Which bit to use ......

It would appear I've made a pigs ear of soldering a ground wire to a Zimo MX 617 decoder.

I am attempting to chip an old Bachmann V1 split chassis loco and wanted to add a stay alive at the same time.

This required a wire to be attached to the "ground" tab on the decoder.  I'm guessing that, in doing so, I've been over zealous with the iron and/or solder.  Looking at it carefully, I think I've touched the thingy next to the tiny ground pad with either the solder or the iron with the result it now seems the decoder is dead.

Testing it on the Zimo MXTAPS decoder tester says "no CV's read".

My question is, what sort of iron and bit do you electronics whizz-kids use when soldering these tiny pads and wires ?

I know you get YouChoos to pre wire your ground connection John (Dew), and I wish I'd done the same but it's too late now.  I have another MX 617 6 pin decoder in stock and have ordered a 6 pin socket with trailing wires but will still need to attach that ground wire……….

Whilst writing, where do you buy liquid flux and what sort is it ?  I'm hoping there's a "no clean" version available.  I have gallons of Bakers Soldering fluid but that's like dipping everything in quick lime ….

Hi Peter,

The small pointy tips focus the heat just where you need it for delicate and accurate wires. Tin the wire and pad, then place the end of the tinned wire on the pad and touch it with a clean iron tip to melt the solder.

As with most things, practice makes perfect, so try soldering tinned wire ends to a strip of scrap brass until you have it off perfectly.

DC Concepts sell a brilliant liquid no clean flux, together with a good range of solders and soldering accessories.

Best,

Bill

Thanks Bill.

For this job, I used an Antex 18w iron with the "pointiest" bit that came in the kit of 4 or 5 bits and lead free 1mm solder wire - the only stuff I coukd source locally.

I have a range of irons including a temperature controlled "station" but this one I think,  had the finest bit.

I have previously succeeded in soldering the odd wire directly to the decoder but yhis pad is both small and doesn't offer much space between other components mounted on the decoder.  That's why I wondered if our regular solderers used something special - higher power for shorter time or low melt solder, maybe even a special bit etc. etc.

I'll have a look at DC concepts site and see what they have on offer - thanks for the heads up.  I'm trying to source more and more in France now having been stung a couple of times with duty and handling costs on imports from UK.  Also, I'm not altogether sure they could post flux - wouldn't it be classed as a "noxious substance" ?

Just had to post this ……..

I looked at DC Concepts liquid flux and the blurb says "use our fluxes and you'll be able to solder almost anything.  Solder will flow into the joint like water………"

Expecting to wade through pages of fluxes, it then said "see our range here - 2 results " !   

Anyway, it's obviously branded by them so unlikely to be sold in France ……. delivery, if possible, is likely to be around £20 at a guess …… :hmm

Ebay is hopefully your friend in France too for small quantities of electronic items, try searching for "flux pen" you should find starting from £4 incl uk shipping (IDK what the French prices will be).

A flux pen is like a felt tip full of flux, you just dab it on the pcb prior to soldering and it helps everything flow, invaluable for SMT components.

Thanks Roger and yes, I've seen those flux pens advertised.  If memory serves me rightly, they hold around 10ml of flux - presumably non-acidic.  Do you think they'd be more economical than a bottle - i.e. does liquid flux "go off" ? 

Obviously one wouldn't use it on every job - for my "everyday" soldering, I use rosin cored solder which shouldn't need additional flux - note "shouldn't" ……………………… :roll:

Hi Peter, the liquid is just as good, maybe goes everywhere a little bit more whereas just like whiteboard pens flux pens eventually dry out (maybe after a good few years).

The main difference between the electronic fluxes is cleanability rather than aggressiveness, if you don't clean after use eventually corrosion can set in, I always wash pcb's I have been working on with a brush and IPA (frowned on in a production environment). BTW if you suspect the pcb is old (not bright) an additional wash/scrub before fluxing does no harm (gives the flux less work to do).

Hi Peter,

Kester no residue flux. Organic (malic acid based In an ethanol/water and SDS surfactant solution I think), so the metal has to be clean. I now use almost exclusively very fine tips on an Amazon cheap iron with adjustable temp. and very small diameter low residue cored solder. Tinning wire usually leaves enough flux. For belt and braces a touch of flux on the tinned wire from a fine paint brush is all you need. I have also been using Soundtraxx decoders that come with a capacitor already wired in. Most times I just remove it, as they make not a jot of difference.

I also use Pb/Sn solder. The ones that are lead-free usually need a higher temperature to melt and an even higher one to remelt after tinning and do not flow as smoothly. You can always use low melt solder to minimise damage. Not the very low one, that's a pain. 

I've also had some success using thin pre-tinned solid copper wire to connect to decoder pads, solder up the wires to the stay-alive first, metal tweezers will work as a heat sink. Easier to work with compared to stranded wire, which at the diameters used is floppy. Usually comes with a soft plastic cover. 

Try using a soft 2B pencil with a sharp point and draw around the edge of the solder pads where they meet the substrate. The wax in the pencil helps keep solder from bridging. IPA and a clean piece of kitchen towel on the end of some fine tweezers to remove. 

Failsafe solution is to have the supplier do the work. I've toasted a few decoders over the years. When it's a $120 one that definitely is  a "would you believe it!" moment.

Nigel