Soft Soldering

Concepts

This is one skill that needs some basics understood, before you can be really achieve good results.

In the years that I spent as a Coppersmith, this was a day to day activity and I was taught how to solder by several gentlemen who have probably forgotten more about the subject than I remembered. But what I do remember are some very basic things that make a real difference.

The key things to remember are;

bulletCleanliness is next to Godliness.
bulletSolder will only go where the heat is.
bulletThe right flux for the job is a must.
bulletBe patient.

Soft Soldering, as we called it, is a simple task if this things are taken into consideration.

Cleanliness

Regardless of what type of material you are trying to solder, if the area you are trying to solder is not clean, you will not get anywhere. Make sure that both parts or the area are free from oil, paint, rubber, etc. Anything that stops you seeing the bare metal.

If it is not, then scrape it, file it, sand it, grind it, do anything you have to get to the bare metal. One word of caution with steel and stainless steel, do not burn or blacken the metal as a result of too much heat. Once this occurs you will get a black oxide skin on the metal and you will not be able to solder over this.

If you do get this, then you have to clean it off with either sandpaper, file or some other abrasive action that does not overheat the metal. You can also use Hydrochloric Acid to "pickle" the metal. This usually gives good results with all metal, but please ensure you have the correct safety equipment. It should also be noted that if you do use acid to clean the metal, you must wash off the residue.

Acid cleaning should only be used for non electrical components. If you are soldering electrical or electronic components, please clean only with the abrasive methods and treat the components with some respect. In general you usually do not have to clean electronic components that much.

Just remember your output will be as good as your input, the cleaner it is the better it will be.

Solder will only go where the heat is.

This is a fact of physics, as you melt the solder with a soldering iron, gas torch or any other means of heat you choose, the molten solder can only travel to the areas that have enough heat to allow it to flow.

If it is not hot enough for the solder to flow of its own free will and you "blob" the solder on to the area, you will not have created a good mechanical bond with the parent metals. The correct heat allows the solder and the parent metal to form an alloy between them, this is the actual joining part of the process. Not enough heat, no join. It should also be noted that too much heat will burn the parent metal and the solder, this will have two effects.

bulletIf the parent metal gets too hot and it is steel based, it will get a black oxide skin on it and you will not be able to solder anything to it.
bulletIf the solder gets too hot, it will burn some of the alloys out of it and you will end up with either a weak or brittle joint.

This is one of the basic mistakes that most people make and the easiest way to deal with it is to be patient and let the heat do its job. 

The right flux for the job is a must.

This is another area that is really misunderstood and has major effects on the quality of the soldering job.

There are basically two families of flux;

bulletAcid Based
bulletResin Based

The Acid based solders use different types of acid that have been chemically treated to give them certain properties for the expected use. They usually come in a liquid form and need to be brushed onto the solder joint.

The Resin (pronounced Rosin) based solders are made up of chemicals that have been treated to make them into a firmer form. Most solder that you by for electronics is resin cored. This fluxes are safe to use with most electronic components. 

Why the difference, many reasons, but mainly heat type, the type of components being soldered.

From a type of heat perspective;

bulletAcid based solders work well when you are using an exposed flame to solder with.
bulletWhere as the resin based solder work best with a soldering iron. This is does not mean you cannot use either them with the other method of heating, but there are limitations.
bulletAcid based fluxes work well with a soldering iron as long as you remember that they will cool the solder area, so a little more heat may be required.
bulletResin based fluxes work ok with a flame, but you cannot put the flame directly on the flux, it burns and usually burns black and sooty. This undoes all your efforts in trying to get the area clean. To use them this way you will need to heat the joint area first and then introduce the flux. You can "play' the flux with the very end of the flame as long as you watch the temperature. 

From a purpose perspective;

bulletElectrical / Electronic components should only be soldered with Resin.
bulletSteel, Stainless Steel, Bright Steel, Iron usually solder better with Acid.
bulletBrass, Copper, Gold usually solder with either it is dependant on the sensitivity of the components. Acid delivers the best results, however if it was anything electrical use resin first.
bulletGalvanised, Zinc, and Cadium Plating try acid first, but watch the fumes.

Be patient.

Take you time and if you are not sure try it on a small area and see what happens. If it works, great, if not then try the other method.

As my first tradesman said to me "son patience is a virtue and you learn it from soldering!"

