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.....