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Soldering tips and tricks for complicated metals Answered

Whether you are just a hobby builder or do your own electronics projects, you know how to solder...
Then one day you find yourself in the position that your solder just does not want to stick...

My first moment of total defeat happened when I was a teenager.
Was building some simple motor with instructions from a book but substituted what I could...
Ended up with some stainless steel contacts and being unable to solder my wires to them...
If you ever had problems like this then read on ;)

What are easy to solder metals?
Basically everything that does not form an oxide layer on the surface and is able to bind with tin, lead or silver.
Copper is one of the easiest metals to solder on but every plumber certainly knows how important a clean and corrosion free surface is.
Any coating or alloy that prevents oxidisation or provides a harder surface usually means with normal, electornics solder we might be lost.
Nickel for example can be a true pain and same for chrome.
So lets start with the hard metals first.

Steel, nickel, stainless...
If the part size does not already mean trouble to get it hot enough, then we face the problem of how to "wet" it with our solder.
Normal steel is usually fine if you give it a fine sanding right before the soldering, however getting the heat onto the part is crucial.
Even something simple like a 5mm thick steel rod can be a pain with a normal soldering iron.
I good way to cheat is to preheat the part or area with a blow torach on a soft flame - not a hot, blue flame.
Try to do this away from the area you need to solder as the temperature difference usually causes some initial condensation on the surface.
Most steels that play a vital role don't like to be overheated as it can affect the hardness an other things, so be careful here.
Rosin core solder works fine on steel and it also indicates when the temperature gets too hot by boiling and smoking badly.
If you still struggle to wet the surface try to scratch it with your solder - if it does not melt the surface is not hot enough.
Nickel coatings are usually very thin and a slight sanding quickly reveals the layer underneath.
If the metal used is not copper already then a copper layer will be electroplated on before the nickel coating.
Either way the key is to get through the nickel without going through the copper, for example if steel contacts were used for durability reasons.
After that soldering is as easy as directly onto copper.
Steinless steel however can be a true pain, same by the way if you need to preserve the nickel coating as best as possible and can sand it off.
Without using chemistry the only way I found is to use a stainless steel tip in the soldering iron.
But as the preperation of one requires chemicals anyway we might start with them first.
The passivating layer of layer or stainless steel can of course be pre-treated by sanding.
Especially very shiny surface benefit from it.
After this I prefer to wet the surface with Phosphoric Acid - you can find it in the harware store as "Rust remover".
It is a food grade acid used in many of your favourite fizzy drinks, so skin contact is not a big deal - just wash it off.
The phosphoric acid is not strong enough to break the oxide layer but it keeps air away.
And once you start scratching the hot metal with your stainless steel soldering tip it will prevent a new oxide layer from forming.
This method however requires a low temperature solder and quick work as the acid boils off quickly.
In the plumbing section of your hardware store your find various fluxes for soldering.
Look for something containing both Ammonium Chloride and Tink Chloride.
Around here a common brand name is Bakers Fluid.
Usually if it has a red danger label on it you will find the above ingredients on the lable somewhere.
Be careful with it as it is very corrosive and harmful to your health!
Good thing is that all remains can be washed off with just running water.
What does it do though?
Unlike the phosphoric acid, the chlrodies directly attack the metal.
Especially once getting hot, so if in doubt wear proper protection as advised on the label!
The oxide layer is not only being eaten away, there is also an ion exchange happening, so a product with more than 30% of zink chloride is prefered here.
The zink binds with the stainless steel or nickel and provides an easier way to bond for the solder.
Key is to work quickly and with precision!
Flux paste is good for brazing but not so good for soldering.
The flux liquid, unlike the paste will start to boil right when the metal get to soldering temperatures.
That is if you use standard lead based solder, most lead free types should be ready a bit sooner.
Start to scratch the metal with the solder and use a soft flame from the other side or close to the soldering area - do not apply the flame directly onto the flux covered area.
Why? Well, the flux isolates the metal from the heat of the flame and it will boil off way before the metal gets hot enough ;)
On smaller parts and when using the soldering iron create a small bubble of solder and keep scratching the surface while it heats up.
In case the flux dries off apply a bit more before this happens!
Once the solder starts to wet the metal a tiny bit it is usually very easy to spread it out to the desired size and shape.
With the heat applied from the underside the solder will always flow to the area of most heat!
Once done it is best to let the part cool down then to give it a good wash under running water to remove all remains of the flux.
Failing to to do so will result in quick and ongoing corrosion, so do it properly...

