When using lead free (silver) solder for the first time, you are probably much disappointed. The solder joint looks not nice, looks unreliable, and badly done. Well, and that is probably just what it is. Lead-free soldering is not more difficult, but mostly it is ANOTHER process, and you probably don't know that. All you need to do is change the way you are soldering, and you can have very nice results. Lead-free soldering was always done to get solder joints of higher stability. So... what keeps you from doing it? .
Here is a typical problem situation:
You want to add a wire to an existing connection. The solder joint gets too big and dirty, and the more solder you take, the worse it gets. The components around it start to over heat, and the solder is not flowing still. So you take some more solder from the roll, but the flux that is inside doesn't do the job very good, and the solder joint looks not good. Higher solder temperature makes it worse, and at lower temperature also.
You want to make a normal joint, but it doesn't look nice. So you take some more solder, and re melt the joint to make it look better. Like as you were used from normal solder. However now comes the surprise. The extra solder doesn't flow nice at all. You re flow it again. And again? After a while the components around it are burned, and/or the the PC board cracks off the solder pads. You remove everything and try it again.
What causes this problem?
Here is the difference: With lead solder, the moment it melts, already little flux will make it l flow nicely. The flux is active from the moment the solder has melted, and lead solder melts at such a "low" temperature, the flux will not burn away quickly, doing it's job all of the time. Lead containing solder is an alloy of tin and lead, and the mixture leads to this pleasant, ideal solder temperature, good solderability, no whisker forming, and little cold flow. (Cold flow means the solder "flows" over the years, when pressure is applied to it, like a wire with pull force, or a free wired part with "spring" force on the wiring will get loose after some 20 years. By adding more lead, the solder becomes harder, but must be soldered at a lot more heat. So for soldering copper pipes,a high lead content solder is used. For electronics, the high tin contents solder is used.
Not so with lead-free solder. This ideal combination of tin with lead, is replaced here by a simply less ideal combination of tin and other metals.
Disposing of the lead, and solder with tin only is not a good idea. Pure tin is too soft. Besides tin forms so called whiskers, which are needle shaped, hair fine crystals, which can cause a short circuit on PCBs, switches, etc. Pure tin can't be used for anything because it re-crytallises over the years, and badly changes it's shape.
So the added metals for the lead free alloy, are replacing the lead as good as possible, make the tin become a "solder" again.
Pleas make note, when creating an alloy, the result is not an average of original metals. So melting temperature, hardness and color may change surprisingly in any direction, and also very small traces particular metal can gibe considerable changes. It seems however silver is the (second) best metal to use, after lead. So that's why lead free solder is also "silver" solder. It does contain also other metals, such as copper, and with Mundorf Supreme Silver-Gold solder, even a small dotation of real gold, to get the required properties.
What stays however, is for electronics a disadvantage: The higher temperature. So it's important soldering is done fast. But here comes the problem, it just doesn't "wet" the surface as easy as lead containing solder. It needs to be soldered even longer. It's just how this is.
For a good result, you have to heat up the joint, until the flux starts to burn dark brown. This may seem like "burned" because of the smoke and the color. In this dark brown condition, combined with heat, the resin gets highly reactive. It can now even take away the oxygen molecules from metal oxide, and that's what we want it to do. The oxide is not just removed from the objects we solder, but also from the solder itself. So a dark, crumbling surface of the melted solder becomes shiny and clean.
The resin smokes away, reducing the oxides to pure metal again. And then the silver solder can penetrate the metal below, and we see the solder "flow" when that happens.
A cold solder joint?
This penetration is only possible with "solderable" metals. Like copper and brass, and nickel. If there was no good penetration, we call it a "cold solder joint". So a cold solder joint may look nice initially, but after some years it gets loose. If you try to solder metals which look like nickel, but is in fact a lower cost alloy, you have a high risk on a cold solder joint. So it looks good only initially, but will come loose after 5...10 years.
The moment, the connection is made.
When the flux goes up in smoke, it is most active. However, this moment is short, since the burning flux is disappearing quickly. This makes the moment of good solder flow so short, and you can only make small solder joints, like solder two little wires together. The moment you need to do something larger, you simple need to apply a great lot of extra flux, or even apply it a few times. I have tried everything, but I find this the only way. For best optical results, you can remove the flux residue with alcohol, and it will look very nice.
This is a liquid flux, of highest possible concentration, so as much as possible will stay on the object. It looks like honey. If for some reason you find it too thick, you can use alcohol to make it thinner.
You can also use it to prepare printed circuit boards before soldering. For this application, mix 1 part of the liquid solder flux with 1 part alcohol. Paint the PCB with this, let is dry on the air for one hour, or dry it with air from a hair dryer. When dry, solder the PCB the normal way. Don't bother how it looks afterwards, you can remove the residue nicely with 100% pure alcohol.
Hint: Resin in a good glue. So keep the bottle clean before you close it. If in any case you glue the lid on, with spoiled residue, you can open it after warming the lid with very hot water, or leave the closed bottle in alcohol a few hours.
How to apply extra flux?
The good old way, to dip the solder iron into some flux is not working with lead free solder. The flux is burning away immediately on the solder iron. The small residue that is left, will not spread over the solder joint insufficient, and you may just burn the components.
Process for larger solder joints:
Apply liquid flux on all surfaces that you want to solder. Use as much as you can apply on it. Better take too much as too little, any residue can be very easily wiped off afterwards with a cloth and some alcohol.
With litz wires, apply liquid flux solder before you solder them. Then pre solder those very thin, before soldering finally into the solder joint that they are intended for.
Solder the normal way. I you have enough solder, but you feel there is not enough flux, let cool down the joint, paint as much liquid flux on is as you can and re solder.
If that will not help, here is another method. Empty half of the bottle content in some small metal container. (Like the metal lid of some bottles) Let it dry in there until you have a sticky paste, of which you can apply as much as you need then. With this it will always work nice.
Smaller joints will need less attention, the above is for larger, more difficult joints.
Once you have this technique under control, you will not understand why you ever had problems with it :)