Soldering Usage Matrix

I have put this simple matrix together to make it a little easier for you;

Soldering Job Suggested Method Notes
Electrical Wire to a PCB Resin Cored Solder and a Soldering Iron  
Two Electrical Wires Resin Cored Solder and a Soldering Iron  
Electronic Components into a PCB Resin Cored Solder and a Soldering Iron  
Steel to Steel Acid Flux, 60%/40% stick solder and a flame. The flux I use is made by BOC is it called 956 Soldering Flux and is the best I have ever used as it is not as sensitive to heat as some are. Use what you can, but get used to it with some scraps.
Copper to Copper Acid Flux, 60%/40% stick solder and a flame.  
Brass to Brass, Acid Flux, 60%/40% stick solder and a flame.  
Stainless Steel to Stainless Steel Acid Flux, 60%/40% stick solder and a flame. This is tough stuff, be really carefull with the heat and you may find you have trouble with the flux if it is not correct.
Silver to Silver Acid Flux, 60%/40% stick solder and a flame. Be careful with the amount of heat, silver melts quickly and you cannot tell how hot is it by looking at it.
Gold to Gold Acid Flux, 60%/40% stick solder and a flame. As above
Steel to Stainless Steel, Copper, Brass Acid Flux, 60%/40% stick solder and a flame.  
Nicad Battery Pack Resin Cored Solder and a Soldering iron Make sure the battery terminal are clean. Give them a light sandpaper even if they look clean as the manufacturers coat them with chemicals that actually make it harder to solder.

Make sure you use a big soldering iron and do not hang around. It is also worthwhile to "tin" the ends of the battery before you start. This will make sure you get a good result. 

Fluxes

I have used many different types of flux, below is an outline of my thoughts

Flux Maker Notes
Comweld 956 British Oxygen Company formally CIG in Australia The best for any flame based soldering. I have not used anything better ever. Get a bottle if you can.
Bakers Soldering Flux Bakers (not sure too many years since I looked at the bottle) It is pretty good, but really suffers when you get it too hot.
Resin Cored Solder Everyone Great on electrical components and has some use with a flame. Not too good if the surface is a little dirty.
Killed Spirits You if you like, it is Sulphuric Acid that has been "killed" by dropping blocks of Zinc into it. Be careful this give off lots of dangerous vapours and should only be done with the correct safety equipment. Really good on heavy galvanised iron sheets, but it does not like the flame too much. You have to use a big soldering iron, most plumbers used this stuff to solder roofing iron.

In an emergency you can take a small amount of battery acid and a zinc coated bolt to get a flux.

Pork Fat You laugh!!!

 

This is an emergency resin flux replacement. It does work but you need to coax it along. Mind you it smells great when you put a flame on it....mmmm...crackling!!
Coca Cola You laugh again!!! This is an emergency flux, don't even try a flame it just burns the sugar. You have to be patient with it and it take a while for the acid in Coke to clean stuff. But you can use it.

Tips and Techniques

Here a few simple things that will improve your success.

Item Notes
Tin the area you want to solder first. If you can coat the area that you want to solder with a thin coat of solder prior to trying to solder anything to it. Just run over the area with the iron or a flame and coat it. This will help ensure that you get a good bond when you go to solder the other parts on.

This is really important with Ni-Cad batteries.

Avoid "Dry" joints A "dry" joint happens when you solder something and it moves while it is setting. This actually partially breaks the joint and can lead to connectivity problems with electrical connections or strength problems with mechanical joints.

Set the joint up that is to be soldered so that you can solder it and not have to hold it with your hands.

Don't use to small of a Soldering iron If you are not sure if your soldering iron is too small for the job, then let it warm up to its normal operating temperature, then try to tin a small area where you want to solder. If you cannot get the solder to flow freely, the soldering iron is to small. If you leave it on the area for a small while and the solder flows then it may be ok, however if it does not, then stop.

You will need to get a bigger soldering iron to do the job properly. if you don't then you will get a bad joint, which may cause more problems.

Don't use too big a Soldering iron This is very rarely a problem, because you can usually control the heat of the area you want to solder by the amount of time you have the iron in touch with the area you want to solder. However be aware that if the solder burnt you will have a weak joint.
 

Copyright 2002 - 2003 Craig Tarlington

This material cannot be used for any commercial purpose without my written approval. 

 

Suggestions? Corrections? Remarks? e-mail - Craig Tarlington.
As I get quite a number of messages, it might take some time until you receive an answer and in some cases I get lost in the flood and you may even receive no answer at all. I apologize for this, and if you have not lost patience, you might want to send me a copy of your e-mail after a month or so.
Copyright
2002 - 2004