Aluminium, the bad metal...
I encountered it first when I could not welding or brazing on a quite small part.
Plus, of course, the problem of having to add a copper wire as well.
Then again when I had to solder some aluminium wire.
Acid won't work, chlorides only make it worse, so don't bother with either for aluminium.
Standard rosin core solder also fails.
But there is a suprisingly simple solution to the oxide problem on aluminium.
Mechanical work...
There are quite few videos out there showing how someone solders onto some aluminium foil.
It is so simple because the foil is thin - use it to test your new skills.
A thing though that is often done wrong is the surface preperation.
It usually starts with a fine sanding - to remove the oxide layer.....
The some oil is applied and soldering starts under the oil cover.
And if pay attention then it is often a painful process of scratching with the soldering iron while trying to make the solder bubble wet the aluminium.
That's why foil is so simple here....
What happened in those videos?
Quite simple: Aluminium oxidises right away while you sand it.
Even if you are quick with the oil it already happened.
So why not do the sanding after the oil was applied?
A fibreglass pen or a stainless steel wire brush (usused on other things!) work quite well here.
The oil prevents the air from attacking the aluminum.
If in doubt use some clay and form a little dam around the soldering area to prevent the oil from running off.
Petroleum jelly, vaseline and all other identical things work fine here same for clean engine oil.
But you have to use rosin free solder, no flux core, just plain solder.
If you don't have it simply melt some normal rosin core solder to a nice drop and clean the rosin off ;)
Since there is no real oxide layer with this way of pre-treating the soldering and wetting happens right once the aluminium get hot enough to melt the solder.
You might find it sticking nice right away but don't be fooled!
You need to heat the aluminium until you actually see the solder forming a nice puddle.
With careful sanding you create very clean boundaries.

Other soldering tricks...
Getting cholired based flux for a single job might be overkill.
If you happen to have one of these tip cleaning stones for your soldering iron then you have what you need ;)
Simply scrape some of it off and dissolve it is a tiny amount of water.
Will only be ammonium chloride and requires more scratching on stainless steel but works...
Preparing a stainless steel soldering tip sunds as easy as finding a suitable piece of wire and grindinga tip onto it.
If you every changed the tip on a soldering iron them you know there is two types.
The simple one for the cheaper irons uses a set screw or similar to hold the tip.
The better ones are hold in place by a collar or other type of screw fitting.
And well, those have a thicker part in their body.
If you need to solder stainless steel more than once or twice it makes sense to buy a cheap but powerful soldering iron and to make sure it uses a straight piece of metal with no thicker parts to hold it in place.
If you can't find some stainless steel wire or round bar of suitable thickness you can go slightly below or much thinner if you require a thin tip.
Just make a copper or aluminium collar for the tip to hold it in place, like a sleeve to go around.
Grind the tip to your desired shape before fitting it in....
You won't need a mirror finnish and it can be helpful if the the surface is quite rough.
After all, you want to scratch around on stainless steel with it and you can't harm it this way.
To get a nice and clean cover of solder onto the tip you need the mentioned flux from above.
Use a small cup and fill some of the flux in it so you can dip the tip of the soldering iron into it.
If there is no temperature control start with a cold iron and the tip sanded off a last time right before dipping it into the flux.
Use some clamps or whatever you feel like to help keeping the tip in place.
If you get flux onto bits you don't want to cover with solder then wash off and try again.
Turn the iron on observe the tip.
As soon as you see tiny bubble forming take it out and quickly start rubbing your solder onto the tip.
It helps to have a thick enough solder so you can apply some pressure here.
And of course the solder should be nice and shiny and not covered by oxides...

Special cases like titanium or othe metals that usually fail to bond with solder....
Let's face it: whenever soldering is not feasable we are happy to revert back to crimping or screwing.
Nothing wrong with it either and often the better option when it comes to being able to do a quick repair at a later stage.
Most of thes special metals, including your favourite heating wire can still be solder using the right surface prep and flux but it really should be avoided if you can.
And real bond like you get when soldering copper would only be on a surface level and mechanical strenght questionable.
On a professional level ultrasonic soldering is used to make the impossible possible.
The cavitation effect breaks through the surface oxides or passivating layers and the solder just wets the surface like it would be copper.
On a hobby level things look different though.
Unless you decide to build your own solar panels from scratch the investment into some low end ultrasonic soldering machine already set you back a few grand....
There is a way to cheat on the cheap though if you are into experimenting and building things....
More on that in my other topic about making an ultrasonic soldering tank. ;)