How to use flux

[dublin78]

Hi

I have forgotten what type of flux if any to use.

I am using a lead/silver solder with no flux inside. Some wires that I am tring to use will not accept any solder, and it simply falls off. Will using flux help? Whay type? Is a flux pen the best solution? I don't want to have to clean afterwrds, as most joints will be hard to get at.

Thanks

 

[dublin78]

It is mains cable that seems to be the hardest to use.

Maybe I should just rub with emery paper, or a fine needle file beforehand?

 

[AndrewT]

The flux boils just before the solder melts. This "boiling" flux very aggressively, chemically removes contamination from the surface to be soldered.
This process requires the flux to be matched to the solder. For electronics work buy flux cored solder. The manufacturer makes sure the flux and solder temperatures are compatible for effective cleaning.

I suggest that all beginners and more experienced DIYers only use a low temperature lead based eutectic flux cored solder. The cheapest and just as effective as most others is 63/37. Buy it in various diameters from 26swg to 16swg. 500gram reels will last a long time.

BTW,
some corrosion on copper wiring is very hard. This is best removed mechanically. Quite difficult on stranded wire and especially difficult on 0.1mm & 0.15mm diameter strands. 7/0.2 is just about cleanable.

 

[DF96]

Is your soldering iron big enough for the job? Most people need 2 or 3 irons: 15-18W for fine PCB work, 20-30W for general work, 50W for really thick cables and tags.

Forgive me asking, but do you know how to solder? The instinct for many is to melt the solder onto the tip, then carry it molten to the work. Wrong! This doesn't work too well even when the right flux is used as the flux is active near the tip but all used up by the time it gets to the work.

I prefer 60/40 solder. Don't buy cheap solder. For years I used a roll of Ersin Multicore. When it ran out I bought a cheap reel from Maplins. Useless stuff! I bought some decent solder and all is well again. Avoid lead-free.

 

[AndrewT]

Warton Metals
http://www.warton-metals.co.uk/coredsolderwire.html

 

[dublin78]

Thanks all.

I have one of these 50W variable solder stations: 50W Solder Station : Temperature Controlled Irons & Solder Stations : Maplin Electronics

I am sure that it was a lot more than £20 when I bought mine. This is great value.

I tend to solder one end, let it cool, then introduce it to the other and heat both till a join is formed. Correct?

 

[richie00boy]

Sounds like you are using plumbing solder. As said, flux needs to be matched to the solder, but also to the job. Use proper electronics solder with flux core.

Warton Metals Autosol RA 2% in 0.7mm is a good choice if you want leaded.

 

[dublin78]

It is some "boutique" silver "audio grade" solder that I bought of an ebay oufit a few years back. The rest of the stuff that he was selling all loked high end, so I assumed that the solder was fine.

Thanks. I will buy the Warton Metals one when mine runs out.

 

[KatieandDad]

Maplin do plenty of solder too.

solder : Maplin Electronics

 

[DF96]

I tend to solder one end, let it cool, then introduce it to the other and heat both till a join is formed. Correct?

Not generally. The key to soldering is to get the two surfaces quickly up to the right temperature, get the flux and solder in, then let them cool down quickly before they damage anything nearby. Pretinning can sometimes help, but unless there is something temperature sensitive nearby (such as wiring insulation, or semiconductors) you may get a better joint by not letting it cool down because then you have to heat it all up again.

Where possible, make a mechanically sound joint first - solder is mechanically weak. Then get heat and solder+flux in. Get the solder to flow in, then pull iron and solder away. Leave the joint to cool down and don't try wiggling it or otherwise testing it or moving it until you are sure the solder has solidified. You can turn a good joint into a bad one by disturbing it too early, as it disrupts the crystalisation of the solder.

 

[simon7000]

I buy liquid flux in gallon bottles. I would be glad to send you an ounce except the postage is about $17! The most commonly sold paste flux is for plumbing. That is acidic and not suitable for wiring. If this is a power cord I would just buy an ounce of rosin core solder and use that. Silver solder usually requires more heat than 60/40 solder. It also does not flow as well. Using it on PVC insulated wire might require enough heat to damage the insulation.

The reason why folks use 63/37 solder is that it goes from liquid to solid so fast you rarely get a cold solder joint.

When the iron is at proper temperature soldering a joint should take 3 seconds. Longer and the iron is too cold. Faster and it is too hot.

 

[Simon B]

I often use rosin flux, bought in a large can from a plumbers merchant, and decanted into a small plastic container.

It's a big help with both assembly and disassembly. I should stress rosin flux, not anything else - other plumbers' fluxes are way too corrosive for electronic/electrical work. Rosin is the flux in flux-cored solder

The problem with the flux core in solder is that it's not reaching the places that need protecting until things are already quite hot, ie already oxidising.

A cocktail stick makes a good applicator - only small quantities are needed

 

[kristleifur]

Is your soldering iron big enough for the job? Most people need 2 or 3 irons: 15-18W for fine PCB work, 20-30W for general work, 50W for really thick cables and tags.

+1

I'd like to add that I've found a 90W iron to be very useful too. Great for the biggest joints and some desoldering.

Re. flux, I always have a bottle of liquid "no-clean mildly activated rosin flux" handy. It makes surface-mount soldering a breeze. Here's a video that shows one use for flux: Surface Mount Soldering 101 Video

 

[kamis]

I buy liquid flux in gallon bottles. I would be glad to send you an ounce except the postage is about $17! The most commonly sold paste flux is for plumbing. That is acidic and not suitable for wiring. If this is a power cord I would just buy an ounce of rosin core solder and use that. Silver solder usually requires more heat than 60/40 solder. It also does not flow as well. Using it on PVC insulated wire might require enough heat to damage the insulation.

The reason why folks use 63/37 solder is that it goes from liquid to solid so fast you rarely get a cold solder joint.

When the iron is at proper temperature soldering a joint should take 3 seconds. Longer and the iron is too cold. Faster and it is too hot.

Silver in lead based soldering alloys , like Sn62/Pb35 /Ag2 in fact decreases alloy melting point compared to non Ag types, and improves wetting and solder flowing. Also, soldered joint is stronger and does not deteriorate over time. It is also eutectic.