder paste across the traces to be soldered.



Hold the wires in place and heat up the solder paste with the tip of the soldering iron. We started by tacking the outer wires by heating the tracks rather than the actual wire. When the cable was held in place, we then put the tip of the iron directly on top of each core of the cable. You can see the solder paste melt and turn shiny. After a second or two, the solder starts to appear on the top of the wires - transferred, perhaps, by some form of capilliary action?



Anyway, whatever the reason, the end result is some beautifully soldered wires, no bridging (any excess solder paste is either burnt off or just runs towards the pre-tinned solder tracks) and a nice, strong, solder joint on each core of the cable.

The last thing to do is to test the continuity between the exposed ends of the cable (the opposite end to the one connected to the PCB) and the traces on the PCB. By testing "outside" each end of the solder joints, we can be confident that if we get continuity, the joint is good.



Any solder paste should do. We used this stuff. It was given to us by Matt from BuildBrighton. He said it was about 12 months out of date but had been hiding away in a cupboard and he had no plans to use it. Although the paste has a slight "crust" when we opened it, the paste underneath was still soft and usable.